Influence of Economics, Interspecific Competition, and Sexual Dimorphism on Territoriality
of Migrant Rufous Hummingbirds
Author(s): Astrid Kodric-Brown and James H. Brown
Source: Ecology, Vol. 59, No. 2 (Mar., 1978), pp. 285-296
Published by: Ecological Society of America
Stable URL: http://www.jstor.org/stable/1936374 .
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Ecology, 59(2), 1978, pp. 285-296
?) 1978 by the Ecological Society of America
INFLUENCE OF ECONOMICS, INTERSPECIFIC COMPETITION, AND
SEXUAL DIMORPHISM ON TERRITORIALITY OF MIGRANT
RUFOUS HUMMINGBIRDS1
ASTRID KODRIC-BROWN
AND JAMES H. BROWN
Department of Ecology and Evolutionary Biology, University of Arizona, Tucson, Arizona 85721 USA
Abstract. Migrant Rufous Hummingbirds (Selasphorus rufus) arrive in eastern Arizona in late
summer and establish feeding territories from which other hummingbirds are excluded. Territories
vary 100-fold in area and 5-fold in number of flowers. A simple cost-benefit model accounts for
observed variation in territory size and number of flowers defended. Both sexes defend territories,
but d d utilize denser flowers than Y Y. These differences appear to be related to sexual dimorphism
in wing disc loading. Selasphorus rufus appears to have sacrificed efficient flight for aggressive ability
as a strategy for competing with resident hummingbird species during its migration. Comparison of
feeding territories of S. rufus and other nectarivorous birds indicate similarities which suggest that
these systems may be subject to similar economic constraints.
Key words: Aggression; Aves; behavior; competition; energetics; foraging; hummingbird; migration; nectar; Selasphorus rufus; sexual dimorphism; territoriality; wing disc loading.
INTRODUCTION
Territoriality is utilized by many animals to exploit
limited resources such as food, breeding sites and
mates. Territoriality is presumed to be adaptive whenever it is economical; when the benefits of exclusive
use of a resource outweigh the costs of its defense
(Brown 1964, Brown and Orians 1970). This concept
of economic defensibility has stimulated empirical attempts to assess the adaptive significance of territoriality by measuring characteristics of organisms and
the resources that determine these costs and benefits.
Characteristics of feeding territories of nectarivorous birds have caused them to be frequently used in
investigations of the economics of territoriality. Most
of these studies have attempted to assess costs and
benefits by estimating the energy budgets of territorial
(and sometimes nonterritorial) individuals and relating
these to energy content of available food (Pearson
1954, Stiles and Wolf 1970, Wolf 1970. 1975, Stiles
1971, Gill and Wolf 1975, Carpenter and MacMillen
1976a, b, see also Lasiewski 1963, Wolf and Hainsworth 1971, Hainsworth and Wolf 1972, Hainsworth
1974). This approach has many advantages. However,
several potential errors in extrapolating laboratory
measurements of energetic costs to field measurements of time budgets make it difficult to determine
the precise conditions when territoriality is advantageous. An alternative approach is to assume observed
territories are economical, to measure variation in the
kind and distribution of resources utilized by territorial
and nonterritorial individuals, and to infer costs and
benefits from these data (e.g., Gass et al. 1976). We
have used the latter approach to study feeding territories of Rufous Hummingbirds (Selasphorus rufus).
1 Manuscript received 4 April 1977; accepted 2 August
1977.
The present study investigates economics of territoriality in this species in the context of intra- and
interspecific competition for food. Selasphorus rufus
migrates through the mountains of the southwestern
United States enroute from its breeding ground in
northwestern North America to its wintering ground
in southern Mexico (Grant and Grant 1967, Phillips
1975, Gass et al. 1976). During migration, individuals
of both sexes temporarily establish territories where
nectar-producing flowers are sufficiently abundant. In
this paper, we describe and discuss the temporal pattern of territory establishment and occupation, correlates of variation in territory size and number of
flowers defended, and differences in aggressive and
foraging behavior between the sexes.
METHODS
The study area
Fieldwork was conducted in the White Mountains
of east-central Arizona during the summers of 19731976. All investigations of territoriality at natural flowers were made in the immediate vicinity of a large
meadow 2,300 m elevation, 6 km NE of Nutrioso,
Arizona, from 2 July-10 August 1975 and 5-17 August
1976. At this primary study site, hummingbirds defended territories where 3 species of red, tubular flowers bloomed in the meadow and on adjacent sparsely
wooded slopes. Some supplementary work was done
in other areas and habitats within a few kilometres of
the primary study site. In 1974, experiments with artificial feeders were performed in open woodland habitat at 2 sites: 1,900 m elevation, 25 km SE of Alpine
and 2,600 m elevation, 6 km N of Alpine, Arizona.
The birds
Two species of hummingbirds, breeding Selasphorus platycercus and migrant S. rufus were abundant
286
ASTRID KODRIC-BROWN AND JAMES H. BROWN
Ecology, Vol. 59, No. 2
in the primary study area. These species were cen- tion, seasonal patterns of flower density and nectar
sused during standardized I-h walks at weekly inter- availability were determined by sampling at weekly
vals during 1975. During all 4 summers, 276 birds were intervals during 1975. Flower densities were ranked
captured in mist nets to sample the pollen they were on a scale from 1(<0.05 inflorescences/m2) to 5(>5
carrying and to measure their body weight (in 1975 inflorescences/m2), and 2 flowers from each of 50
and 1976 only), wing length and bill (culmen) length. plants of each species were sampled for nectar from
Territories of 20 male and 4 female S. rufus were 1400-1600 h.
mapped at the primary study site in 1975; territories
Experimental manipulations of territories
of 19 males and 5 females were mapped in the same
area in 1976. Territory size was determined by obIn 1975, 2 kinds of perturbations of natural territoserving defense and foraging behavior of resident ries were attempted. The number of flowers on 3
birds, marking boundaries with flagging tape, and cal- mapped territories was reduced by cutting off approxculating the included area. In most cases, boundaries imately half of the inflorescences. Then, 48 h later,
of territories, and age and sex of residents were de- territories were mapped and available nectar in flowtermined accurately by observing territorial individu- ers was sampled. One artificial hummingbird feeder
als. In the few cases where the boundaries of adjacent that supplied a 20% sucrose solution was placed in
territories overlapped significantly, the area of overlap each of 6 mapped territories. One resident bird began
and included flowers were divided equally between the using its feeder almost immediately; its territory was
2 birds. In 1975, 6 territorial birds (3 males and 3 fe- mapped and nectar in flowers was sampled 48 h later.
males) were captured in mist nets and individually The other feeders never were used, although they remarked with small pieces of colored flagging tape mained in place for 7 days.
Because of an exceptional drought in 1974, so few
glued to the crown. This procedure facilitated mapping
of large territories and provided some data on duration flowers were in bloom we were unable to observe susof territorial occupancy. In 1976, 7 birds (3 females tained territorial defense of native flowers. We studied
and 4 juvenile males) were captured to verify age and some aspects of territoriality by observing behavior of
sex (Stiles 1972).
birds at artificial feeders. At each of the supplemental
study sites described above, we placed several glass
The flowers
feeders containing 150 cm3 of 20% sucrose solution,
Hummingbirds utilized 3 species of red, tubular which the birds removed from the bottom through a
flowers, Castilleja integra, Penstemon barbatus and single glass tube. Feeders were hung in open areas
Ipomopsis aggregata as primary food sources. The from the lower branches of trees at a height of -=1.5
number of flowers of each species within a territory m and separated by distances of at least 5 m. Most of
was counted and flowers were selected arbitrarily for the hummingbirds using the feeders were captured in
measurement of available nectar. From 1400-1600 h, mist nets and individually marked as described above.
the entire nectar contents of these flowers were mea- Utilization and defense of feeders was recorded by
sured with 20-,ul calibrated micropipettes. In 1975, thorough censuses conducted at least once each day.
when P. barbatus was the most abundant flower on
RESULTS
territories, available nectar was measured in 2 flowers
and
Seasonal
interspecific interactions
patterns
on each of 30 plants of this species on each territory.
In 1976, when I. aggregata was more abundant than
Specialized red, tubular hummingbird flowers were
P. barbatus on most territories. we measured available available and used by hummingbirds throughout the
nectar in I flower on each of 30 plants of each species summer (Fig. 1). Selasphorus platycercus utilized
(if abundances permitted) on each territory.
these flowers early in the season.
Other floral characteristics of the 3 species were
Selasphorus platycercus were displaced abruptly by
measured on a large scale to obtain values character- migrant S. rufus in early July (Fig. 1). The date of the
istic of the study area as a whole. These included (1) first observed S. rufus was remarkably consistent from
corolla length, measured with calipers to the nearest year to year: 7 July 1973; 3 July 1974; 4 July 1975; not
0.5 mm; (2) nectar concentration, measured as equiv- observed in 1976. The migrants appeared to arrive in
alent percent sucrose using a temperature compensat- waves that coincided with the onset of clear weather
ed hand refractometer; (3) 24-h nectar production, de- following cloudy and rainy periods. The first arrivals
termined by covering inflorescence with nylon mesh were exclusively adults, and males appeared to outbags, measuring the nectar accumulated after 24 h, number females. Juveniles were rarely seen or capand correcting for the number of new flowers opening tured in mist nets before mid-August, when they began
each day and the amount of nectar in control flowers to arrive in numbers. The newly arrived S. rufus were
on adjacent plants sampled at the time of bagging; and intensely aggressive, both intra- and interspecifically.
(4) diel patterns of nectar secretion, measured as de- The first arrivals established territories on the densest
scribed above except that bags were left in place for patches of flowers. By mid-July, all flowers on the
2-h periods distributed throughout the day. In addi- study site had been incorporated into territories of S.
Early Spring 1978
,
O_:
cr~
287
TERRITORIALITY OF HUMMINGBIRDS
Characteristicsof the nectars of 3 species of humflowers present on territories
mingbird-pollinated
TABLE 1.
A
S. rufus
*
24-h secretion'
S. p atycercus0_
-
.
Species
Concentration
(%sucrose)
Castilleja integra
Penstemon barbatus
Ipomopsis aggregata
41.75 (3 1)2
33.89 (41)
27.54 (52)
0
_
(r
;i-
xx-2
--
I U)
10
dashedin
an
qu
y------a
I
3
Penstemonintereat_obarbatus r
Capoostilia
t:2
c
t
from3 W)
_,0
2 LL
DAT E
Glwrahstol
~
~
~
rufus. Selsphmous plratyr
2.56
2.37
=
24-h accumulation in
Ipomopsis aggregata respectively.
Sample sizes are given in parentheses.
_j~~~~~~
in w95
mtgra?-o
CharacteristicsofJly26l
2.08
baggedflower, x = amountin control flower at time of bagging, and p = proportionof new flowers opening each day.
Empiricallydeterminedvalues of p were approximately0.5,
0.33 and 0.5 for Castillejaintegra, Penstemon barbatus and
2
z
8.10 (100)
8.59 (100)
1 24-hsecretion(y) was calculatedaccordingto the equation
A = py/z + (1 - p)(x + y) where A
te
(Al)
6.45 (96)
(mg
sucrose)
Augurst2Auu
e Wregat omteFThre1 esonaepatersof hummingbird
abwes cunrdance
Flowers of all 3 species had similar color, shape and
corolla length, and they offered similar nectar rewards
(Table 1). This was particularly true of P. barbatus
and I. aggregata, which also displayed their flowers
at comparable heights in loose racemes. Flowers of all
3 species secreted similar quantities of sucrose each
day, but the concentration of nectar varied among species. Diel patterns of nectar secretion were studied
only in P. barbatus and I. aggregata. Both species
produced nectar continuously throughout the daylight
hours at a rate of =-0.2 mg sucrose/h.
Characteristics of territories
of S. rufus
Size and number offlowers.-Territories
rltvdestofflowerson4
3hofuntilthspecrrtoies,
(circles%connce
bny differed in size (area) and in number of flowers beteri2
ory
toothe
lef
the
ae.8
anfI
FIn1.PeStemsona
humigbrdgabuanoftnoc-,
dahe
lerinoywes),andbaratuers
averaesuatty
ofvavailablel
nectaropers
tween years (Table 2). In 1975, territories were larger
rangedlfrom <0.1a flow
sparshedlyinstributd.
Dvraensiatieso
and contained more flowers, but the density of flowers
was lower than in 1976.
lycluded. Ocaesiostnd,allyug they were
hado semeniatNumber of flowers within a territory varied much
bothuP.bearbatrus
teptingl
ditoibfeed,.butnS.irufs
chasged thmfrom
10flw
pandyIcagregataereamspectivelyDeneless than territory size or flower density (Table 2). The
number of flowers varied by a factor of 3.6 in a single
.
Chh
arbacterstics
of floreala
resourctiesyDn
where
its dlyrenrsiy
a
ol 0ig.5flo
Jurittory
Aguse
year and by a factor of 5.6 for both years combined.
flowers
sthreseies of
humingir
occua
achswthntrredtonie
In contrast, territory size varied 99-fold in 1 yr and
fower to ug
were sometes
uretidinfdense snds
species
100-fold for both years combined. Hummingbirds deths
pce
nterritories
defneib.rfus.e
Castilejaentyegars fended a relatively constant number of flowers, but
fr
thrpeie
to5atndforu<10%no
197. flers/ie
fore they were able to exploit a wide range of flower denIns97ua.lbrbtu
abundanto
el/em2
brbtu
than totagmae
wasIcorned
usually.
raemIcouned
then
totalbhistog
gregata on 4122 of the48
2he48territories, whereas%
in17
flowers
ande>50%
on onlyth
they sities by varying territory size. This is demonstrated
sities
ol these
pa
whesre wasy
Ithin0fowingbot
tereriate
wasrOwerseinall
exatly renstversed.Thi
dramatically by a plot of territory size against flower
seritutony
density (Fig. 2). The fitted regression does not differ
in thenst2
obyrtu totefatta
were
and
ing97,rerritaories
ured in toomied, sutaS. Tfres
ambaunanc
eatsie
o
significantly from the line describing the perfect hystudied ltrinthne sead,astonghwhen more somggegtame
hthese species ontertiomines differe occurbee y
perbola that results from plotting the average number
wearselditinbbloo.Hummngbirdes visitdP.o barbatusland
of flowers for all territories (738) as density values over
btiP
aggregataulmst indiscagrimiatarselytandinappoxthe observed range of territory sizes.
matielyothesproportos in whichl
iore
Nectar resources. -Territories varied significantly
ptcheyiti occr.
in availability of nectar. Despite modest variation in
number of flowers defended, amount of nectar per
flower increased markedly with number of flowers in
a territory (Fig. 3). It is interesting to note that flowers
apparently accumulated more nectar in 1976 than in
1975. Rates of nectar secretion are remarkably con-
288
ASTRID KODRIC-BROWN AND JAMES H. BROWN
TABLE
Ecology, Vol. 59, No. 2
2. Characteristics of hummingbird territories
Flowers (n)
Number
and
sex
1c
26
36d
4 Y---->
6
5Y
66
7 Y-*
86
96
106
lid
126
136
146
156
166
176
186
196
206
21 dI
1262
1363
226
23Y
24Y
25 Y-26 Y
276
286
296
30Y
316
326
33c
346
356
366
376
386
396
406
416
426
43 Y
446
456
.
466
47.
First
captured
Last
observed
Area
20Jul
20 Jul
...
18 Jul
20Jul
...
...
...
...
31 Jul
29 Jul
...
27 Jul
31 Jul
...
...
...
...
...
...
...
...
...
...
27 Jul
...
...
...
...
...
240
570
162
863
3,263
628
792
378
406
516
567
483
680
888
625
800
376
550
420
920
646
483
162
900
3,200
1,255
1,385
2,827
452
560
346
1,650
127
95
113
32
127.2
452
300
195
254
360
396
516
900
314
360
213
325
...
...
...
...
...
...
23 Jul
...
...
...
...
...
...
...
...
28 Jul
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
. ...
. . .
. . .
. . .
. . .
. . .
. . .
...
...
...
31 Jul
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
...
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
. .
. .
. .
. .
. .
. .
. .
(m2)
Penstemon
barbatus
x
available
nectar/
flower
(mg
sucrose)
Castilleja
integra
Total
20-31 July 1975
933
4
714
0
1,039
2
669
22
424
0
614
7
440
0
1,609
1
24
1,194
0
1,254
875
26
387
0
391
235
398
15
633
98
573
53
842
34
540
69
653
12
811
281
17
1,480
388
0
607
5
334
365
103
301
0
18
21
76
126
1
29
89
63
16
52
116
139
436
18
44
70
117
0
12
100
93
26
6
297
937
732
1,062
767
550
622
469
1,699
1,281
1,270
953
503
765
849
749
670
946
726
665
1,104
1,597
481
638
705
701
3.90
1.20
6.56
0.82
0.15
0.99
0.53
4.49
3.16
2.46
1.68
1.02
1.13
0.96
1.20
0.84
2.52
1.32
1.58
1.20
2.47
1.00
3.94
0.78
0.22
0.258
0.220
0.573
0.378
0.145
0.187
0.264
0.701
0.698
0.473
0.227
0.331
0.204
0.405
0.426
0.378
0.504
0.423
0.277
0.613
0.930
1.238
0.299
0.536
0.149
4-6 August 1976
90
386
486
118
98
235
19
648
17
899
33
547
184
96
8
497
0
683
143
295
0
571
4
422
102
594
132
198
135
251
0
815
97
425
0
745
0
574
130
203
0
628
34
375
0
1,086
0
823
106
187
158
16
0
0
21
18
15
26
0
15
23
0
0
0
3
0
8
44
0
0
2
8
582
791
491
683
916
580
301
523
698
464
571
441
719
330
386
815
525
745
582
377
628
409
1,088
831
0.46
0.57
0.17
1.51
1.64
1.68
0.18
4.12
7.35
4.11
17.84
3.47
1.59
1.07
1.98
3.21
5.25
1.88
1.13
0.42
2.00
1.14
5.11
2.56
0.527
0.608
0.340
0.399
0.577
0.436
0.210
0.374
0.325
0.348
0.333
0.245
0.635
0.229
0.315
0.402
0.551
0.421
0.311
0.127
0.319
0.428
1.137
0.853
Ipomopsis
aggregata
Flower
density
(n/M2)
1 After reduction of flowers by 50% on the adjacent territories 86 and 96, one bird (216) took the densest stands of
flowers on both territories and the other bird apparently abandoned its territory. Neighboring residents also expanded their
territories somewhat to incorporate the remaining flowers and area of the 2 original territories.
2 After a feeder was placed in the territory and it was being utilized frequently by the resident.
3 After reduction of flowers by -50c.
stant within and between years for local populations
of plants, so that it is likely that this difference reflects
differences in the economics of territorial defense that
are the result of differences in the average density of
flowers on territories or some unmeasured parameter
such as intruder pressure.
Because flowers secrete nectar continuously and the
average rate of secretion is known, the amount of nec-
Early Spring 1978
TERRITORIALITY OF HUMMINGBIRDS
10,000
12
5,000
10O
289
1975
-
9
E
N
V)
A= 738 D-'O
\\
-1,000
/
V .
A=649D082
500
0.6
_
0 0.4
_
9
:cd
cr
0
Y=-0 034 + 0 0005X
r= 0.79
C1
(0.2
3.10.0
0
_J
100.
* . 4
, , .
<
1.2
Z
1.0 _
dr
1976
50
/l
<
01
0.5
10
5
10
50
J 0.8
FLOWERDENSITY (NUMBER/M2
Y = -0.095
r= 0.80
+ 0.00088
0.6
X
FIG. 2. Inverse relationshipbetween territory size (A)
and flower density (D). Sexes of males (cc) and females
d
0.4 _d0
o
(Y Y) are indicated; territorieswhere the resident was replaced by an individualof the opposite sex are shown by
combiningthe symbols. Territoriesmappedin 1975and 1976
0.2 ?
are shown by shaded and unshaded symbols respectively.
/9
The fitted power equationand the linear correlationcoeffi0
200
400
600
800
1,200
1,400
1,000
1,600
1,800
cient (r) for log10transformeddata are given. This relationNUMBER OF FLOWERS
ship (continuousline) does not differ significantlyfrom that
which would be obtainedif the average numberof flowers
FIG. 3. Increasein availablenectarper flower with numwere defendedregardlessof territorysize (dashedline).
ber of flowers on territory.Territoriesof residents indicated
as in FIG. 2. Equationsfor the least squaresregressionlines
and their correlationcoefficients(r) are given.
tar per flower can be used to calculate the frequency
with which flowers on a territory are visited. These
calculations indicate that in 1975, a bird defending 500 the rate of nectar production and consumption on terflowers harvested 0.22 mg sucrose each time it visited ritories containing many flowers.
If variation in the quantity of nectar/flower reflects
a flower, and it visited each flower approximately
every hour if it were the only bird foraging on the the economics of foraging and defense, experimentally
territory. Similarly, a bird defending 1,500 flowers har- manipulating the numbers of flowers should produce
vested 0.71 mg sucrose at each visit, and intervals predictable changes in the amount of nectar/flower acbetween visits averaged >3.5 h.
cording to the relationship in Fig. 3. Number of flowWe can estimate the energy value of nectar pro- ers was reduced on 3 territories (Fig. 4). On I of these
duced on territories with small numbers of flowers. If territories, the resident bird remained and quantity of
we assume an average nectar production of 2.5 mg nectar/flowers declined to predicted levels. The other
sucrose per flower per day (Table 1) and 16.7 J/mg 2 territories with reduced flowers were adjacent, and
sucrose (Brody 1964), then nectar production is 42 J I resident abandoned its territory while the other inper flower per day. By these calculations, territories creased its holdings to include the flowers formerly
produced 12.5 to 70 kJ/day. This technique provides defended by both birds; nectar levels in these flowers
a reasonably accurate estimate of the energy con- assumed expected values. It is difficult to add to the
sumed by both residents and intruders on territories number of flowers on a territory, but nectar availabilwith <500 flowers, because birds visit flowers fre- ity can be increased if the resident can be induced to
quently and thus harvest most of the nectar produced.
use an artificial feeder that dispenses large quantities
For territories containing many flowers, the method of sucrose solution. Only I of 6 feeders placed on terseriously overestimates nectar produced and con- ritories was utilized by the resident, but on that relasumed by birds for 2 reasons: (1) secretion rate ap- tively poor territory, the amount of nectar/flower inparently decreases as flowers accumulate nectar; and creased 7-fold (Fig. 4), dramatically supporting the
(2) flowers contain significant quantities of nectar prediction.
Defense of artificial feeders.-In
contrast to 1973,
when they fall off the plant. Since we do not know the
exact relationship between secretion rate and quantity 1975, and 1976 when flowers were abundant, 1974 was
of nectar present, we have not attempted to estimate a year of exceptional drought and the number of flow0.0
LI
I
290
ASTRID KODRIC-BROWN AND JAMES H. BROWN
ers blooming on the study area was <1% that in the
other years. In 1974, few birds remained to establish
territories on the study area and many must have
starved. Although nectar was secreted at similar rates,
foraging by hummingbirds was so intense that quantities of nectar in the few flowers in bloom averaged
<20% of the values in other years. Although birds
attempted to defend these flowers, interactions with
intruders were so frequent that no sustained territoriality was observed. An emaciated female S. rufus
was found torpid at midday. Birds so weak that they
could barely fly frequently were observed probing at
practically all kinds of red objects. The same feeders
that were only occasionally utilized in other years
were discovered, fed from and defended within a few
minutes of being set up.
During 1974, we made numerous observations of
birds at feeders, but only those that seem relevant to
territorial defense of natural flowers are summarized
here. In particular, feeders provided data on inter- and
intraspecific aggressive superiority that were similar
to those patterns observed at flowers. Selasphorus rufus was almost always dominant over all other hummingbirds, and males of S. rufus usually were dominant over the females. Nine feeders were set up in and
adjacent to a meadow 6 km N Alpine on 3 July. Initially they were utilized extensively by several individuals of S. platycercus, but these were rapidly excluded as S. rufus migrated into the area and set up
territories centered about feeders. By 6 July, 3 male
and I female S. rufus each defended a feeder, and a
male S. platycercus defended and courted females at
another; the remaining 4 feeders were heavily utilized
by several S. platycercus of both sexes. By 11 July,
I individual S. rufus controlled each of the 9 feeders.
Eight of these birds (6 males and 2 females) almost
completely excluded hummingbirds from their feeders, but I female simply was overwhelmed by the large
number of S. rufus and S. platycercus (as well as 2
Stellula calliope) that fed with considerable success
from her feeder.
Eleven feeders were left in place in an open woodland 25 km SE Alpine from 1-9 August. On the first
day, each was defended by an individual S. rufus (7
males and 4 females). Two days later, nonterritorial
birds of 4 species (numerous S. rufus, S. platycercus,
several S. calliope and at least I Archilochus alexandri) had overwhelmed defenders of 2 nearby feeders
and were using them collectively. By 5 August, all
territorial female S. rufus had been displaced, 9 male
S. rufus each defended a feeder, and -30 birds of 4
species fed collectively from the remaining 2 feeders.
Duration of residency.-We have some information
on the length of time individual marked S. rufus remained on their territories. Since a bird may have been
resident for an undetermined period before it was
marked, the longest records probably reflect most accurately duration of residency. Of 6 marked birds with
Ecology, Vol. 59, No. 2
1.4
A
1.2
t
o
(
I
1.0
'R2+3
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o
,J LL
tz
tr0,6
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~~~~~~9
9
4
0.c
0
200
400
600
800
1,000
1,200
1,400
1,600
1,800
NUMBER OF FLOWERS
FIG.4. Availabilityof nectar on experimentallymanipulated (shaded symbols) and unmanipulatedterritories (unshaded symbols). When the resident of territoryA fed extensively from a feeder, the flowers that he continued to
defendaccumulatedmorenectarthanany unmanipulatedterritory (dashed arrow). When the numberof flowers of territory RI was reduced 50%, availabilityof nectar in the remainingflowersdecreasedto expected values (arrow).When
the numberof flowers in 2 adjacentterritories,R2 and R3,
was reduced 50%, 1 resident disappearedand the other incorporatedthe flowers into his own territoryso that nectar
availabilityremainedhigh (convergentarrows).
territories at natural flowers in 1975, 1 male and I female were resident for 12 days and I male and I female
were resident for 10 days. Of 7 marked birds defending
territories centered around artificial feeders in 1974,
1 male was resident for 7 days and another for 5 days.
Of 8 marked nonterritorial birds using feeders collectively, 1 female was present for 8 days, I male for 7
days and 3 females and I male for 6 days.
We frequently noticed that marked birds were obviously fatter and more sluggish during the last day or
2 of their residency than they had been earlier. Unfortunately, birds were almost impossible to recapture, so we were unable to measure the rate at which
marked birds gained weight during their residency.
From maximum differences in body weights among
birds, we surmise that individuals of both sexes deposited 0.5-0.8 g of fat during the 7-14 days they were
probably resident on their territories.
Differences between sexes.-Both sexes of S. rufus
were strongly territorial, but they defended different
sizes of territories and densities of flowers. Males defended smaller territories that were centered around
denser stands of flowers, and there was little overlap
between the sexes (Figs. 2, 5). Of 9 female territories,
3 were taken over by males after periods of 2-10 days.
All of the latter had flower densities within the narrow
range of overlap between the sexes (Fig. 2). The sample size for female territories is small; large territories
with low flower densities probably are underrepresented because of practical difficulties in marking res-
Early Spring 1978
TERRITORIALITY OF HUMMINGBIRDS
0.70F
-Z
COST OF DEFENDING
ADDITIONAL
RESOURCESEXCEEDSBENEFIT;,,.v,-,,-
1,500
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291
SUFFICIENT RESOURCES
TO SUPPORT
TERRITORIAL
INDIVIDUAL_'\'7\,\
1,000
1,500
2,000
2,500
3,000
3,500
TERRITORY SIZE (M2)
FIG. 5. Variation in number of flowers defended as a function of territory size. Symbols as in FIG. 2. Number next to
each symbol gives quantity of available nectar (in mg sucrose) per flower on that territory. Note that all territories lie between
2 thresholds (dashed lines bordering stippled areas), which apparently represent the limits of economically defensible resources. See text for further explanation.
idents and mapping boundaries. All flowers on our
study area appeared to be incorporated in a territory,
and the sparsest stands invariably were defended by
females.
As could be expected from their success at defending dense flowers, males usually excluded females
from artificial feeders. In 1974, 14 males and 6 females
defended feeders immediately after they were placed
out. Five of these females and at least 5 of the males
subsequently were replaced by other males, but in
only I instance did a female replace a male. The maximum length of time a female defended a feeder was
3 days, whereas the 2 males with longest residency
defended for 7 and 5 days.
These intersexual differences in territorial defense
are correlated with differences in wing disc loading
(the ratio of supported body mass to area swept out
by the wings) which influences the metabolic cost of
hovering and forward flight (Pennycuick 1968, WeisFogh 1972, Epting and Casey 1973, Feinsinger and
Chaplin 1975). The 2 sexes have almost identical body
weights, but wings of males average 8.6% shorter than
those of females. This corresponds to a 17%difference
in wing disc loading and a 9.5% difference in the estimated metabolic cost (power output) of hovering
flight (Table 3). Immature male S. rufus, which arrived
later in the season than adults, defended densities of
flowers greater than those in female territories and indistinguishable from those of adult males (Table 2).
Wing disc loading of our small sample of immature
males was intermediate between adult males and females.
Wing disc loading of both sexes was higher than that
of other hummingbird species that S. rufus dominated
and excluded from its territories. Table 3 gives values
for S. platycercus and Archilochus alexandri; small
samples of Stellula calliope conform to this pattern.
Behavior of territorial and nonterritorial birds
Although all flowers on the study area appeared to
be included within a territory, not all birds possessed
territories. There were conspicuous behavioral differences between territorial residents and nonterritorial
birds. Residents spent most of their time sitting on
prominent perches. Occasionally they flew over the
territory or patrolled the boundary. When other birds
approached their territory they initially gave a characteristic twittering call and flashed their gorget (highly conspicuous in adult males and present in reduced
form in adult females and juvenile males) toward the
potential intruder. Usually this was sufficient to prevent trespass on the territory, but if not, the resident
vigorously chased the intruder beyond the boundary.
Nonterritorial birds were of 2 kinds: challengers and
nectar robbers. Challengers were noisy and aggressive. They engaged residents in numerous, conspicuous chases and actual fights. The latter often were
extremely violent; birds hovered, striking each other
with bills and wings, and occasionally dropped to the
ground locked in combat. Sometimes challengers remained in the vicinity of a territory and repeatedly
harassed the resident. One male, which successfully
displaced a resident female, engaged in such tactics
for more than a day.
292
ASTRID KODRIC-BROWN AND JAMES H. BROWN
Ecology, Vol. 59, No. 2
3. Measurements of wing length and body weight and calculated wing disc loading and power output (in milliwatts)
for hovering flight for 3 species of hummingbirds
TABLE
Species
Wing disc
loading1
(g/mm2)
Wing length
(cm)
Body wt
(g)
3.5 (13)
3.5 (13)
3.4 (39)
3.8 (21)
.044
.038
YY
4.07 (118)3
4.42 (97)
4.91 (12)
5.15 (10)
YY
4.25 (27)
4.59 (22)
3.1 (15)
3.3 (16)
.037
.034
Sex
Selasphorus rufus
YY
Selasphorus platycercus4
Archilochus alexandri5
.031
.031
Power output2
(J/h)
407 [= 113.1 mW]
372 [= 103.3 mW]
335 [= 93.1 mW]
367 [ 101.9 mW]
332 [= 98.2 mW]
342 [= 95.0 mW]
Wing disc loading (LWD)calculated from wing length (1) and body weight (W) using the equation LWD
W/[l7r(l+ 0.404
106)2](Feinsinger et al. in press; Greenewalt 1975).
2 Power output for hovering flight (PHOV) calculated from wing disc loading (LwD), body weight (W), and air density (p)
using the equation PHoV = 19.44 W(LwD/p)05,where p was assumed to equal 0.0122 (Feinsinger et al. in press).
3Sample sizes are given in parentheses.
4Includes body wt of 39 d d and 21 Y Y from N. M. Waser (personal communication).
5 Includes wing lengths for 20 d d and 20 Y Y and body wt for 15 d d and 16 Y Y from Stiles (1973).
In contrast to residents and challengers, nectar robbers were submissive and inconspicuous. When trespassing they rarely vocalized, consistently remained
near the ground and vegetative cover, and usually fed
on flowers far from the perch of the resident. Although
robbers sometimes perched for long periods and visited many flowers during several feeding bouts, they
fled immediately when discovered and chased by the
resident. Although adult males occasionally behaved
in this manner, they seldom were successful robbers,
probably because of their conspicuous color and distinctive sound while in flight. Most robbers were females and (later in the season) juveniles. Because of
their secretive behavior, robbers were difficult to observe and unfortunately we have no estimates of their
abundance or their impact on nectar resources.
DISCUSSION
Economics of territoriality
In recent years, much emphasis has been placed on
the economics of territoriality, and nectar-feeding
birds frequently have been used for empirical studies
because of the relative ease with which the costs and
benefits of their territorial defense can be quantified.
Most previous workers have attempted to evaluate the
economics of territoriality by determining time-energy
budgets for territorial (and sometimes nonterritorial)
individuals (e.g. Stiles 1971, 1973, Wolf and Hainsworth 1971, Gill and Wolf 1975, Wolf 1975, Carpenter
and MacMillen 1976a, b). We have used a different
approach; it is more indirect, but has certain advantages. We assume that the hummingbird territories we
observe are profitable for their owners. This is reasonable because the small size and high metabolic rate
of hummingbirds prevents them from surviving for
long if energy expenditure exceeds income; birds with
uneconomical territories should abandon them and
either become nonterritorial or move to another area
where resources can be defended economically.
Therefore, variability in size of territory and number
and density of flowers is assumed to indicate the range
of conditions permitting economical defense. Analysis
of variation in these parameters and in the quantity of
nectar accumulated in flowers should help elucidate
the adaptive strategies by which individuals economically defend diverse resources.
Territories vary greatly in size and number of flowers, but this variation is confined within definite limits
that appear to be determined by economic constraints
(Fig. 5). A minimum of :300 flowers is required to
support a territorial bird when flowers are dense. The
threshold number of flowers to support a resident increases with territory size, so that the largest territories contain a minimum of :500 flowers. This increase
presumably reflects increasing costs for the resident
as territory size increases. Distances flown while foraging and repelling intruders should increase with territory size. The fact that the largest territories are occupied exclusively by females, which have less costly
flight but inferior aggressive ability in comparison to
males, suggests that the costs of foraging become of
primary importance when flower densities are low.
There is also an upper threshold number of flowers
above which cost of defense apparently exceeds benefit. This threshold value decreases rapidly with territory size, presumably because distance of foraging
and aggressive flights (and perhaps also the frequency
of successful nectar robbing by intruders) increases
rapidly with area.
Between the limits defined by the thresholds is a
range of territory sizes and number of flowers that is
economically defensible (Fig. 5). There is great variation in the number of flowers in small territories, but
this variability decreases rapidly with increasing territory size. When flowers are dense, birds can obtain
the benefits of increased amounts of nectar per flower
by increasing the number of flowers defended with
Early Spring 1978
TERRITORIALITY OF HUMMINGBIRDS
,CL
., -
z
w
m
CH
,,'B
0
X,,
0
0
a7RL>
/-)RH
NUMBER OF FLOWERS DEFENDED
FIG. 6. A cost-benefit model of territoriality which accounts for variation in the number of flowers defended. Two
cost curves (CHand CL).for high and low densities of flowers
respectively, intersect a single benefit curve (B). The hatched
areas between the intersecting curves represent the ranges
(RHand RL)of number of flowers birds are expected to defend
at high and low densities respectively.
little increase in territory size and hence in defense
costs. The more flowers a bird defends, the greater
the accumulation of nectar within each blossom (Figs.
3 and 5). Therefore, the resident needs to visit fewer
flowers per unit time to obtain the same rate of energy
intake. Since the amount of nectar per flower varies
as much as 5-fold when flower densities are high, the
potential benefits of defending many flowers is substantial. However, it is uncertain to what extent birds
can profit from such increased nectar availability. As
the number of defended flowers increases, they can
collect nectar at a greater rate and spend a smaller
proportion of their time foraging. This may be advantageous because it permits more rapid accumulation
of fat reserves, and provides more time to watch for
intruders and predators. For a migratory bird, rapid
deposition of fat probably is advantageous, but there
must be a maximum rate at which food resources can
be converted to adipose tissue. Birds also may benefit
by reducing foraging time. Foraging birds probably are
more susceptible to predators and nectar robbing by
intruders than birds sitting on their conspicuous perches which afford panoramic views. We observed several birds missing all of their retrices, which suggests
that predation on hummingbirds may be more frequent
than is generally thought (Mayr 1966). When flowers
are sparse, there is little variation in the number defended; most territories contain few flowers in excess
of the minimum threshold. Presumably, it is uneconomical to defend more because this would entail large
increases in territory size and hence in the cost of
foraging and aggressive flights for relatively little benefit.
We present a simple cost-benefit model (Fig. 6)
which accounts for the pattern of variation in territory
293
size and number of flowers defended (Fig. 5). If benefit
(B) is defined as the quantity of floral nectar which can
be utilized by a resident, we can plot a single benefit
curve which must begin at the origin, increase directly
with number of flowers defended, and then level off
as the capacity of the bird to harvest nectar becomes
saturated. If we define cost (C) as total energy expended for maintenance, foraging and territorial defense, then we can plot a cost curve which must begin
at some positive value (maintenance cost) and increase
with number of flowers to reflect energy spent on foraging and defense. We show cost as a linear function,
but some other form of continuously increasing curve
is possible. Since cost of defending flowers depends
on density, and sexual dimorphism in territorial strategies indicate it is more expensive to defend sparse
flowers than dense ones, we plot 2 cost curves with
different slopes. At each flower density, the most advantageous territory contains a number of flowers
such that B-C is maximized. We suggest that birds
should never defend more than this number, because
they could increase their net profit simply by giving
up some of their flowers. Competition with other birds
may force some individuals to defend territories containing fewer flowers than the optimal number, but
they should only be territorial if they can remain in
positive energy balance (B-C > 0). The model indicates that birds can be expected to defend a range of
territory sizes such that there is more variation in
number of flowers when they are dense than when
they are sparse. The model also accounts for the observation (Fig. 5) that minimum number of flowers
defended decreases with increasing flower density, but
varies less than maximum number.
Economics of territoriality undoubtedly are influenced by factors we have not analyzed here. We present values for average density of flowers on territories, but flowers were not uniformly distributed.
Usually, small territories were centered around I
dense patch, but if a patch was large, it was often
subdivided among several birds. Even the largest territories contained small stands of much denser flowers
than average. Territories also varied in shape. Most
were compact and approximately circular, but occasionally the distribution of flowers and other territories
resulted in more irregular and elongate shapes. We do
not know how intruder pressure varied with the other
characteristics of territories. Intruder pressure can affect the economics of territoriality in 2 ways: (1) by
eliciting energetically costly agonistic displays and patrolling and aggressive flights and; (2) to the extent
intruders are successful in robbing nectar, by reducing
the amount available to residents. These factors and
others we have not considered, probably account for
some of the variation in the relationships depicted in
Figs. 2, 3 and 5.
Territoriality and other kinds of interference competition that have evolved in response to economic
294
ASTRID KODRIC-BROWN AND JAMES H. BROWN
tradeoffs in resource utilization can result in food limiting populations such that some individuals can starve
while others have access to food in excess of their
needs. This probably happens in our study area, at
least in years of average to low flower abundance.
Many individuals of S. rufus are unable to secure territories, while others defend territories containing several times the amount of nectar required to support a
resident and enable it to deposit sufficient fat to continue migration. For territorial organisms, it probably
is unnecessary to invoke group selection (Wynne-Edwards 1962) or special mechanisms other than selection for individual economic benefit to account for the
apparent "self regulation" of populations below the
limit determined by available resources.
Wing disc loading, sexual dimorphism and the
strategy of migration
Territoriality enables migrant individuals of S. rufus
to gain access to floral resources and replenish lipid
reserves. This species has the longest migratory route
of any North American hummingbird. Its migration
follows the blooming of hummingbird-specialized
flowers (Grant and Grant 1967, Gass et al. 1976). From
its winter range in southern Mexico, it migrates north
along the Pacific Coast where lowland and foothill
flowers bloom in response to winter rains. It breeds
from Oregon and Idaho to southern Alaska and the
Yukon. Its southward migration in late summer follows inland mountain ranges where flowers bloom at
intermediate to high elevations. The pattern of migration appears to consist of a series of long flights interrupted by intense foraging that usually involves territoral defenses of dense flowers for periods of several
days (Armitage 1955, Grant and Grant 1967, 1968,
Dunford and Dunford 1972, Gass et al. 1976). Longdistance migratory flights may be a common characteristic of temperate-zone hummingbirds. Archilochus
colubris is known to fly =1,000 km across the Gulf of
Mexico (Lasiewski 1962, 1963), and the exhausted,
emaciated appearance that we observed in newly arrived S. rufus (and also Stellula calliope) suggests that
other species may fly comparable distances.
Migrant hummingbirds face the problem of competing for floral resources not only with other members
of their own species but also with breeding and resident hummingbird species during migration and on the
wintering ground. Such competition may be particularly severe during migration because at each stop the
energy-depleted individual must rapidly and effectively compete with established birds which are utilizing
the local flowers. This may account for the apparent
paradox that males of S. rufus, the species with the
longest migratory route, have the highest wing disc
loading and hence, the energetically most costly flight
reported for any North American hummingbird (Feinsinger and Chaplin 1975, present study: Feinsinger and
Chaplin [1975] give a higher value of wing disc loading
Ecology, Vol. 59, No. 2
for male Selasphorsus sasin [0.0466 g/cm2] than for
male S. rufus. However, they used body-weight data
for captive individuals of S. sasin, and such birds tend
to be more obese than free-living ones. Recalculating
using data from Stiles [1973] gives a value of 0.0403
g/cm2 for male S. sasin, which is significantly lower
than our value [0.0428] for male S. rufus).
Several authors have noted that metabolic cost of
both hovering and forward flight varies directly with
wing disc loading (Pennycuick 1968, Hainsworth and
Wolf 1972, Weis-Fogh 1972, Epting and Casey 1973,
Greenewalt 1975). Feinsinger and Chaplin (1975),
however, point out that there is a tradeoff between
energetically efficient flight and aggressive ability.
They note that territorial hummingbird species have
higher wing disc loading than traplining species, and
they suggest that selection for speed and maneuverability in aerial encounters has favored the evolution
of relatively short wings in territorial hummingbirds.
Our observations support this pattern. Both sexes of
S. rufus have higher wing disc loading than the 2 species of breeding hummingbird (S. platycercus and A.
alexandri) that are displaced from flower patches and
feeding stations when aggressively territorial S. rufus
migrated in our study area. We suggest that S. rufus
has sacrificed efficient flight for aggressive ability, enabling it to compete successfully with resident hummingbirds during migration and perhaps also on the
wintering ground.
It is interesting that the virtually absolute dominance of S. rufus over resident hummingbird species
observed at our study area is not characteristic of its
interspecific competitive ability in other habitats. Our
study area was located at intermediate elevation and
contained relatively dense flowers. Observations in
other habitats indicate that S. platycercus is not completely excluded by S. rufus at high elevations (>2,500
m) nor is A. alexandri completely excluded at low elevations (>1,500 m). We suggest that at high elevations, low air density (which increases flight costs;
Feinsinger et al., in press) and low temperature
(which increases total metabolic costs) increase the
advantages of efficient flight and enable S. platycercus
to compete successfully with S. rufus. In the desert
habitats at low elevations, flowers tend to be sparsely
distributed which favors their exploitation by A. alexandri with its low wing disc loading. It appears that
differences in aggressive ability and foraging behavior,
which are related to differences in wing disc loading,
play a major role in mediating competition among temperate North American hummingbird species, and
these may be particularly important during migration.
It is during migration that these species, which have
largely nonoverlapping breeding ranges but are all
about the same size and able to utilize the same flower
species, come into greatest contact and have greatest
potential for direct competition.
The sexes of S. rufus differ conspicuously in wing
Early Spring 1978
TERRITORIALITY OF HUMMINGBIRDS
disc loading and in size of territory and density of
flowers defended. Males apparently use the aerial
agility conferred by their short wings to aggressively
defend flowers that are sufficiently dense to pay their
high foraging and defense costs. Females, because
their longer wings permit more efficient flight, are able
to forage and defend flowers that are too sparse to be
defended economically by males. The result is subdivision of floral resources on the basis of density with
only a narrow overlap between the sexes (Figs. 2, 5).
Most of the successful nectar robbers which we observed were females. Feinsinger and Chaplin (1975)
have reported other examples of intersexual differences in foraging and territorial behavior, in hummingbirds, that are correlated with differences in wing
disc loading.
The adaptive basis of sexual dimorphism in wing
disc loading in S. rufus is not clear. Although these
differences provide the basis for resource subdivision
and consequent reduction of competition during migration, it is likely that selective pressures related to
the different roles of the sexes during the breeding
season are primarily responsible for the evolution and
maintenance of sexual dimorphism. We suggest 2 possibilities. First, males may be selected for high wing
disc loading because they aggressively defend territories of dense flower patches that are used for courtship and mating. Superiority in aerial aggressive encounters conferred by relatively short wings may
increase reproductive success by affecting the outcome of intrasexual competition for mates. Evidence
against this hypothesis is the observation that the related species, S. platycercus, has similar territorial
breeding behavior, but low wing disc loading in both
sexes (Table 3). We favor the alternative hypothesis
that low wing disc loading is advantageous to female
S. rufus on the breeding grounds. For a period of several weeks, activity of females must be centered
around the nest. Because the abundance of flowers in
nesting habitats of S. rufus may be low (W.A. Calder,
personal communication), energetically efficient flight
should be particularly advantageous to females while
they are nesting and feeding young. We suggest that
wing disc loading of females reflects a compromise
between selection for efficient flight during the breeding season and selection for aggressive territoriality
during migration. Because males do not participate in
incubation or care of young, they are not subject to
this compromise and can respond to a much greater
extent to selection for aggressive ability.
Feeding territories of nectarivorous birds
Selasphorus rufus resembles many other hummingbirds, African sunbirds, and Hawaiian honeycreepers
in defending feeding territories during the nonbreeding
season (Pitelka 1942, Grant and Grant 1968, Wolf
1969, 1970, Stiles and Wolf 1970, Wolf and Hainsworth
1971, Stiles 1973, Gill and Wolf 1975, Feinsinger and
295
Chaplin 1975, Carpenter 1976, Feinsinger 1976, Carpenter and MacMillen 1976a,b). These feeding territories have been investigated intensively because they
provide excellent systems for field studies of avian
behavior, territorial economics, and plant-pollinator
interaction. Results of these studies invite searches for
common patterns and mechanisms.
Although some hummingbirds use traplines to exploit sparse floral resources and apparently are not
strongly territorial, many species aggressively defend
feeding territories. Feinsinger and Chaplin (1975; see
also Feinsinger et al., in press) pointed out the relationship between wing disc loading and foraging and
territorial behavior that appears to be widespread in
hummingbirds and to account for the interspecific and
intersexual interactions of S. rufus. Several authors
(e.g., Pitelka 1942, Cody 1968, Stiles 1973, Feinsinger
1976) have discussed the prominent role of interspecific aggression and territoriality in subdividing food
resources among coexisiting hummingbird species.
Gass et al. (1976) used an approach similar to ours to
study territoriality of migrant S. rufus. Although they
did not comment on many of the patterns reported in
this paper, they observed precise regulation of territorial size in response to flower density (Fig. 2) virtually
identical to that shown in Fig. 4 of their paper. It is
interesting that only females and juvenile S. rufus utilized their study area in northwest California during
July and August, but these birds defended small territories with high densities of flowers comparable to
those defended by adult males on our study site.
Sunbirds in the Old World and honeycreepers in
Hawaii have coevolved mutualistic relationships with
specialized flowers that they pollinate while foraging
for nectar. Convergent similarities between these systems and the hummingbird-plant associations of the
New World include frequent defense of feeding territories (Gill and Wolf 1975, Carpenter and MacMillen
1976a,b). Thus, Gill and Wolf (1975) report that the
African sunbird Nectarinia reichenowi defended feeding territories varying in size from 6.7 to 2,300 m2 and
containing 1,000 to 5,000 flowers of Leonotis nepetifolia; these flowers produced -1 mg of sucrose/(flower-day) or 17 to 85 kJ/(territory-day). These values can
be compared to those we obtained for S. rufus, which
defended territories varying in size from 32 to 3,200
m2 and containing from 300 to 1,700 flowers of 3 species; these flowers secreted -2.5 mg of sucrose/(flower-day) or 12.5 to 70 kJ/(territory-day). Although these
2 coevolved mutualistic systems of nectar-feeding, territorial birds and bird-pollinated flowers appear to be
convergent in many respects, several differences are
apparent. Nectarinia reichenowi (body weight, 15g) is
-4x larger than S. rufus and, like other sunbirds, it
differs from hummingbirds in rarely hovering while
foraging.
The striking similarity between the pattern of territory sizes and number of flowers defended by S. rufus
296
ASTRID KODRIC-BROWN AND JAMES H. BROWN
(Fig. 5) and comparable data for African sunbirds (Gill
and Wolf 1975; see Fig. 4 which, however, is not discussed in the same context) suggests that these evolutionarily convergent territorial systems are governed
by similar economic constraints. These similarities
and others mentioned above indicate that it may be
possible to develop general models of territorial economics which are widely applicable to nectarivorous
birds and perhaps to other organisms as well.
ACKNOWLEDGMENTS
We thankKevin and KarenBrownfor assistanceand companionshipin the field, R. Vestal for help with data analysis,
S. B. Chaplin,P. Feinsinger,C. L. Gass, F. B. Gill, F. R.
Hainsworthand L. L. Wolf for criticallyreadingthe manuscript, and numerouscolleagues includingZ. Abramsky,T.
C. Gibson, H. R. Pulliam and N. M. Waser for valuable
discussions. The study was supportedin large part by the
NationalScience Foundation(GrantsGB 39260and DEB 7609499).
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