AMER. ZOOL.. 18:753-763(1978).
Proximate Costs of Competition for Nectar
FRANK B. GILL
Academy of Natural Sciences of Philadelphia,
Philadelphia, Pennsylvania 19103
SYNOPSIS. Food competition among coexisting nectarivorous birds is conspicuous and often
intense, affecting patterns offlowerchoice, daily behavior budgets, and timing of successful reproduction. Exploitative competition involves loss of accumulated nectar to other
individuals that visited a flower first. Preliminary data support the use of Poisson models of
the frequencies of point-source visitation and overlap for determining the probabilities of
actual competitive events. Nectar losses from monitored flowers can be estimated in terms
of time intervals between visits weighted by flower-specific nectar production and birdspecific nectar removal capabilities. Foraging time budgets then provide a meaningful
common denominator for assessing the impacts of competitive nectar losses, because
compensatory increases in foraging time are required to maintain a balanced energy
budget. Flexibility in foraging time budgets made possible by high efficiency foraging and
predictably low competitive losses may be an important determinant of reproductive
timing and success in nectar feeding birds.
Aggressive displacement of competitors and territorial defense of flowers are common
forms of interference competition in nectar-feeding birds. Aggression has definable caloric
costs that ultimately must relate to caloric gains. Defense of flowers increases the
aggressor's exclusive use of nectar, increases the predictability of a nectar supply, and
increases the average amount of nectar obtained per flower. Simple cost-benefit models of
territoriality define conditions when net benefits of territoriality are greater than those of
alternatives.
INTRODUCTION
Resource competition usually is viewed
as some combination of exploitation or
co-utilization of a limited resource, and
interference or interactions which affect
viability or access to a resource (Miller,
1967; Case and Gilpin, 1974; Gill, 1974).
Competition of both kinds is a conspicuous
aspect of the biology of nectar-feeding
birds (Pitelka, 1951; Skead, 1967; Cody,
1968; Stiles and Wolf, 1970; Wolf and
Hainsworth, 1971; Gill and Wolf, 1975;
Carpenter, 1976), largely because nectar is
a caloric resource that is produced in a
finite number (often small) of conspicuous,
stationary, easily located containers
(flowers). Nectar also is renewed at predictable rates in flowers. Competing indi-
The development of this paper has been supported
by the National Science Foundation (GB 76-04152)
and stimulated by frequent discussions with L. L.
Wolf, R. Horwitz, and D. Gill. J. Brown, F. R.
Hainsworth, S. Lanyon, L. Wolf, and an anonymous
reviewer made helpful comments on earlier drafts.
viduals can find the flowers easily and
revisit the same resource sites. Individuals
often compete aggressively for access to
flowers with nectar when this resource is
economically defensible (Brown, 1964; Gill
and Wolf, 1975; Carpenter and MacMillen, 1976a).
Here I examine some patterns and
proximate costs of competition in nectar
feeding-birds in terms of what might be
called microeconomics. This approach
emphasizes proximate impacts on individual nectar feeding birds of caloric losses
resulting from both exploitative and interference competition. It is a mechanistic
and highly reductionistic approach to the
process of competition that ultimately may
influence higher levels of community organization. I assume that small, nectarfeeding birds are often food and energy
limited, and thus are subject to various
degrees of competition in the course of
their annual cycles (Wolf, 1970; Stiles,
1973; Yeaton and Laughrin, 1976). I shall
be concerned primarily with interactions
that influence an individual's access to
753
754
FRANK B. GILL
nectar supplies essential to maintaining
balanced daily energy budgets (Wolff/ id.,
1975). I will suggest that regardless of the
overall abundance of nectar the loss of
nectar to other individuals feeding at the
same Mowers may constitute competitive
interactions of ecological and evolutionary
significance.
EXPLOITATIVE COMPETITION
Exploitative competition for nectar involves probabilities of nectar loss from
particular Mowers visited. This first involves probabilities of sequential visits to
specific sites, usually within a large array of
alternative sites. Second, site visits may he
weighted by their resource value, which is
a function of time and the rate of nectar
accumulation within the brief productive
life of Mowers. If the nectar in them is not
consumed, it may be resorbed or else discarded with the wilted flower. The probabilities of Mower visits are analogous to
prey encounter rates (Schoener, 1971)
while temporary changes in resource value
are analogous to individual survivorship
and to increased size due to age-specific
growth of prey.
The probability (P) of k visits to a Mower
in time / by a randomly foraging individual
can be modelled as a Poisson process
P(k) =
k!
where /JL is the mean visitation rate. This
means that some proportion P of the available Mowers will be visited k times in a given
time period. The mean visit rate /x is
defined as the total number of visits to
Mowers (or inflorescences) in that time
period divided by the number of Mowers.
By monitoring visits to marked Mowers we
can compare the observed frequency distributions of Howers visited k times with the
expected distribution (Fig. 1). The available data for non-territorial sunbirds and
about half the data for territorial sunbirds
are statistically indistinguishable from the
expected Poisson distribution (Gill and
Wolf, 1977). Territorial individuals
sometimes biased visits towards unvisited
inflorescences over relatively short time
60-
PO-YX
29 JULY
P'05
e
i
£
'
8
6
Z
20-
s
0
No. of Visits Per Paw
FIG. 1. Distribution of visit frequencies to inflorescences (paws) of Leonotis nepetifolia. The observed frequencies (open circles) by a marked sunbird, Nectarinia reichenowi, are compared to the frequencies
expected from a Poisson distribution (closed circles).
The data on the left are not significantly different
(P > .05) from the expected distribution and were
the typical result in non-territorial situations. The
data on the right show significant biases toward previously unvisited paws (0 visit category) as sometimes
accomplished by territorial males. Details of this study
are available in Gill and Wolf (1977).
periods. The Hawaiian Honeycreeper,
Lnxops viren.s, shows similar nonrandom
foraging patterns (Kamil, 1978). But apparently the restricted feeding areas, high
mobility and foraging intensity of nectarfeeding birds often combine to equalize
probabilities of visits to flowers (Gill and
Wolf, 1977), making random models of
flower visit probabilities useful as hypotheses for field studies.
Prior visits to Mowers do not seem to be
detectable by birds until they actually
probe a Mower. Therefore, if individuals a
and b feed randomly in the same place fx
will increase as fx,a + fjih. I define a competitive event as a pair of successive visits to
the same Mower by different individuals.
The probability of successive visits in the
time period /, P,,,,, will be the product of the
separate probabilities of one visit each by
those individuals:
Pa(l)Pb(l) =
p ( - fJL ) fJL ]
= [exp To some degree Pah is a function of
overlap on some predefined resource gradient. But fJ.,, + fji/i are functions also of (1)
Mower density, (2) flower abundance, (3)
rates of nectar production, (4) Mower quality, (5) caloric requirements of the birds
and hence their foraging rates, and (6)
h
h
755
COMPETITION FOR NECTAR
total numbers of birds in an area. Average
/Aa+b should vary seasonally and thus could
1 Flower
- - ^ _ - —
Wilts
provide a comparative index to the level of
consumer pressure on available nectar resources. For example, our studies of Ken/ ^ b
yan sunbirds show \x for all species com- (D a
t
c
bined averages 0.4 visits inflorescence"1
hour ~' when nonbreeding species aggrec'
_—
c"
gate at Leonotis nepetifolia in the lowlands.
In contrast, values of /i. from breeding
t
t
t
Th ird visit
First visit
situations when flowers are conspicuously
Second visit
more abundant average less than 0.1.
Time
Pab decreases exponentially as fJL deFIG. 2. Temporal changes in resource value of a
creases, so that slight reductions in average nectar-producing flower. The amount of nectar in a
visit probabilities can greatly reduce the flower and therefore available to a nectar-feeding
probability of competitive events. Pab is bird is a function of the nectar in the flower when
thus a rather precise measure of the prob- is first opened, (a0), the nectar accumulation (b) per
period / and the amounts removed (c, c\ c") on
ability that a particular resource item will time
preceding visits.
be lost to a preceding competitor. This
joint probability may be small simply as a
result of moderate numbers of flowers and (Fig. 3; Gill and Wolf, 1978ft) using averlow feeding rates or as a result of behaviors age field measurements of starting nectar
that actively reduce exploitative competi- volumes, nectar accumulation rates, and
tion, such as patterning visits relative to proportions of available nectar removed
encountered variations in Hower quality (Fig. 2; Gill and Wolf, 1978c). Exploitative
that are the result of previous foraging nectar losses by nonbreeding male Nec(Gill and Wolf, 1977; Kamil, 1978), tarinia reichenowi feeding at Leonotis
segregation of foraging efforts (Colwell ei
«/., 1974; Feinsinger, 1976), and excluding
competitors from certain flowers (interfer7•
N. reichenowi 1973-75
ence). Competitive interactions also will be
/
o non-territorial males
reduced in frequency if flowers disappear
• non-territorial females
6
(for example wilt) within the time period.
The more transient the resource items the
lower the probability of joint site overlap
o
/ .
and exploitative loss.
a
o
4
The amount of nectar lost to com- =>
(0
/ o
• o
o
petitors is the fraction of the potentially
o
accumulated amount that was removed by
•°
o
prior visitors. This is a function of visit
/
o
o
•
timing (Fig. 2). Summed over all flowers
?
8 * °°°
4
visited, we can express the total nectar lost
.* . •
o
«
o o
as a percent of potentially available nectar
'
/
in visited flowers. The individual that gets
to a flower first obviously minimizes loss
0
1
2
3
4
5
6
7
and negatively affects the foraging effiPotential Nectar Uptake (ul/fl)
ciency of subsequent visitors. The percenFIG. 3. Nectar losses from undefended Leonotu
tage loss of potential intake should in- nepetifulia
flowers, by the sunbirds, Sectarinia
crease as the interval between sequential reichenowi (from Gill and Wolf, 19786). Calculation of
visits decreases and as the time before the actual intakes involves subtracting nectar removed by
first visit increases. We can estimate the preceding visitors from total potential accumulation
percentage of potentially available nectar at each flower. Each point is an average of at least 100
visits during a four-hour morning observation
that was actually lost to preceding visitors flower
period.
o
0
•
J,
m
756
FRANK B. GILL
nepetifolia when alternative nectar sources
were limited were as high as 83% and
averaged 36%. Females lost significantly
more than males, averaging 45%. In contrast, territorial defense greatly reduced
actual losses to an average of 8% (Fig. 4).
Comparable data are not yet available for
sunbirds on breeding territories but competitive losses there are low (Gill and Lanyon, unpublished).
Optimal timing of visits is a function of
the probabilities of visits by competitors
and of nectar renewal rates. Waiting
longer before visiting a Hower may increase the amount of potentially accumulated nectar, which sets up a positive feedback system of reduced foraging times and
more accumulated nectar (Wolf et ai,
1975). But waiting also increases the probability that the flower will be visited by a
competitor. Any aspect of foraging that
biases visits to least recently visited Hovvers
obviously should be advantageous, but
under conditions of high competitor pressure or relatively ineffective territorial defense, a bird should revisit flowers frequently to minimize losses. Shorter inter-
potential
a.
"^ 4
Q)
._ 3
a>
actual
Q.
C
<D
2
CD
Territorial
Nonterritorial
KIG. 1. Comparison of net tar lo-ses b\ territorial and
non-territorial male Xictanm/t itnlnmnvi feeding at
Lt'(>iioti\ tii'pttjfttlia. The data for the two conditions
were obtained in different \e.ns u-Heiting diflt'ient
levels of net tar a\ailabilit\ ;<'H! the ad\anlaues of
l)eing tei I ilouai or non-tei ittoi :.ti.
vals between foraging bouts and shorter
foraging bouts (because of less crop or
stomach depletion) would be expected.
Yeaton and Laughrin's (1976) observations
of Calypte anna and Selasphorus sasin agree
with this prediction, although they
explained sasin's shorter, more frequent
foraging at high density Zauschneria with
many intruders in terms of morphologically smaller crop volumes than anna,
which fed at lower density flowers with
little intruder pressure.
Evaluating the consequences of nectar
loss requires a common currency that is
biologically meaningful for different sized
species. Ultimately we are interested in the
impact of the loss of nectar on an individual's survival and reproduction. To
some degree these must be inversely proportional to the minimum amount of time
required for self-maintenance especially
the foraging time required to maintain a
state of energy balance (Schoener, 1971;
King, 1972; Wolf, 1978). "Surplus time"
can be used for storing energy, avoiding
predators or climatic extremes, and/or reproducing, all of which have positive
fitness value. Thus minimum foraging
time budgets may be one indirect way to
estimate the effects of resource loss on
fitness. Birds losing nectar to competitors
must compensate for this by increasing
foraging time or else suffer a drain on
energy reserves. Elsewhere we have calculated the relationship between the average
amount of nectar obtained per flower and
the foraging time required to balance daily
energy expenditures (Gill and Wolf, 1975,
1979r/; Wolf et al., 1975). This gives us a
needed tool for analyzing exploitative
competition in natural systems in a biologically meaningful and comparative way.
Required foraging time budgets also
provide a common denominator for different species since differences in
metabolic costs and foraging efficiencies
are included. We not only have a common
currency, but an index that is weighted in
terms of meaningful species-specific
translations of unitary nectar losses. Colwell and Futuyma (1971) stressed the importance of appropriate biological
weighting in a .somewhat different context,
757
COMPETITION FOR XECTAR
but one advantage of nectar-feeding bird
systems is that realistic weighting factors
are derived easily for almost any situation.
In particular, smaller species have reduced
foraging costs, but these do not seem to
compensate fully for poor extraction
abilities at some resources (GiH and Wolf,
1979/;). Consequently competitive nectar
losses should have greater impacts on subordinate generalists with intrinsically low
extraction efficiencies than on dominant
specialists with intrinsically high extraction
efficiencies.
Using data from July 22, 1974, as an
example (Table 1) we see that percent
increases in the foraging time budgets
projected as compensation for competitive
nectar losses ranged from 25% for
reichenowi, to 42% for venusta, to 67% for
famosa. For example, N. famosa should have
obtained 3.8 /ul per flower, which necessitates foraging 2.1 hours per day. Instead
famosa lost 1.4 /xl per Hower to other sunbirds and obtained 2.4 /AI per Hower, increasing its minimum foraging time by 1.4
hours to 3.5 hours. In another example,
Figure 5 summarizes the average foraging
time increases attributable to exploitative
nectar loss experienced by non-territorial
1
2
3
4
5
6
7
8
9
Average Nectar Obtained Per Flower (ul)
FIG. 5. Increases in minimum foraging time required
to compensate for loss of nectar to competing sunbirds by non-territorial male and female Nectannia
reichenowi feeding at Leonotis nepetifolia. Right hand
points indicate foraging time calculated for intake
projected for visited flowers if no competing sunbirds
had visited these flowers previously. Nectar loss of 1.7
/ul per flower (6 6) indicates compensatory foraging
time increases of 70% (from Gill and Wolf, 1978A).
is poor, the increase due to competitive
losses may be substantial, as at Leonotis
Wolf, 1979f/). Minimum daily foraging nepetifolia in the winter months in Kenya.
times of Kenyan sunbirds feeding at L. If the increase is so great that the indinepetifolia often doubled due to competi- vidual cannot maintain a balanced energy
tive nectar losses (Gill and Wolf, 1979r/).
budget in the available daylight foraging
The flexibility a bird has for other ac- hours, it must leave the area or shift to
tivities, including reproduction, will de- another nectar source or deplete reserves.
pend on the requirements for self- Alternatively, a species may feed at its
maintenance. When foraging efficiencies preferred flower (at high densities) which,
are intrinsically low and flower availability combined with predictably low competitive
N. reichenowi at Leonotis nepetifolia (Gill and
TABLE 1. Nectar loss among sunbirds feeding at flowering Leonotis nepetifolia.*
Flower
Species
Famosa
Venusta
Reichenowi
Kilimensi\
Senegaten\i\
Amethyslina
visits
416
1389
73
102
138
24
Uptake (Ml/H)
Potential Actual
3.8
2.9
3.9
3.8
3.6
4.7
2.4
2.2
3.2
3.0
'2.3
4.7
Minimum requited
foraging times
(hr/dav)
Potential
Actual
2.1
3.1
1.6
3.5
4.4
2.0
Losses (7r of potential uptake) to
F
V
R
K
S
5
7
0
18
7
19
14
0
—
23
—
0
2
0
0
0
—
4
9
0
0
13
—
1
7
0
5
0
—
* Representative but unpublished data collected on 22 July, 1974, near Lake Naivasha Kenya. Foraging times
not calculated for three species because of inadequate extraction efficiencies.
758
FRANK B. GILL
ritorial defense of a particular space containing essential resources (usually flowers
in the case of nectar-feeding birds). Nectarfeeding birds shift easily from one pattern
of spatial defense to another, presumably
due to economic considerations (Wolf,
1978). So far, however, nearly all study of
interference competition in nectar-feeding
birds has been on "territoriality."
The territories of nectar feeding-birds
clearly relate to food resources. Several
studies (Gill and Wolf, 1975; Gass et al.
ing Ribes• speciosum (Stiles, 1973), for Selas- 1976; Kodric-Brown and Brown, 1978)
phorus Jiammula with Tropaeolum (Hains- establish that feeding territories of parworth and Wolf, 1972), for Panterpe insignis ticular species include rather predictable
with Macleania glabra (Wolf and Stiles, numbers of flowers, or if more than one
1970; Wolf el al., 1976), perhaps for flower species is involved, territories have
Oreotrochilus estella with Cajophora (Car- equivalent resource values despite large
penter, 1976), and for Nectarinia reichenowi variations in area and flower numbers (Fig.
with Leonotis mollissima (Gill and Lanyon, 6). As expected, a strong inverse relationunpublished). Opportunistic breeding may ship exists between flower density and teroccur at preferred flowers when competi- ritory size (Fig. 7). Generally, the resource
tive loss is unpredictable, depending espe- value of the territory is similar to or, in
cially on the presence and activity of a some cases, greater than the total daily
social dominant (e.g., Wolf and Wolf, 1976; energy costs of the defending individual
Pitelka, 1951). I don't mean to overly (Gill and Wolf, 1975; Gass et al., 1976;
simplify the complex nature of breeding Wolf and Wolf, 1976; Kodric-Brown and
seasonality but merely am suggesting that Brown, 1978). Territories will be abancomparative studies of foraging efficiency
and competition costs at different seasons doned when the number of flowers is
are both practical for nectar feeding birds reduced below levels required to sustain a
and may be important to understanding territorial individual (Gill and Wolf, 1975;
Kodric-Brown and Brown, 1978). Furtheir reproductive timing and success.
thermore intruders may be tolerated
losses, insures that self-maintenance requires little foraging time. Competitive losses may he minimized either through social
dominance or maximal segregation of
species during peaks of resource availability (Feinsinger, 1976). Under such conditions, each species should have the greatest
flexibility for activities other than selfmaintenance and should breed (Stiles,
1973; Wolf and Wolf, 1976). This seems to
be the situation, for example, in Calypte
anna breeding in conjunction with flower-
1NTERKKRKNCK (OMl'K 11 I ION
Cost of Defending Additional Resources Exceeds Benefit
Interference prevents or reduces access
of a competitor to some it source required
by both (Miller, 1969). In nectar-feeding
birds, interference is via aggression rather
than poisoning or other chemical inhibition (Case and Gilpin, 1974; Gill, 1974).
The effectiveness and consequences of the
aggression are determined by the dominance relationships of the individuals
(Morse, 1974). But interference of some
kind is a logical behavioral and evolutionary response to situations involving
exploitative competition (Case and Gilpin,
1974; Gill, 1974). Aggressive interference
may involve simple, temporal) displacements of subordinates, or sustained ter-
l 500
' Insufficient Resources to Support Territorial Individual
0
500
1000
1500
2000
2500
3000
3500
Territory Size (mJ)
FIG. 6. Relation of territory size to number of defended flowers in the Rufous Hummingbird (from
Kodric-Brown and Brown, 1978). Closed circles were
territories defended by males; open circles were territories defended by females which held larger territories with lower flower densities. Territory size was
flexible to accommodate a certain number of flowers
required to satisfy the daily energy requirements of
the territorial individual.
COMPETITION FOR NECTAR
759
tential intruders than scattered flowers
(Yeaton and Laughrin, 1976). As noted
above though (Fig. 7), territory size decreases as flower density increases and,
therefore, the distance and cost of the
average chase should decrease. Unfortunately we still lack data on the relation
between intruder pressure and territory
size. In one case (Stiles and Wolf, 1970),
the rate of chases increased exponentially
as a function of the number of hummingbirds in a densely flowering tree. The
cost of increased numbers of chases on
small, flower-rich territories ought to exceed the savings of costs per chase, which
should be proportional to the radius of
0.5
1.0
5.0 10.0
50.0
territory size. If intruder pressure de2
Flower Density (number /m )
creased with flower density the cost of
FIG. 7. Inverse relation of flower density and terridefending a small Hower-rich territory
tory size in the Rufous Hummingbird (from Kodriccould be greater than defending a large
Brown and Brown. 1978). Open and closed circles as
flower-poor one.
in Figure 6. Solid line regression calculated for all
points; dashed line for male territories only.
The percent of the daylight hours spent
chasing intruders ranges from less than 1
within a "defended" area by a territorial to 14 (mean about 5) in different systems
bird as long as they do not leed at the (Table 2). There is no consistent difference
territorial individual's regularly visited in the time costs of breeding versus feedflowers (Wolf <7 «/., 1976). Thus, territorial ing territoriality, partly because feeding
defense of flowers often relates to compe- territoriality may be abandoned when the
costs of chasing intruders become excestition for food.
The costs and benefits of territoriality of sive (Cill and Wolf, 1975; Wolf and Wolf,
1976). The percent of total daily energy
nectar-feeding birds can be partitioned
costs
used for aggression ranges from 3-20
and analyzed separately. The costs are
primarily the time and energetic costs of percent (average about 10). These investflying towards and chasing intruders. ments are probably important aspects of
There may be additional costs of territorial the biology of nectar-feeding birds, given
advertisement but these are difficult to their delicate energy balances (Lyon el al.,
distinguish from other concurrent func- 1977; cf., Ripley, 1959; Murray, 1971;
tions (though see Carpenter and MacMil- Emlen, 1973). The investments can be relen, 19767;). The costs of flight increase covered from increased likelihood of an
almost linearly (in0-97) with increasing individual's balancing its daily energy
weight of the birds (Tucker, 1974). This costs.
means that each flight can cost a larger
Contingent upon degree of effectiveaggressor more than a smaller "displacee," ness, the benefits of flower defense involve
though large size may often be essential for improved nectar availability of two kinds.
winning an encounter.
First, aggressive displacement of others
The costs of territoriality also are pro- reduces their foraging at particular flowers
portional to the frequency of attempted (Yeaton and Laughrin, 1976; Carpenter
intrusions or the intruder pressure. In- and MacMillen, 19766) and increases the
truder pressure may vary greatly and we nectar per flower to the defender (or its
need information on this. We generally dependents) (Gill and Wolf, 1975). Losses
assume that dense flowers attract more from defended flowers are less than from
nectar feeders and, therefore, more po- undefended flowers (Gill and Wolf,
760
FRANK B. GILL
TABLE 2. Time and energy costs ofterritoriality in nectar feeding birds.
Species
Vestiariu coccinea
Sectarinia iricheno-wi
Xecta rin ia fam osa
Cahple anna
Oreotrochilu.s eslella
En la mpi.s jugnla ris
Eugenes fulgens
Panterpe insignis
Colibri thalassinus
Kind of
territory"
Time c
Energy11
<1
3
7
2
6-8
14
8
3
2
1-3
<1
1 (19f
7
13
3'
9-14
20
7-9
6-14"
3-5
2-3
<1
F
F
B
F
B
F
B(9)
BF
B
B
B
Reference
Carpenter and MacMillen, 1976o
Gill and Wolf, 1975
Wolf, 1975
Stiles, 1971
Stiles, 1971
Carpenter, 1976
Wolf and Hainsworth, 1971
Wolfrf til., 1976
Wolfe/ nl, 1976
W'oH'elal, 1976
a
Based on field measurements of actual aggression involved in territorial defense, with one footnoted
exception.
b
F = feeding territory, B = breeding territory.
c
Percent of daylight observation hours spent chasing intruders.
d
Percent of estimated 24-hour energy expenditures attributable to aggressive Mights except where noted.
e
Value in parentheses includes extra costs of territorial advertisement through exaggerated foraging
movement.
' 24-hour energy budget based on estimate of 7.8 kcal from MacMillen and Carpenter, 1977.
B
Daylight energy budget only, data not available for 24-hour conversions.
1979a). This means that the rate of net
energy gain while foraging can be higher
for territorial than for non-territorial individuals at the same time and place. To the
degree that territorial individuals defend
the densest flowers, their foraging costs
will be lower than those of non-territorial
birds feeding at more scattered flowers.
The second principal benefit of flower
defense is increased predictability of a
nectar supply (Gill and Wolf, 1975; Carpenter and MacMillen, 1976V/; Charnov el
al., 1976). The variance in contents of defended flowers may be less than in undefended flowers so the chances of finding
some nectar in each flower visit are increased. Put another way, the chances of
doing badly at a flower are reduced. Also,
territorial individuals may have the option
of patterning their Hower visits in ways that
avoid previously visited flowers, thus increasing the predictability of encountering
rich flowers, (Gill and Wolf, 1977; Kami),
1978).
The benefits of aggressive interference
depend on its effectiveness. This can be
considered in terms of net loss levels sustained by aggressive versus non-aggressive
individuals in "controlled" situations, or in
terms of the percent of the flower visits or
total foraging time that is attributable to
the aggressive individual. Few quantitative
data are available on the effectiveness of
territorial defense, even though some authors (Carpenter and MacMillen, 1976c/;
Lyon el al., 1977; Wolf, 1978) have noted
variations in percent intruders chased.
Territorial Allen Hummingbirds defending high density Zauschneria accounted for
only 62% of the total hummingbird feeding time at the defended flowers (Yeaton
and Laughrin, 1976). On nearby territories of Anna's Hummingbirds with low
density Zauschneria flowers, intruders accounted for only 8% of the feeding activity. This suggests greater effectiveness of
territorial defense by Anna Hummingbirds, but effectiveness may be simply
an inverse function of intruder pressure.
Intruders accounted for 15% of the visits
to Leonotis nepetifolia flowers defended by
territorial Xectarinia reichenowi and 13.69c
of the nectar uptake from all defended
flowers (Gill and Wolf, 1979c/). However,
the actual amount of nectar lost in a fourhour period by territorial sunbirds to intruders averaged only 89? so to some degree the intruders and the territorial sunbirds visited different flowers during the
observation period as expected from probabilities of point source overlap.
A territory holder mav chase- al! intrud-
761
COMPETITION FOR XECTAR
9
ers from some parts of a territory but not
from others. The breeding territories of
Calypte anna include a well-defined core
that is rigorously defended and a large
buffer zone that is less well-defended
(Pitelka, 1951; Stiles, 1973). When a territorial male sunbird (N. famosa) was
stressed (i.e., when its foraging time budget
approached 40%) it defended peripheraJ
parts of the territory less intensely, especially when several intruders appeared
simultaneously (Wolf, 1975). This could
both increase foraging efficiency in the
core area and reduce expensive defense
costs.
The variables defining costs and benefits
of flower defense can be integrated into
models of economical defensibility as a
determinant of territoriality (Brown, 1964;
Brown and Orians, 1970). The models
(Gill and Wolf, 1975; Carpenter and
MacMillen, 1976«; Kodric-Brown and
Brown, 1978) develop the concept that
territoriality should occur when the net
benefits of the behavior are positive and
exceed those of alternative behaviors, presumably enhancing individual fitness.
Carpenter and MacMillen (1976a) argued
that thresholds for territoriality in relation
to general nectar availability are specified
by the relation between the added costs of
territoriality and the resulting increased
nectar uptake. But the details of their
model only specify a threshold between
existence and starvation mediated through
territoriality and do not include the
economics of alternative behavior required
for a quantitative theory of optimal social
behavior (Pyke, 1978) that embraces a continuum concept of aggressive interactions
(Wolf, 1978).
Alternatively the relation between undefended and defended nectar volumes
mediated through foraging time requirements can be used to predict combinations
of nectar availability and defense costs for
which territoriality should be more
economical (Gill and Wolf, 1975). Because
there may be more nectar in defended
flowers, territorial individuals can spend
less time feeding and potentially save
energy that compensates for caloric costs
of defense, since the metabolic costs for
foraging are at least 2-3 times those of
sitting (Gill and Wolf, 1975). When average undefended nectar volumes are low,
minor increases of encountered volumes in
defended flowers make possible the recovery of substantial energetic investments in
defense. To the degree that defended volumes are raised above these threshold values, territoriality should be advantageous.
Kodric-Brown and Brown (1978) recently developed a more comprehensive
cost-benefit model for territoriality in
hummingbirds. Assuming that benefits
and costs are positive but different functions of the number of flowers defended,
the model illustrates roughly when the net
benefits of a particular size territory should
be positive. Qualitative predictions are
supported by their data on the Rufous
Hummingbird. This model has considerable promise for experimentation. Variations in territory size also can be examined
in relation to time and energy budgets of
the territorial individual in terms of optimality criteria. Preliminary work of this
sort suggests that an individual may
minimize daily energy costs through territoriality and thereby increase survivorship or reproduction (Pyke, 1978). We are
only beginning to develop the potential of
such cost-benefit analysis for interference
competition in nectar feeding birds.
CONCLUSIONS
There are obvious problems in extending the micro-economics of competition
for nectar to broad patterns of community
structure. It may be a matter of philosophy
and faith, but I doubt that it is as difficult
as trying to build an understanding of
development through biochemical studies
of gene action. Certainly it is going to be
relatively easy to develop simple, comparative, and biologically relevant field estimates of competition based on real probabilities and costs of point resource use,
which will be an improvement over the
rough overlap/alpha value studies of the
past decade.
There undoubtedly was selection in nectar-feeding birds for sensitivity to the relation between benefits and costs, as well as
flexibility in behavioral alternatives. I think
762
FRANK B. GILL
we are increasingly impressed with the
subtlety of behavioral alternatives used
daily by nectar-feeding birds to maintain a
state of energy balance. Understanding
this inevitably will incorporate learning
theory and physiological feedbacks, but we
must also include the probabilities, patterns, and micro-economics of competitive costs.
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