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873
The Condor98:873-876
0 The CooperOrnithologicalSociety1996
SEASONAL CHANGES IN DIET, DIGESTIVE MORPHOLOGY AND
DIGESTIVE EFFICIENCY IN THE RUFOUS-COLLARED SPARROW
(ZONOTRZCHZA CAPENSZS) IN CENTRAL CHILE’
F. FERNANDO NOVOA, CLAUDIO VELO.W AND M. VICTORLA MPEZ-CAJLEJA
Departamentode CienciasEcol6gicas.Facultad de Ciencias, Universidadde Chile,
Casilla 653, Santiago, Chile
FRANCISCO
BOZINOVIC~
Departamentode Ecologia. Facultad de CienciasBiolbgicas,P. VniversidadCatdlica de Chile,
bio.puc.cl
Casilla 114-0, Santiago, Chile, e-mail:fbozinov@genes.
Key words: Zonotrichiacaper&; diet;digestive
tract;
seasonalchanges.
Seasonalchangesin birds’ energyrequirementsas well
as in environmental food availability and quality, influencebirds’ capability to obtain and digestfood (Karasov 1990). Increasedmetabolic rates, hypertrophy of
the gastrointestinaltract, and increasedabsorption of
nutrientsmay be viewed asresponsesofwintering birds
to ohvsical and biotic seasonalhabitats (Siblv
~ I 1981.
Dyksira and Karasov 1992).
In central Chile, the climate is seasonalwith warm,
dry summersand cool rainy winters;mean annual temperature is 22.1”C and mean annual minimum temperature is 7.7”C (di Castri and Hajek 1976). One of
the most conspicuousresident birds of the mediterranean environments in central Chile is the Rufouscollared Sparrow Zonotrichia capensis(Emberizidae).
This speciesis opportunistic,feeding on seedsand insectsand showing seasonaldietary shifts accordingto
food availability (Lopez-Calleja 1995). Individuals of
this speciesalso exhibit seasonalchangesin metabolic
rates and thermal insulation (Novoa 1993). Consequently, Z. capensislives under marked seasonalvariations in energyavailability and requirements.Specifically, when nutritional demandsincreaseduring winter, birds may respond to the seasonalenvironment
through behavioral, anatomical and physiological
changesthat allow the maintenanceof a positive energy
budget and consequentlypermit survival. The aim of
this work is to examine the relationship between seasonal changesin diet, digestivetract morphology, and
digestiveefficiencyof Rufous-collaredSparrowsin central Chile.
MATERIAL AND METHODS
Zonotrichia capensiswere collected bimonthly from
Januaryto November 1992 in central Chile (33”17’S,
71”ll’W) by shooting. Digestive tract contents were
preservedin 70% alcohol for dietary analysisof gross
items (proportionof seedsand insects)followingMpezCalleja (1995). After capture, the digestive tract was
I Received 9 April 1996. Accepted 21 June 1996.
2 Correspondingauthor.
removed and the dry massof digestadetermined. Digestive organswere washed with physiologicalsaline
solution.We measuredthe fresh length(L) (+ 0.0 1 cm)
and dry mass(DM) (+ 0.001 g). The following organs
were measured:crop, small intestine, large intestine,
and ceacum (see Bozinovic et al. 1990 for methodologicaldetails). Given the allometric relationship between body mass (mb) and L, digestive tract length
valueswere standardizedby rnb”.“, and digestivetract
dry mass values standardized by rnbl.O(see Karasov
1990).
To determine the effectsof seasonalchangesin diet
and digestivetract morphologyon digestiveefficiency,
birds were captured every two months in 1992 and
transported to the laboratory. There we conducted
feeding trials for five days, offering an equal amount
of commercial bird seed(Phalaris sp.) to each animal
and collectingtheir fecesand remaining food. Feeding
trials were begunimmediately after the birds were captured. Collectedfecesand remainingfood were weighed
and stored,after dryingthem at 60°C to constantweight.
Apparent digestibility was calculatedas: [(Qi - Qe)/
Qi] x lOO%, where Qi = daily weight of dry matter
consumptionand Qe = daily weight of dry fecesproduction. Digestibility is apparent becausethis method
underestimatesdigestiveefficiencyby the contribution
of metabolic and urine wastesand nonreabsorbedsecretions of the digestive system (see Karasov 1990).
The significanceof the effects of seasonson diet,
digestivetract morphology,and digestiveefficiencywas
assessedby a one-way ANOVA. To determine significanceof differencesbetweenvariables after the ANOVA, we used the a posterioriTukey test for multiple
comparisons.Relationshipsbetweenvariableswere established with a least-squareslinear regression(Steel
and Torrie 1985).
RESULTS AND DISCUSSION
Our seasonaldietary analysesshowed that seedsaccount for about 80% of the diet. However, during July
(winter) insects increased significantly, representing
about 50% of the diet (seeTable 1). On the other hand,
massof digestacontentsincreasedsignificantlyduring
winter (May-July) being between 40% to 58% higher
than during spring-summer(November-January) (Table 1).
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0.2
r-
I 0.0’
E
0
10
14
12
Total
digestive
16
tract
16
mass
20
(mg/g)
v)
:
5
0.2
r
b
0.0%
5
Total
6
digestive
7
tract
length
8
875
According to Sibly’s (198 1) model of optimal digestion, one of the advantagesof increasingdigestivetract
size is that it allows an increasein digestamean retention time, which increasesdigestibilityif food ingestion
rate is constant, or it allows a constant digesta mean
retention time which maintains digestive efficiency if
food ingestion increases.Alternatively, if food ingestion rate increases,increaseddigestivetract size allows
a constantdigestamean retention time, which maintains digestive efficiency. Coincident with the winter
season,Z. capensisswitched to a higher-energy diet
(insects)and still had to increasetheir gut capacityand
meal size. Ordinarily, under constantenergydemand,
the switch from seedsto insectswould promote a decreasein gut size. Thus, they made at least two adjustments to the winter season.It is unclear whether
the switchto insectsis due to increasedenergydemands
by low ambient temperaturesor simply increasedinsectavailability/decreasedseedavailability. Nevertheless,digestivetract increasesmay allow an increasein
the amount of food carried while still maintaining the
effectivenessof food processingand digestion.
We thank F. M. Jaksic, P. A. Camus and two reviewers for useful comments on the manuscript. Our
researchwas funded partially by Fondo de Desarrollo
Cientilicoy Tecnol6gico(FONDECYT) grants1950434
and 1950394.
(cm/g)
FIGURE 1. Relationshipsbetween:(a) total digestive
tract mass and digesta mass content (Y = 0.89, P =
0.04) and (b) total digestive tract length and digesta
mass content (Y = 0.60, P = 0.15) in Zonotrichiu capensis.Each point representsbimonthly means f SD,
r and P-values are from a linear regression.
Results of morphologicalanalysesof digestive tract
between seasonsshowed that both L and DM of the
digestive tract changed seasonally(Table 1). During
winter, crop L and DM increased by 9% and 27%,
respectively.During winter, the small intestine L and
DM also increasedby 8% and 45%, respectively.Both
large intestine and ceacum L and DM did not exhibit
significant seasonalchanges(Table 1). Apparent digestibility was not significantlydifferent between seasons, with an annual average of 79.8 f 1.0% (Table
1).
A positive and significantregressionwas found between seasonaltotal digestive tract dry mass and digestadry mass (y = 0.0084x - 0.64, r = 0.892, P =
0.04, Fig. la) but not betweentotal digestivetract length
and digestadry mass (y = 0.049x - 0.25, r = 0.602,
P = 0.15, Fig. lb).
Seasonalchangesin digestivetract morphologyhave
been demonstratedin many small endotherms inhabiting seasonalenvironments (Gross et al. 1985, AlDabbaghet al. 1987,Green and Millar 1987, Bozinovic
et al. 1990, Walsbergand Thompson 1990, Nagy and
Negus 1993). Environmental conditions that generate
an increase in avian nutritional or energeticrequirements could be related to the observedincreasein size
of digestivechambers.Suchchangesmay tend to maximize ingestionrate and the effectivenessof energyuse.
LITERATURE
CITED
AL-DABBAGH,K. Y., J. H. JIAD, AND Y. N. WAHEED.
1987. The influenceof diet on the intestine length
of the White-cheeked Bulbul. Omis Stand. 18:
1501-1502.
Bozmovrc, F., F. F. NOVOA, AND C. VELOSO. 1990.
Seasonalchanaesin enerav exnenditure and digestive tract of Abrothrix&d&
(Cricetidae) in
the Andes ranae.
Zool. 63:1216-1231.
- Phvsiol.
.
DI CASTRI,F., AND E. R. HAJEK. 1976. Bioclimatologia de Chile. Ediciones Universidad Catolica,
Santiago,Chile
DYKSTRA,C. R., ANDW. H. KARAsov. 1992. Changes
in gut structure and function of House Wrens
(Trbglodytesaedon) in responseto increasedenergy demand. Phvsiol. Zool. 65:422-442.
G~EENY-D.A., AND J.-S. MILLAR. 1987. Changesin
gut dimensionsand capacityofPeromyscusmaniculutusrelative to diet quality and energy needs.
Can. J. Zool. 65:2159-2162.
GROSS,J. E., Z. WANG, AND B. A. WUNDER. 1985.
Effectsof food quality and energy needs:changes
in gut morphology and capacity of Microtus ochrogaster.J. Mammal. 661661-667.
KARAsov,W. H. 1990. Digestion in birds: chemical
and physiologicaldeterminantsand ecologicalimplications. Stud. Avian Biol. 13:39l-4 15.
LOPEZ-CALLEJA,
M. V. 1995. Dieta de Zonotrichia
capensis(Emberizidae) y Diuca diuca (Fringillidae): efecto de la variaci6n estacional de 10srecursestr6ficos y la riqueza de aves granivoras en
Chile central. Rev. Chil. Hist. Nat. 68:321-33 1.
NAGY, T. R., AND N. C. NEGUS. 1993. Energy acquisition and allocation in male collared lemming
(Dicostomixgroenlandicus):effects of photoperi-
876
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od, temperature and diet quality. Physiol. Zool.
66537-560.
NOVOA, F. F. 1993. Ecofisiologiade Zonotrichia capensis:cambios estacionalesen el gasto y la adquisici6n de energia. Ph.D. diss., Universidad
Chile, Santiago.
SIBLEY,
R. M. 1981. Strategiesin digestionand defecation, p. 109-139. In C. R. Townsend and P.
The Condor 98:876-879
0 The CooperOrnithological
Calow [eds.],Physiologicalecology:an evolutionary approach to resourceuse. Blackwell, Oxford.
STEEL, R. G. D., ANDJ. H. TORRIE. 1985. Bioestadistica:principios y procedimientos.McGraw-Hill,
Bogota.
WALSBERG,
G. E., AND C. W. THOMPSON.
1990. Annual changesin gizzard size and function in a frugivorous bird. Condor 92~794-795.
Society 1996
WINTERING SWAINSON’S HAWKS IN CALIFORNIA’S
SACRAMENTO-SAN JOAQUIN RIVER DELTA’
SEBASTIANK. HERXKY
Department of Wildlife, Fish, and ConservationBiology, Universityof California at Davis, Davis, CA 95616
Key words: Swainson’s Hawk; Buteo swainsoni;
natural history; winter distribution;Sacramento-San
JoaquinRiver Delta.
The Swainson’sHawk (Buteoswainsoni)is considered
a summer resident breeder on lame Darts of North
America, leaving in winter to migratesouth to South
America (AOU 1983, Palmer 1988). Wintering Swainson’s Hawks in North America had been confirmed
only from southernFlorida, where somejuveniles were
found during most winters, and from the southwestern
United Statesasoccasionalmigration dropouts(Palmer 1988).
Swainson’sHawks in California breed chiefly in the
Central Valley and on the Northeastern Plateau (California Department of Fish and Game 1993), and the
speciesis listed as threatened in the state. Until 1990,
few crediblewinter recordshad been reported from the
state(e.g., Browning 1974, McCaskie 1985, Morlan et
al. 1987, McCaskie 1989), most of which were considered to representvery late or early miarants. Several
reports from California’s Sacramento-San Joaquin
River Delta (LeValley and Rosenberg1984, Campbell
et al. 1986, Morlan et al. 1987, Campbell et al. 1988,
Yee et al. 1989, Erickson et al. 1990) suggestedsome
Swainson’s Hawks were winterina locallv. Durine the
1990/1991 winter, 28 hawks were cont%med ig the
Sacramento-SanJoaquin River Delta, mostly consisting of adults (Yee et al. 1991). A population of comparable size was reconfirmed in the two subsequent
winters (W. Holt and D. G. Yee, unpubl. data).
Holt and Yee (unpubl. data) observedthe population
irregularly during all three winters. Thev discovereda
communal roost on Andrus Island (Fig. 1) and ob-
I Received 19 June 1995. Accepted 31 July 1996.
z Current address:Foundation for Tropical Research
and Exploration, P.O. Box 74431, Davis, CA 95617,
e-mail: [email protected]
served foraging flocks of up to 16 Swainson’s Hawks
on five Delta islandsprevina on small rodents in close
associationwith farming operations. The objective of
the present study was to systematically monitor the
population to determine wintering seasonchronology,
size, composition, distribution, and foraging ecology.
STUDY AREA AND METHODS
The study area was in the Sacramento-SanJoaauin
River Delta in the CentralValIey ofCalifornia (38005’N,
121”35’w) (Fia. 1). Historicallv. the Delta was an extensive freshw%er marsh. Naturally deposited levees
had been forming regularlyflooded islandsdominated
by tule (Scirpusspp.), cattail (Typha spp.), and reeds
(Phragmitesspp.)(Her-boldand Moyle 1989). Sincethe
185Os,> 80% of the Delta’s wetlands were converted
to mostly agriculturallands (California Department of
Water Resources 1987). The area’s main crops were
corn, grain, and hay. Accesswas limited becausemost
Deltaislandswereprivately ownedand few publicroads
led through the study area. Boats could not be used
becausehigh leveesand elevationsbelow sealevel limited visibility of islands.
Swainson’s Hawk population surveys were conducted by car on 23 islands from 25 September 1993
to 11 March 1994. I first surveyedareaswhere hawks
were seen previously (Holt and Yee, unpubl. data).
Surveyswere extended to other accessibleislands and
conducted4-6 times a week by mid-October and reduced to twice a week by late February. Survey frequency was determined by logistic factors such as accessibilityand remoteness.I conductedfrom 5 1 to 92
surveysper island on Andrus, Brannan, Bouldin, and
Terminous, from 31 to 50 surveysper island on Empire, King, Staten,and Twitchell, from 11 to 30 surveys
per island on Sherman and Venice, and from one to
ten surveysper island on Bacon, Bethel, Byron, Holland. Jersev, Lower Jones. Medford. Palm. Roberts.
Upper Jones,Veale, Victoria, and Webb. Potentialroost
sites were located during initial surveys and visited
regularlyfrom mid-November to early Februaryin late