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8-1-1995

Foraging Behavior of American Redstarts in Breeding and Wintering Habitats: Implications for Relative Food Availability

Irby J. Lovette Dartmouth College

Richard T. Holmes Dartmouth College

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Dartmouth Digital Commons Citation Lovette, Irby J. and Holmes, Richard T., "Foraging Behavior of American Redstarts in Breeding and Wintering Habitats: Implications for Relative Food Availability" (1995). Open Dartmouth: Peer-reviewed articles by Dartmouth faculty. 3902. https://digitalcommons.dartmouth.edu/facoa/3902

This Article is brought to you for free and open access by the Faculty Work at Dartmouth Digital Commons. It has been accepted for inclusion in Open Dartmouth: Peer-reviewed articles by Dartmouth faculty by an authorized administrator of Dartmouth Digital Commons. For more information, please contact [email protected]. The Condor97:X32-791 0 The CooperOrnithological Society 1995

FORAGING BEHAVIOR OF AMERICAN REDSTARTS IN BREEDING AND WINTERING HABITATS: IMPLICATIONS FOR RELATIVE FOOD AVAILABILITY l

IRRY J. LOVETTE~ AND RICHARD T. HOLMES Department of Biological Sciences,Dartmouth College,Hanover, NH 03755

Abstract. We investigatedfood availability for a long-distancemigrant species,the Amer- ican Redstart (Setophagaruticilla), in both its summer breeding habitat in New Hampshire and in its winter habitat in Jamaica. We used four components of foraging behavior (prey attack rate, foraging speed, time spent foraging, and foraging maneuver use) as indicators of the relative availability of prey in the two seasons. Redstartsattacked prey at a significantlygreater rate in summer than in winter, indicating that foragingbirds encounteredprey more frequently in summer. The winter prey-encounter rate was low even though redstarts moved almost twice as fast while foraging in winter as in summer. Male redstartsalso spent more time foraging in winter (85%) than in summer (43-65%), possiblyto balancethe low rate at which they encounteredprey. In winter, redstarts used more foraging maneuvers that were directed towards small flying prey, whereas in summer they usedmaneuvers that resultedin the capture of relatively large and presumably energy-richprey suchas lepidopteranlarvae. That wintering redstartsforaged faster, attacked prey lessoften, and spent more time foragingthan thosein summer indicatesthat the winter is a period of relative food scarcity for this species,whereas the breeding seasonis a period of greater resourceabundance. Key words: Foraging behavior;American Redstart;Setophaga ruticilla; Neotropicalmi- grant; prey availability; Jamaica.

INTRODUCTION small, mainly insectivorous species. No direct Food is often considered the resourcemost likely method of estimating prey availability is free from to limit bird populations (Lack 1966, Wiens potential biases (Cooper and Whitmore 1990) 1989) and has been implicated as an important and these biases may be exacerbatedwhen com- limiting resourcefor passerinetaxa as diverse as parisons are made between seasons,habitats, and corvids (Hogstedt 1980, 1981), sparrows (Pul- prey communities. Furthermore, insectivorous liam and Dunning 1987, Arceseand Smith 1988), birds usually consume a variety of prey types, warblers (Rodenhouse and Holmes 1992) and which often differ in motility, crypsis, size and finches (Smith et al. 1978, Schluter 1982). Mi- palatability (Sherry 1984, Holmes and Schultz gratory birds could experience food limitation in 1988, Hutto 1990). Prey availability is affected winter (Alerstam and Hogstedt 1982, Fretwell by all of thesefactors, as well as the overall abun- 1986), in summer (Probst 1986, Holmes et al. dance of prey items. Thus, no prey sampling or 1986, Martin 1987, Sherry and Holmes 1992), census method directly assessesthe prey avail- or during both periods (Cox 1985, Sherry and ability actually experienced by foraging birds Holmes, in pressa). To date, however, there have (Wolda 1990). been few attempts to compare summer and win- An alternative way to obtain information on ter food availability for any long-distance mi- food availability is to examine how the birds gratory species. themselves searchfor and capture food, because The lack of information on food availability foraging behavior often reflects in predictable for migrant songbirds largely results from the ways the types and abundances of available prey difficulties of measuring prey abundance for these (Hutto 1990). At least four components of for- aging behavior can provide information on food availability. First, prey attack rates are an indi- cation of how often prey are encountered by for- ’ ’ Received 12 December 1994. Accepted 24 April 1995. aging individuals (Davies and Houston 198 1, z Present address:Department of Biology, Univer- Price 198 1, Blancher and Robertson 1987, sity of Pennsylvania, Philadelphia, PA 19104. Thiollay 1988). This relationship has been ver-

[7821 REDSTART FORAGING BEHAVIOR IN SUMMER AND WINTER 783 ified by experimental work with insectivorous imately 13 km west of Black River, St. Elizabeth warblers, which demonstrated that attack rates Parish, Jamaica (Holmes et al. 1989). This study increase with increasing prey abundance (Hutto area was located within a monospecific black 1990). mangrove (Avicennia germinans) woodland that Second, the rate at which birds move while had a dense, virtually contiguous canopy be- foraging (“foraging speed”) can be used as an tween 8 and 15 m above the ground, but little indicator of the intensity of foraging effort (Rob- vegetation at lower strata (Parrish and Sherry inson and Holmes 1982). By foragingfaster, birds 1994). The site was submergedby up to one me- searcha greateramount of substrate,and thereby ter of water in October-November, but was increase their prey encounter rate. Third, the mostly dry in March. Although the leafy canopy amount of time allocatedto active foragingshould cover within the mangroves decreasedin the late increase both during times of high energy de- winter dry season(Parrish and Sherry 1994) the mand and during periods of low food availabil- mangroves retained substantial green foliage ity. Estimates of the time spent foraging should throughout the year. Previous work has dem- thus provide an indirect measure of prey avail- onstrated that mangroves contain high densities ability. Fourth, birds use different classesof at- ofadult male American Redstartsrelative to oth- tack maneuvers to catch different types of prey: er Jamaican habitat types (Holmes et al. 1989, for example, many warblers use sallies to capture Sliwa 1991) and thus our winter study site was flying prey such as adult Diptera, whereasgleans presumably located within a high quality win- are often used to obtain sedentary prey such as tering habitat (Sherry and Holmes, in pressb). lepidopteran larvae (Bennett 1980, Robinson and Redstarts at both Hubbard Brook and Luana Holmes 1982). The proportional use of different Point were individually color-banded and their types of foraging maneuver thus gives informa- territories mapped. To reduce variance due to tion about the types of prey taken. between-sex differences in redstart foraging be- In this study we examined the foraging behav- havior (Holmes 1986) we report observations ior of the American Redstart, a long-distance only on male redstarts. Similar proportions of migrant, paruline warbler, in its temperate adult and yearling males were observed in all breeding habitat in New Hampshire and in a sampling periods. Although yearling male red- tropical wintering area in Jamaica. We reasoned starts are female-like in appearance, these indi- that if we found seasonal differences in redstart viduals could often be identified as male by foraging behavior, we could identify periods of plumage characteristics(Pyle et al. 1987) or by relatively low food availability by a conjunction song behavior during the breeding season. Data of low attack rates, high foraging speed, greater from the few female-plumaged birds that could time spent foraging, and low prey quality. While not be visually sexed in the field were excluded none of these behavioral parameters alone is a from analyses. sufficiently reliable index of resourcelevels, when Each of our observations were assignedto one considered in concert they provide insight into of five sampling periods, “early winter” (late Oc- differencesin the relative availability of food re- tober-early November) and “late winter” sourcesbetween seasons. (March), and three summer periods, “prebreed- ing, ” “incubation” (including the nest building METHODS and laying periods), and “nestling” periods. Data Summer observations were made from 20 May on summer males whose breeding statuswas not to 6 July 1990 on a 180-ha plot in the Hubbard known were not included in analyses. Brook Experimental Forest, West Thornton, New To obtain foraging and time budget behavior Hampshire (Sherry and Holmes 1985). The veg- sequences,we moved systematicallythrough each etation at the Hubbard Brook site consisted of study plot until we encountered a male redstart. mature second-growth northern hardwoods for- We then followed that individual for as long as est dominated by sugarmaple (Acer saccharum), it remained in sight, and dictated a constant nar- American beech (Fugus grandzjbfia), and yellow rative of its movements, prey attacks, and other birch (Betula alleghaniensis). behaviors into a portable tape recorder. The re- Winter observations were made in March and corded data on movements, prey-attacks, and in October-November 1990 on three gridded other behaviors were later transcribed, using a plots totaling -25 ha near Luana Point, approx- stopwatch to measure the time between events. 784 IRBY J. LOVETTE AND RICHARD T. HOLMES

Multiple observations of attack rates and for- sampled time budgets acrossthe daylight period aging speedsfrom a single individual were av- in both seasons,our total observation time was eraged within sampling periods, so that each in- short relative to the total time available for red- dividual was represented in statistical compari- start activity, and thus our time budget figures sons of these parameters by only one mean value should be considered as approximations of red- within each period. Individuals were often ob- start time allocation. served again in subsequentperiods within a sea- Foragingmaneuvers. Prey-attack maneuvers son, but no single individual was seen in both were classified into five categoriesafter Remsen summer and winter. In both seasons,observa- and Robinson (1990): (1) gleans (attacks from a tions were obtained throughout the daylight stationary or hopping position directed towards hours, but were concentrated between 06:00- stationary prey items); (2) sally-hovers (attacks 12:oo. from a hovering position directed towards sta- Attack rate. The rate at which redstarts at- tionary prey items); (3) sally-strikes (attacksfrom tacked prey was determined for each observa- a flying position directed towards stationary prey tional sequenceby dividing the number of prey items, without a hovering pause);(4) sallies(flights attacks by the amount of time spent actively from a perch to attack flying prey items); and (5) moving about in search of prey. We defined a flutter-chases (flying attacks directed towards a “prey attack” as a directed movement towards flying or falling prey item which has been flushed a potential prey item (i.e., one of the five foraging or dislodged). As an analysis of the frequencies maneuver classesdefined below). Observational of transitions between maneuvers showed that sequencesduring which fewer than two attacks there was little sequential dependenceamong the took place were not included in attack rate cal- maneuver types recorded within a single obser- culations. vational sequence (Lovette, unpubl. data), we Foragingspeed.Movements made by redstarts pooled all the maneuvers we observed within while searchingfor prey were classifiedinto three each sampling period. categoriesfollowing Robinson and Holmes (1982, Comparisons of attack rates, foraging speeds, 1984): hops, short flights (I 1 m), and long flights and foraging time among sampling periods and (> 1 m). The rates(number/min foraging) ofeach between seasons were made using analysis of of these three categorieswere summed to deter- variance (ANOVA) and multivariate analysis of mine the total rate of movement while foraging, variance procedures (MANOVA; SAS 1987). which we define as “foraging speed.” Where a single degree of freedom is indicated in Foragingtime. The amount of observationtime comparisons of summer and winter data, the allocated to foraging was estimated for each in- analyses were performed after the observations dividual by recording the predominant activity within each season were pooled. Foraging time during successive five second intervals within frequency data were arcsine transformed before each observation sequence. This short, stan- analysis. Differences in foraging maneuver use dardized sampling interval was necessary be- were tested by chi-square on the total counts of cause redstartsrapidly switched between behav- all maneuvers observed within each sampling iors, and we wished to avoid making subjective period. For comparisons between seasons, we judgments about exactly when a given behavior pooled within-season observations even though began or ended (Tyler 1979). Redstartswere con- significant within-season variation existed in sidered to be foraging if they were actively mov- some cases(see below). ing about in search of prey. Non-foraging activ- ities included stationary singing, aggressiveor RESULTS courtship behavior, preening, and stationary vig- We quantified the attack rates, foraging speeds, ilance. Seasonal differences in the conspicuous- time budgets, and foraging maneuver use of 33 ness of male redstart behaviors such as singing male redstarts in summer and 58 male redstarts may have influenced the detectability of the in- in winter. dividuals we studied. We avoided biasing our samples towards conspicuous behaviors by fol- PREYATTACKRATES lowing individual males for as long as possible, Within-seasonvariation. In summer, foraging and by observing the behavior of many different male redstarts attacked prey at a mean rate of male redstarts.Note, however, that although we about 5 attacks/minute (Fig. 1A). There was no REDSTART FORAGING BEHAVIOR IN SUMMER AND WINTER 785

8 A. AttackRate 7

1

0 B. ForagingSpeed 25

0 31C. ForagingTime 90

80 9 (1214) 70 9 (2055) 960 15 (5865) Z50 B i= 40 * 30 20 10 0 Prenesting Incubation Feeding &lY Late 13Young Winter winter t- BreedingSeason (New Hampshire)- -Winter (Jamaica)- FIGURE 1. Prey attack rates (A), movement rates while foraging (B), and proportion of time devoted to foraging (C) by male American Redstarts in winter and summer habitats. Sample sizes for foraging time are number of individuals observed and total number of secondsof observation (in parentheses).Sample sizes for prey attack and movement rates are given in Table 1. significant difference in attack rate across the three We found more variation in attack rates within stages of the breeding cycle (F = 0.02, df = 2, P the winter than within the summer: attack rates < 0.98). This mean summer attack rate is similar differed significantly between the early and late to that reported by Robinson and Holmes (1982), winter sampling periods (F = 4.70, df = 1, P < who found that redstarts at Hubbard Rrook at- 0.04). Male redstarts attacked prey at an average tacked prey at a mean rate of 5.6 attacks/minute. rate of 3.2 attacks/minute in the early winter, 786 IRBY J. LOVETTE AND RICHARD T. HOLMES

TABLE 1. Foraging speedsof male American Redstarts in summer at Hubbard Brook Experimental Forest, New Hampshire and in winter at Luana Point, Jamaica.

Movement rates(mean number of movements/min k SE) Short I-Qx? F0r&llg PWiCd WY Hops fights flights sped Summer: Pre-nesting 10 (2,119) 7.5 + 1.3 2.0 + 0.5 2.4 + 0.5 11.9 + 1.5 Incubation 15 (5,460) 7.0 + 1.5 1.4 + 0.3 1.4 * 0.3 9.8 + 1.9 Nestling lo (1,443j 15.3 + 3.6 4.4 +- 1.1 1.9 + 0.4 21.6 f 4.5 Winter: Early winter 22 (10,059) 18.0 + 1.6 3.7 f 0.3 1.9 + 0.2 23.7 + 1.5 Late winter 36 (21,609) 16.3 + 0.9 2.5 & 0.3 1.6 + 0.1 20.4 + 1.0 * n = numberof individuals observed. b f = cumulative numberof secondsof observation. and 2.8 attacks/minute in late winter, after the periods (Table 1). Short flight rates were signif- onset of the dry seasonat our Jamaican site (Fig. icantly greater in early winter (F = 6.36, df = 1, 1A). P < 0.02), but rates of hops (F = 1.24, df = 1, Between-seasoncomparison. Although attack P < 0.27), long flights (F = 1.16, df = 1, P < rates varied within the winter, they were lower 0.29), and overall foraging speed (F = 3.70, df in both winter periods than during any of the = 1, P > 0.05) did not differ between the two summer periods (Fig. 1A). The pooled summer winter periods (Table 1). mean attack rate was significantly greater than Between-season comparison. The foraging both the early winter (F = 5.29, df = 1, P < speedsof male redstarts in winter were similar 0.025) and the late winter (F = 19.40, df = 1, P to those of summer males feeding young, and < 0.00 1) attack rates. Overall, winter attack rates were almost twice as fast as the foraging speeds were about 60% of those in summer (Fig. IA), of the earlier parts of the breeding season (Fig. suggestingthat in winter, foraging redstarts en- 1B). When the pooled summer stageswere con- counter potential prey items at a considerably trasted with the pooled winter stages,two of the lower rate than they do in summer. four movement categorieswere significantly fast- er in winter: hop rates (F = 24.47, df = 1, P < FORAGING SPEED 0.001) and total foraging speed (F = 9.37, df = Within-season variation. Redstartsmove rapidly 1, P < 0.005). Short flight (F = 1.37, df = 1, P while foraging,and their movement ratesare high > 0.25) and long flight rates (F = 0.3 1, df = 1, even compared with other Hubbard Brook war- P > 0.58) did not differ between seasons. blers (Robinson and Holmes 1982). In this study, we found that the foraging speed of male red- FORAGING TIME starts varied both within and between seasons. Within-season variation. In summer, male red- In summer, there was significant variation across starts spent the least amount of time foraging breeding stagesin hop (F = 4.05, df = 2, P < during the incubation period (44%) and spent 0.027) and short flight (F = 6.16, df = 2, P < more time foraging both earlier in the season 0.005) rates, but not in long flight rates (F = 1.37, during the pre-nesting period (59%) and late in df = 2, P < 0.27). The overall foraging speed the seasonduring the nestling period (65%). This (which represents the sum of all three types of variation among the summer periods ap- movement) roughly doubled during the nestling proached statistical significance (F = 2.82, df = period relative to the earlier stagesof the breed- 2, P < 0.075). In winter, the amount of time ing cycle. This increaseduring the nestling period spent foraging did not differ between the two to the rapid pace of 20 movements/minute con- sampling periods (F = 0.8 1, df = 1, P > 0.372). tributed to the significant variation in foraging In winter, male redstartssearched for prey about speed we found among the summer breeding 85% of the time (Fig. 1C). stages(F = 5.08, df = 2, P < 0.01; Fig. 1). Between-season comparison. Male redstarts In winter, movement rates were more consis- spent significantly more time foraging in winter tent between the early and late winter sampling than in summer (F = 65.94, df = 1, P < 0.001). REDSTART FORAGING BEHAVIOR IN SUMMER AND WINTER 787

TABLE 2. Proportional use of foragingmaneuvers by male American Redstartsin summer in New Hampshire and in winter in Jamaica.

Maneuvers (%) A&d sally- sally- Plutter- attacks Period HOWX SAY Strike Chase (W

Summer: Pre-nesting 129 40 12 23 4 26 Incubation 130 34 6 34 4 38 Nestling 17 8 32 6 All summer observations 11 24 25 4 Winter: Early winter 389 22 18 22 19 19 40 Late winter 692 29 18 29 13 12 41 All winter observations 1,081 26 18 26 15 14 41

. Attacksdirect4 towardsflying prey (sallies+ flutter-chases). b n = numberof maneuversobserved.

Even when provisioning young, male redstarts Between-seasoncomparison. The pooled sum- spent only 65% of their time foraging, compared mer maneuver frequencies differed significantly with about 85% in the winter (Fig. 1C). from those in winter (x2 = 54.9, df = 4, P < 0.01). Maneuvers directed at aerial prey were FORAGING MANEUVERS more frequent in winter (41%) than in summer Within-seasonvariation. The proportional use of (28%). These differences resulted from changes the five classesof foraging maneuvers differed in the proportional use of three types of maneu- significantly both within summer (x2 = 22.16, df vers: gleans were more common in summer than = 8, P < 0.01) and winter sampling periods (x2 in winter, whereassally-hovers and flutter-chases = 23.13, df = 4, P -C 0.0 1; Table 2). In summer, were more common in winter (Table 2). the proportional use of sallies differed the most In both seasons,most prey items were too small between stagesof the breeding cycle: sallies were to be visually identified in the field, and therefore common early in the seasonduring the pre-nest- we did not attempt to quantify the actual prey ing and incubation periods, when they repre- types captured. However, in summer we often sented 23% and 34%, respectively, of the ma- observed redstartsconsuming large prey such as neuvers observed, but they were rare during the lepidopteran larvae and adult tipulid flies, nestling period (8% of maneuvers observed). whereas in winter prey items were rarely ob- Similarly, attack maneuvers directed towards ae- served (Lovette, pers. observ.). This difference is rial prey (sallies and flutter-chases) were more consistent with stomach content analyses from frequent in the first two summer periods. the two sites, which have indicated that redstarts The proportional use of foraging maneuvers consume small prey such as dipterans, hyme- found in the present study differ from those re- nopterans, homopterans, and adult Lepidoptera ported by Robinson and Holmes (1984). These in winter, and more large Diptera and larval Lep- differencesmy stem from annual variation in the idoptera in summer (T. W. Sherry, unpubl. data). types of prey taken by breeding redstarts, or it may reflect differencesin the classificationof ma- DISCUSSION neuvers between the two studies. In particular, Many migratory birds experience seasonalshifts we used an additional maneuver category (sally- in food resourcesas they move between breeding snatches) not used by Robinson and Holmes and wintering areas. Some species,such as the (1984), and we may have included in this cate- Bay-breasted Warbler (Dendroica castanea; gory maneuvers that they would have lumped in Greenberg 1984) and Worm-eating Warbler their “flutter-chase” category. (Helmitheros vermivorus;Greenberg 1987), con- In winter, attack maneuvers were more con- spicuously switch between different food sources sistent, with no single type of maneuver showing in the two seasons.Other species, such as the a large shift in frequency between early and late Yellow Warbler (Dendroicapetechiaaestiva), use winter (Table 2). similar foraging methods in both summer and 788 IRBY J. LOVETTE AND RICHARD T. HOLMES

Prenesting Incubation Feeding Early Late Young Winter Winter FIGURE 2. Ratio of foragingmovements to prey attacksfor male American Redstartsin summer and winter. Movement : attack ratio was derived by dividing mean foraging speed within a period by the mean attack rate within that period. winter (Wiedenfeld 1992). In this study, we found had lower attack rates than did speciesforaging that although American Redstarts use a similar on similar types of prey at temperate latitudes, general foraging strategy in both seasons,there which he attributed to lower prey availability in are differences between seasonsin their attack the tropics. In contrast, Wiedenfeld (1992) found rates, foraging speed, amount of time spent for- that Yellow Warblers had similar attack rates in aging, and types of prey consumed. This varia- their temperate breeding habitats and the trop- tion in redstart foraging behavior provides in- ical mangrove forests where they winter, sug- sights into differences in prey availability be- gestingthat prey availability for this speciesmay tween summer and winter. be more constant between seasonsthan it is for Prey attack rates of male redstarts were sig- redstarts. nificantly lower in winter than during any part In summer, the movement rates of male red- of the breeding season.Although attack rateswere starts appeared to vary in concert with the en- low in winter, redstartsforaged at greater speeds ergetic demands imposed by feeding young, as and hence presumably searchedmore substrate male redstarts more than doubled their foraging per unit time than in the breeding season. By speed after they began provisioning nestlings. dividing the mean foraging speed by the mean Their winter foraging speed was similar to this attack rate for each sampling period, we esti- rapid nestling-stagerate, a further suggestionthat mated the number of foraging movements re- winter food resourcesmay be low relative to de- quired for a bird to encounter a prey item in each mand. season.This movement : attack ratio was highest Although time budget sample sizeswere small, in winter, intermediate during the nestling stage, the data suggestthat male redstartsdevote more and lowest in the early stagesof the breeding time to foraging in winter than in summer. This season(Fig. 2). seasonal difference may stem in part from the Both the low winter attack rate and the high time constraints of mate attraction, pairbond winter movement : attack ratio implicate the maintenance, and territoriality that are present winter as a period of relatively low prey avail- in summer but not in winter; during the summer, ability for redstarts. Similarly, in a comparison male redstarts devoted much of their time to of temperate and tropical guilds of insectivorous stationary singing, especially in the prenesting birds, Thiollay (1988) found that tropical birds and nesting stages of the breeding season. Al- REDSTART FORAGING BEHAVIOR IN SUMMER AND WINTER 789 though redstartsengage in agonistic interactions The four behavioral indices of food availabil- in winter, they do not sing, but rather emit chip- ity, taken together, suggestthat redstarts face a notes while foraging (Holmes et al. 1989) and reduced insect availability in winter relative to thus in winter relatively less time is devoted ex- summer. However, male redstarts presumably clusively to territory maintenance. have reduced energetic costs in winter relative Time budgets of insectivorous birds resident to summer: in winter they are rarely exposed to in the tropics indicate that these species may temperaturesbelow their thermoneutral zone, and spend up to 95% of their time foraging (Ricklefs they do not need to allocate resourcesto mate 197 1, Marcotullio and Gill 1985). Insectivorous attraction or raising young. The increased day- birds at temperate latitudes, however, seem to length redstarts experience during the breeding spend less time foraging, even while breeding. seasonalso increasesthe amount of time avail- For example, adult juncos (Bunco phaeonotus) able for foraging. A more detailed knowledge of devote less than 75% of their time to foraging this between-seasonvariation in energetic costs when feeding nestlings (Weathers and Sullivan is needed to determine the fitness consequences 1989), and male Willow Flycatchers (Empidon- of low winter food availability for redstarts. ax trailii) spend a proportion of their time during This study was limited to only one habitat type the breeding season“loafing” (Ettinger and Ring in each season. However, the low winter food 1980). Redstarts appear to follow the time bud- availability we inferred from these foraging be- get pattern of tropical residents while wintering havior analysesis consistentwith known redstart in Jamaica, when they spend about 85% of the demographic patterns. In winter, habitats vary time foraging, and the temperate pattern while in suitability for redstarts, as indicated by dif- breeding, when the proportion of time they spend ferences in densities, age and sex composition, foraging ranges between 43% and 56%. and overwinter survival between habitats This inter-seasonal shift in time allocation (Holmes et al. 1989, Sherry and Holmes, in press probably reflects differences in the composition b). Removal experiments have shown that red- and/or availability of prey resourcesbetween the starts compete strongly for winter territories in two seasons.The small flying insectstaken more preferred habitats such as the mangrove forest often during the winter (T. W. Sherry, unpubl. site where this study was conducted (Marra et data) probably require more time to find and al. 1993). However, further information on the capture, and provide less energy per prey item factors influencing the overwinter survival of than do the types of prey taken in the breeding redstartsis required to evaluate whether the low season.Lepidopteran larvae and tipulid flies, the winter resource levels reported here have de- types of prey fed most frequently to redstart nest- mographic consequencesfor American Redstarts lings (Omland and Sherry 1994) are large prey and other Neotropical migrants. items that are energy-rich relative to the smaller prey that predominate in the redstart’s winter ACKNOWLEDGMENTS diet. We are grateful to the Jamaican Natural Resources Insects attacked in winter were flushed from ConservationAuthority, and particularlyto Miss Yvette their resting places on foliage more often than Strong, for facilitating our work in Jamaica, to the PetroleumCompany of Jamaicafor permissionto work the insects attacked in summer. When foraging, on their property, and to the Northeast Forest Exper- male redstartsfrequently usea characteristic“ tail- iment Station, Radnor, PA, for permission to work at fanning” behavior that flashes brightly-colored Hubbard Brook. We thank Peter Marra and Tom Sher- patches of their plumage, which may flush prey ry for fruitful discussionsand companionship in the field; Peter Hunt, Russ Greenberg, Peter Marra, Paul items from hiding (Robinson and Holmes 1982). Nealen, Tom Sherry, and an anonymous reviewer for Although we did not quantify these “tail-fans,” their comments on this manuscript;and Mark McPeek they appeared to be more common in winter and Peter Petraitis for their advice on statisticalanal- when the “flutter-chase” maneuvers were more yses. This project was supported in part by a REU frequent. The “tail-fanning” behavior of foraging supplement to NSF grants to Dartmouth College and Tulane University. redstarts may thus be particularly advantageous in the winter when prey are relatively scarce,and LITERATURE CITED when those most available are small flying prey ALERSTAM, T., AND G. HCKSTEDT. 1982. Bird mi- which can be flushed by the rapid movement gration and reproduction in relation to habitats rates of foraging redstarts. for survival and breeding.Omis Stand. 13:25-37. 790 IRBY J. LOVETTE AND RICHARD T. HOLMES

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