Determinants of Dietary Preference in Yellow-Rumped Warblers

6 Pages·2002·4.1 MB·English

by  Kim I Frazer

Wilson Bull., I 14(2), 2002, pp. 243-248
DETERMINANTS OE DIETARY PREFERENCE
IN
YELLOW-RUMPED WARBLERS
KIM I. FRAZER' AND SCOTT R. McWILLIAMS' ^
ABSTRACT—Warblers are selective in what they eat, yet little is known about the dietary cues used by
warblers as they decide what to eat. Semisynthetic diets may be useful for investigating how specific dietary
cues, such as appearance or nutrient composition of food, influences diet preference of warblers because the.se
dietary cues can be easily and systematically modified with semisynthetic diets. We offered Yellow-rumped
Warblers (Dendroica coronata) paired choices of live waxworms (Galleria mellonella) and a waxworm mash,
or waxworm mash and a semisynthetic mash. Birds strongly preferred live waxworms over waxworm mash,
suggesting that natural appearance of food strongly influences diet preference of warblers when the nutrient
composition ofdiets is similar. When birds initially were offered the two mash diets, they consistently preferred
waxworm mash over semisynthetic mash within the first 15 min with food, suggesting that they were using
dietary cues that provided rapid feedback as would be provided by a cue such as taste. This initial preference
forwaxworm mash was maintained forthe first twodays, but then the warblersate similaramountsofwaxworm
mash and semisynthetic mash during the last two days of the experiment. The decrease in preference for
waxworm mash over time probably occurred because at least some ofthe cues used by the birds in determining
their diet preference(s) required days for reliable feedback. Thus, diet preferences of warblers apparently were
influenced by dietary cues that provided immediate and delayed, postingestional feedback. These results support
the use of semisynthetic diets in studies of avian diet preferences and highlight the importance of adequate
acclimation time on test diets. Received 19 June 2001, accepted II March 2002.
Descriptive studies of the diet and foraging birds may use a variety of dietary cues to de-
behavior of warblers (family Parulidae) have cide what to eat, including sensory cues such
provided the foundation for testing important as appearance, taste, smell, and texture of the
ecological theory related to niche partitioning food (Willson et al. 1990, Willson 1994) and
(MacArthur 1958; Morse 1980, 1989), com- the nutritional adequacy of the food itself
petition (Wiens 1989), and optimal foraging (Murphy and King 1987, Cipollini and Levey
(Zach and Falls 1976, 1978). Most such de- 1997, Lepczyk et al. 2000). The relative im-
scriptive studies have compared food use and portance of certain cues in determining diet
availability, and have demonstrated that war- preferences can be inferred from measure-
blers are selective in what they eat (Morse ments oftemporal changes in diet choice since
1989). However, predicting the diet of war- dietary cues differ in how rapidly they can be
blers also requires an understanding of their detected. For example, dietary cues such as
diet preferences which can be investigated by color are immediately apparent to birds
allowing an animal equal access to certain whereas postingestive cues related to the nu-
food types and measuring relative use (John- tritional adequacy of the diets may take days
son 1980, Litvaitis et al. 1996). Diet prefer- for reliable feedback.
ence likely is related to the nutritional ade- Since natural foods often differ markedly in
quacy of foods whereas diet selection is a nutrient composition and other dietary cues,
function of the interaction between diet pref- we used semisynthetic diets to identify which
erence and the availability of alternative dietary cues were important in determining
foods. Thus, an understanding of the dietary diet preferences of Yellow-rumped Warblers
cues used by warblers as they decide what to {Dendroica coronata). Semisynthetic diets are
eat is necessary for accurately predicting both useful in studies of avian diet preference and
diet preferences and diet selection ofwarblers. nutritional ecology because the exact nutrient
Free-living warblers and other passerine composition of such diets is known, and be-
cause semisynthetic diets are easily and ac-
curately replicated or manipulated (Murphy
Rho‘ dDeeptI,slaondf, NKaitnugrsatlon,ReRsIou0r2c8e8s1,SUciSeAn.ce, Univ. of and King 1982). Although semisynthetic diets
^Corresponding author; can be formulated to match the nutrient com-
E-mail: [email protected] position of the bird’s natural diet, they often
243
)
244 THE WILSON BULLETIN • Vol. 114, No. 2, June 2002
TABLE 1. Composition (per 100g)oftwo mash dietsandofwhole waxworms^{Galleriamellonella).These
three diets were used to determine how specific dietary cues such as appearance or nutrient composition offood
influences diet preferences of Yellow-rumped Warblers {Dendroica coronata).
Experimental
diet Ingredients Wetmas5i Drymass
Semi-synthetic mash Carbohydrates (D-glucose) 1.13 10.04
Protein (casein'’) 6.80 52.30
Fat (olive oil) 2.61 20.08
Vitamin mix‘" 0.22 1.67
Salt mix‘‘ 0.76 5.86
Agar 1.31 10.00
Water 87.00
Waxworm mash Carbohydrates 0.70 6.92
Protein 3.35 33.00
Fat 4.42 43.63
Calcium 0.01 0.03
Ash 0.20 1.93
Agar 1.47 14.49
Water 89.85'’
^Compositionofwholewaxwormsisthesameaswaxwormmashwithoutagarandaddedwater.
^Casein = (highN)Teklad,U.S. BiochemicalCorp.,Cleveland,Ohio.
AIN-76VitaminMix,ICN Biomedicals,Inc.
‘'Saltmix = ICN Biomedicals,Inc.(Spivey-Foxand Briggs I960).
®11.49%ofwaterisfromwaxworms.
are quite different in appearance (e.g., mash of waxworm mash and three live waxworms and the
diets used by Afik and Karasov 1995, Afik et other dish contained 20 g of semisynthetic mash and
al. 1995, McWilliams and Karasov 1998). In three live waxworms. Each day we switched the lo-
this study, we used live waxworms {Galleria cation of the food types to ensure food position did
not bias the preference results (.see Jackson et al.
mellonella), minced waxworms, and a semi- 1998). This acclimation period was designed toreduce
synthetic mash diet to determine how visual any effect of previous short term experience on diet
appearance and nutrient composition of diets preference of warblers. However, if diet preference is
influenced the warbler’s choice of diets. determined primarily by the duration of time spent
feeding on a diet over months rather than days, then
METHODS warblers should preferthe .semi.synthetic mash and live
netSsttuodycap.tsuubrjeec2t0s Yaenldlowm-airnutmepneadnWcaer.—bleWresiunsseoduthmeirsnt wwaoxrwmormamssh(f(efdedtotot—htehseesebibridrsdsfofror33modnatyhss).) overwax-
Rhode Island during October 1998. Prior to the exper- Preference tests. In experiment 1, we offered 15 g
iments, birds were maintained in the laboratory for 15 of waxworm mash in one dish and 20 live waxworms
weeks and were fed a semisynthetic mash (Table (about 15 g) in another dish to each of the 20 birds.
and live waxworms.Thissemisynthetic mash issimila1r We weighedeach dish with 20waxwormstodetermine
to the nutrient composition of insects (10% carbohy- total mass ofwaxworms offered. Forexperiment 2, we
drates, 52% protein, and 20% fat; Bairlein 1987) and offered 20 birds 15 g of waxworm mash in one dish
has been used to maintain Yellow-rumped Warblers in and 15 g of .semisynthetic mash in another dish. In
the laboratory for months (Afik and Karasov 1995, both experiments, the food was available from 08:00-
McWilliams and Karasov 1998). Birds were weighed 12:00. After the 4-h test period, the food dishes were
and given fresh food and water between 07:30 and weighed and then replaced with two dishes each con-
09:00 EST each day. Birds were housed individually taining 10 g of semisynthetic mash. Subsamples of
in stainless steel cages (60 cm X 20 cm X 35 cm) in food were dried at 100°C toestimatedry matterintake.
a room with 10L:14D light and 21°C temperature re- All birds had food remaining after the 4-h test period
gimes. and by morning, so that the feeding regime each day
During Eebruary 1999, we acclimated all 20 birds was ad libitum.
to the paired dish protocol and the three food types In both experiments, weobserved some birdsduring
used in the preference tests (Table 1). Throughout the the first hour (08:00-09:00) with food to determine
3-day acclimation period andduringbothexperiments, which food type was eaten first and visited most fre-
birds always were offered food in two dishes on op- quently. For experiment 1, we randomly selected 10
posite sides of the cage (about 10 cm apart). During of the 20 birds for observation on day 1 and then we
the 3-day acclimation period, one dish contained 20 g ob.served the same 10 birds on day 2. For experiment
Frazerand McWilliams • WARBl.ER DIET PREFERENCE 245
TABLE 2. Captive Yellow-rumped Warblers (Dendroica coronata\ n = 20) ate predominately live wax-
worms when offered a choice of live waxworms and minced waxworms (experiment 1), and eventually ate
similar amounts ofeach diet when offered a choice of minced waxworms and a semisynthetic diet (experiment
2). Values for food intake are means ± SE.
Foodintake(gdry/4h)ofwarblersgivenachoiceoftwodiets
Live Waxworm Semisynthetic Total food
waxworms mash mash intake^ p
Experiment
1
Day 1 1.48 ± 0.09 0.08 -h 0.01 1.56 ± 0.07 20.95 <0.01
Day 2 1.45 ± 0.06 0.16 4- 0.01 1.56 4- 0.06 24.45 <0.01
Experiment 2
Day 0.64 -1- 0.04 0.30 ± 0.04 0.94 0.04 4.42 <0.01
1
Day 2 0.78 4- 0.04 0.28 ± 0.06 1.06 ± 0.05 5.73 <0.01
Day 3 0.74 4- 0.06 0.78 ± 0.25 1.52 -h 0.09 -1.10 0.77
Day 4 0.52 4- 0.04 0.68 ± 0.47 1.20 0.17 -0.30 0.28
5.7®5,ToPtal=f0o.o0d02i)n.take was similarbetween days in experiment 1 (P > 0.90), whereas total food intake increasedacrossdays inexperiment2(P3.57 =
^Two-tailedpairedr-testwith 19dfcomparingfoodintakeoflivewaxwormsandwaxwormmashinexperiment 1 orwaxwormmashand.semisynthetic
mashinexperiment2.
23,. Wweeombasdeervoebdsealrlva2t0iobnisrdfsroonmdbaeyhi1ndanadbalginadinthoant dhaayd t2.w2e7e,nPday=s 0w.a0s18s)tabtiesctaicuaslelybsiirgdnsifaitceantsl(ifg,h9tl=y
been in the room for two weeks prior to the experi- more of the waxworm mash on day 2 (Table
ment.
Food preference was determined by comparing the 2).
amount offood that had been consumed during the 4- In experiment 2, total food intake during
htestperiod, thenumberofbirdsthatvisitedeachfood the 4-h test period increased across days as
type first, and the mean frequency of visits to each more semisynthetic mash was consumed on
food type during the first four 15-min periods with days 3 and 4 (Table 2). The proportion offood
food each day. — intake composed of waxworm mash declined
useSdtataisttwioc-atlaialneadlypsaiisr.edFr-otrestbottohdeptreerfmeirneenciefttheesrtse,wwaes significantly across days (F357 = 6.94, P <
a significant difference between days in the amount of 0.0001). Birds ate significantly more wax-
tewaoc-htafioloeddtpyapiereedatr-etne.stIntoedxepteerrimmiennetif1,twheerealwsaosuasesdiga- w1o(r6m8.m5a3s%h±tha0n.0s4e)miasnydnthdeatyic2ma(s7h5.o6n3%da±y
nificant difference between days in the proportion of 0.04), but showed no significant difference on
food intake composed of live waxworms. We used a day 3 (53.34% ± 0.05) or day 4 (53.62%;
chi-square test to determine if more birds fed first on 0.05; Table 2). We calculated the statistical
one food type than the other, assuming no preference.
mIneaesxupreersimAenNtO1VaAndtoexdpeetreirmmeinnte2,ifwteheupsreedfearreenpceeasteodf pinotwaekre atsosduemtiencgt aa 0=.100.0g5 adinfdfeurseinncgetihne fesotoid-
birds changed during the first hour with food. For ex- mated within-group variance from our exper-
periment 1, the dependent variable was the proportion iment. The power in this case was 95.5%.
of food intake composed of live waxworms. For ex- Thus, there was a 95.5% probability of de-
popfeerrfiiommoeednntti22n,,tatwkheee cdueospmeedpnodaseernedtpevoaaftreiwdaablmxeewawosarusrmetshmeaAsphNr.oOpVoInrAtieoxt-no ttewceteinngtahe0.t1w0ogmadisfhfedrieentcseaitntfhoeod5%intlaekveelbeo-f
determine if the preferences or total food intake of significance.
birds changed across days. For all statistical tests, a P In experiment 1, all 10 birds that were ob-
value <0.05 was considered significant. All resultsare served during the first hour with food ate a
presented as mean ± SE. live waxworm first on both day 1 and day 2.
In experiment 2, similar numbers of birds ate
RESULTS
first from the waxworm mash (12 birds) and
=
In experiment 1, birds ate significantly semisynthetic mash (8 birds) on day 1 (x“
more live waxworms than waxworm mash on 0.80, df = 1, P = 0.39), whereas significantly
both day 1 (95.09% ± 0.01) and day 2 more birds ate waxworm mash first (19 birds)
(92.66% ± 0.01; Table 2). This difference be- on day 3 (x^ = 16.25, df = 1, P < 0.0001).
246 THE WILSON BULLETIN • VoL 114, No. 2, June 2002
In each of the four 15-min periods during diets. Apparently when two mash diets are of-
the hrst hour with food, birds in experiment 1 fered, the warblers must base their preference
visited the dish with live waxworms more of- on properties of the foods that are not detect-
ten than the dish with waxworm mash (time able prior to ingestion so that sampling is nec-
effect: F354 = 1.36, P = 0.26; day effect: Fj essary. Such sampling of foods with similar
= 0.06, P = 0.81; day X time effect: F354 = appearance also has been observed in bum-
0.23, P = 0.87). On both day 1 and day 2, blebees {Bombus spp.) choosing flowers (Os-
birds visited the dish with live waxworms ter and Heinrich 1976) and in mammalian her-
86% ± 9.3 of the time during the first 30 min bivores (e.g., equines, ungulates, and lago-
with food compared to 68% ± 14.9 of the moiphs) choosing rangeland plants (Westoby
time during the next 30 min. During the first 1974, 1978). Given that the warblers’ diet
15-min period on day 1, birds in experiment choice did not change after the first 15 min
2 visited the dish with waxworm mash as of- with food, the postingestive cues used by the
ten as the dish with semisynthetic mash. How- warblers provided rapid feedback as would be
ever, during the next three 15-min periods on provided by a cue such as taste.
day 1 and during all four 15-min periods on Despite a consistent preference for wax-
day 3, birds visited the dish with waxworm worm mash over semisynthetic mash during
mash a mean of81% ± 5.2 oftotal visits com- the first hour with food, Yellow-rumped War-
pared to the dish with semisynthetic mash blers did not consistently eat more waxworm
(time effect: F3,,4 = 3.06, P = 0.031; day mash than semisynthetic mash during the en-
effect: F,38 = 2.61, P = 0.11; day X time tire 4 h with the diets on all four days of ex-
effect: F3.,',4 = 2.74, P = 0.046). periment 2. Warblers preferred waxworm
mash during the first two days of experiment
DISCUSSION
2 but they ate similar amounts of waxworm
,
Yellow-rumped Warblers consistently ate mash and semisynthetic mash during the last
live waxworms first, they visited the dish with two days ofexperiment 2. The change in pref-
live waxworms more frequently during the erence across days occuiTed primarily because
first hour with food, and >90% of their diet warblers increased their consumption ofsemi-
during each 4-h test period was live wax- synthetic mash as well as total intake between
worms. Given that live waxworms and wax- the first two and the last two days of experi-
worm mash have the same nutrient composi- ment 2. Such a temporal change in diet pref-
tion, the preference for live waxworms is like- erence across days may occur (a) ifbirds must
ly due to the familiar appearance of the live become familiar with the diets (Greenberg
caterpillars to the insectivorous warblers. Sim- 1983, Murphy and King 1987, Avery et al.
ilarly, granivoroLis White-throated Sparrows 1995), (b) ifbirds can not satisfy their nutrient
{Zonotrichia leucophns) preferred a semisyn- requirements on only one ofthe diets (Murphy
thetic mash shaped like seeds to the powdery and Pearcy 1993), or (c) ifthe process ofsam-
form of the same diet (Murphy and King pling and choosing diets involves cues that re-
1982). Thus, natural appearance offood influ- quire days for reliable feedback.
ences diet preference ofbirds, especially when The first two hypotheses are unlikely to ex-
the nutrient composition of alternative diets is plain the temporal change in diet preference
similar. across days that we observed. In general, Yel-
When Yellow-rumped Warblers initially low-rumped Warblers eat a diversity of foods
were offered a choice between the two mash in the wild (Hunt and Flaspohler 1998) and
diets, they sampled both diets during the first so are unlikely to exhibit feeding neophobia
15 min with food and then settled on a con- as observed in some species of warblers and
sistent preference for waxworm mash over sparrows that are feeding specialists (Green-
semisynthetic mash. Such short term sampling berg 1983, 1990). In addition, the design of
of diets at first offering was not evident when our experiment included at least three days of
the birds were offered live waxworms and acclimation on the experimental diets so that
waxworm mash. Denslow et al. (1987) also birds were familiar with those diets. Further-
found that birds required more sampling time more, the warblers used in this experiment
when offered mash diets compared to natural have maintained body mass for weeks when
Frazerand McWilliams • WARBLER DIET PREFERENCE 247
fed only semisynthetic mash (this study) or lan and Willson 1994, Wilfson 1994) and this
waxworm mash (SRM unpubl. data), both may reduce the time delay associated with us-
supplemented with several live waxworms. ing other cues that provide delayed feedback.
Thus, both diets apparently are nutritionally In summary, these results support the use of
adequate, and both diets were familiarto these semisynthetic diets in studies of avian diet
warblers prior to the experiments. preferences and highlight the importance of
The similar appearance and consistency of adequate acclimation time on test diets.
the waxworm mash and semisynthetic mash
ACKNOWLEDGMENTS
apparently required warblers to extensively
sample the diets offered and use cues that re- We thank B. Wilson and M. L. Whitman for useful
quired days for reliable feedback. The birds insights on the design of the project and C. S. Hitch-
initially may have preferred waxworm mash ener, M. Capirchio, and H. M. Hamilton forhelpingto
in experiment 2 because they had just com- care forand maintain the birds. This study was funded
pleted experiment 1 in which they were of- sineapracrht Gbryanat,Unainv.d obfyRUhSodDeAIsGlraanndtUNnod.er5g3r8a7d4u8a.teTRhei-s
fered live waxworms and waxworm mash. Af- is Contribution No. 3937 from the Rhode Island Ag-
ter two days of sampling both diets, however, ricultural Experiment Station.
warblers ate equivalent amounts of waxworm
mash and semisynthetic mash, suggesting no LITERATURE CITED
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