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Comparison of Roost Use by Three Species of Communal Roostmates

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Comparison of Roost Use by Three Species of Communal Roostmates The Condor 92~405-412 0 The CooperOrnithological Society 1990 COMPARISON OF ROOST USE BY THREE SPECIES OF COMMUNAL ROOSTMATES DOUGLAS W. MORRISON Department of BiologicalSciences, Rutgers University,Newark, NJ 07102 DONALD F. CACCAMISE Department of Entomology, Cook College,Rutgers University,New Brunswick,NJ 08903 Abstract. Our previous studies of communally roosting European Starlings (Sturnus vulgaris)revealed that each bird fed daily for months on its own “diurnal activity center” (DAC) and commuted to a variety of nearby and distant roosts. Since these observations contrast sharply with the predictions of most foragingexplanations for communal roosting, we wanted to determine if this DAC-centered roostingpattern occurredin other communally roosting species.In this study we used radiotelemetry to monitor feeding and roosting sites used by starlings,Common Grackles (Quiscuh quiscula),and American Robins (Turdus migratorious),three speciesthat share communal roosts in central New Jersey. Our goals were to determine (a) whether avian speciesthat roost together use the roosts in similar ways, and (b) when and why individuals change roosts. Foraging patterns were similar in all three species;individuals fed daily on their DACs for many weeks. Roosting patterns were similar for gracklesand starlings;individuals switchedamong nearby and distant roosts. In contrast, robins always roosted near their DACs, changingboth roosts and DACs at the end of the breeding season.Predation rates at roostswere extremely low and did not explain the use patterns of large and small roosts. We argue that (a) DACs and DAC-centered roostingare probably widespreadamong communally roosting species,and (b) DAC-based individuals select roosts primarily on the basis of their proximity to good sourcesof food. Key words: Communal roosting:foraging; diurnal activity center;predation; European Starling; Stumus vulgaris; Common Grackle; Quiscalusquiscula; American Robin; Turdus migratorious. INTRODUCTION individuals actually used a variety of roosts but Several investigators have hypothesized that returned to feed on their own “diurnal activity communal roosting by birds evolved to facilitate center” (DAC, an area of l-2 km?) day after day foraging in various ways. For example, roosting for months. Contrary to the assumptions of ear- in a central location can reduce average com- lier models, foraging and roosting were centered muting costs when food patches are short-lived around a stable feeding area (DAC) rather than (Horn 1968). Communal roostsmight also serve around a roost. as “information centers” at which individuals Foraging and roosting behaviors of starlings can learn about new or better food sourcesby change in midsummer. During most of the year, following successfulroostmates (Ward and Za- starlingsfeed exclusively on their DACs and join havi 1973). These and other “roost-centered” small (50-500 bird) communal roostsnearby (< 2 models (e.g., Weatherhead 1983) are based on km away). During late summer and fall, starlings the assumption that the roost is a relatively stable begin to commute to rooststhat are much larger base of operation from which individuals exploit (5,000-50,000 birds) and more distant (4-12 km). less stable food sources. These larger roosts do not form because of a A very different picture of communal roosting shortageof acceptable roosting sites (Lyon and emerged when roostmates were individually Caccamise 198 l), and they are not premigratory marked (Morrison and Caccamise 1985). Radio- aggregations,because they form months before tracking studies of communally roosting Euro- migration (Caccamise et al. 1983). pean Starlings (Sturnus vufguris) revealed that When using distant roosts, starlings continue to feed primarily on their DACs, but also stop briefly at food sourcesnear the distant roosts.By 1Received 26 July 1989. Final acceptance 7 Feb- roosting overnight near food patches far from ruary 1990. their DACs, starlingscan reducecommuting costs t4051 406 DOUGLAS W. MORRISON ANDDONALD F. CACCAMISE to supplemental food patchesby as much as 50% from cotton shoelace(Morrison and Caccamise (Caccamise and Morrison 1986). Recent obser- 1985). After an initial period of adjustment (6- vations of individual starlings en route to and 48 hr), the bird’s behavior was not noticeably from roosts support this “patch sitting” aspect affectedby the transmitter; the radio-taggedbirds of communal roosting (Caccamiseand Morrison flew and foragednormally for months. The range 1988). Is the DAC-centered roosting pattern of of the transmitters was greater than 1.0 km for starlings unique or do other communally roost- birds roosting in trees and less than 0.5 km for ing specieshave similar patterns? birds foraging on the ground. Transmitter life To investigate the relationship of communal averaged 90 (SD = 15) days. roosting to DACs, we undertook a comparative To determine whether the birds had DACs study of starlings, Common Grackles (Quisculus (i.e., spatially clustereddiurnal sightings),we used quiscula),and American Robins (Turdus migra- an automobile with a rooftop antenna to locate torious),three speciesthat sharecommunal roosts each bird during the day, 6 days a week, at ran- in central New Jersey (Caccamise and Fischl domly predetermined times between 06:OO and 1985). This is the first study to monitor both 18:O0. Attempts to locate the birds were suc- roosting and foraging behavior of heterospecific cessfulover 85% of the time for all three species. roostmates. By radio-tagging individuals cap- This successrate was somewhat lower than in tured from the same feeding area, we controlled previous studies(Morrison and Caccamise 1985) for the effectsof habitat heterogeneity. We found becauseof a lack of experienced field assistants that DAC-based foraging behavior is not unique in 1987. to European Starlings and may be widespread For interspecific comparisons, spatial cluster- among communally roosting species.We argue ing was quantified as the mean distance from that DAC-based birds select among suitable each diurnal sighting to the use-weighted center roosting sitesprimarily on the basis of their prox- (i.e., mean x and y coordinates) of all diurnal imity to good feeding areas. sightingsmade on that bird. SeparateDACs were recognizedwhen all sightingsin one cluster were METHODS >2 km from all sightingsin the other cluster. In We radio-tagged nine European Starlings, nine practice, the few cases of multiple DACs in- Common Grackles, and nine American Robins, volved separations of several kilometers. all adults, in central New Jerseyduring the local We located the roost usedby each radio-tagged roosting season (June-November) of 1987. All bird six nights a week. Attempts to locate roost- starlingsand grackleswere postbreedingindivid- ing sites were successfulover 93% of the time. uals. Five of the robins were captured toward the The birds could almost always be found by cen- end of their breeding seasonand may have been susingafter dark the many traditionally usedroost reproductively active for the first 1O-20 days of sites in the study area. Distances from DACs to the 46-102 days (X = 77 days) that they were communal roosts were measured from the use- tracked (Bovitz 1990). weighted center of the bird’s DAC. All birds were captured during the day in the To describe long-term patterns of roost use, horticultural display garden of Rutgers Univer- we restrictedour analysisto the 17 birds for which sity, New Brunswick, New Jersey. Robins were we had greater than 40 days of observations: six mist-netted; starlings and grackleswere captured starlings (four males, two females), four grackles using decoy traps and baited walk-in traps. Birds (three males, one female), seven robins (four selectedfor radio-taggingappeared in goodhealth, males, three females). Radio contact with the with well-maintained plumage and body weights other 10 birds (X = 20 days) was much shorter. above the mean for that species and sex. We Four had transmitters with defective batteries; taggedand releasedall birds at the site of capture after only l-2 weeks in the field, their pulse rates 53 hr after they were trapped. The birds sub- increased almost two-fold, symptomatic of a sequentlyfrequented an urban-suburban area that voltage-drop in lithium batteries. The other six was primarily residential and commercial (8 1%) transmitters lost their antennas, reducing their but contained a few agricultural fields (4%) and effective range to < 100 m. woodlots (2%) (Fischl and Caccamise 1985). Estimates of predation rates were based on the The transmitter package (< 5 g) was attached fates of 63 adult starlings tracked during four to the back of the starling (7 l-85 g), grackle (95- summer roosting seasons(1983-l 986). Each bird 131 g) or robin (73-84 g) with a “vest” made was assignedto one of five categories:eaten, dead AVIAN COMMUNAL ROOSTING 401 but not eaten, battery expired, premature trans- munal roosts at a total of 15 different sites. The mitter failure, and unknown. The eaten category three specieswere frequently found roosting to- may have overestimated predation rate because gether. The six starlings(n = 487 nocturnal sight- it may have included birds eaten after dying of ings) and the four grackles (n = 279 nocturnal other causes.The battery expired category was sightings)joined roosts known to include both assignedonly when the signal was lost (a) after starlingsand gracklesover 97% of the time. Rob- the expiration date calculated on the basis of ins (n = 462 nocturnal sightings) roosted with battery capacity and transmitter current drain, starlings and grackles at least 57% of the time. and/or (b) after a significant (1.5 to 2.0-fold) Of the 15 roosts, 12 were minor roosts of increase in transmitter pulse rate, indicating a ~2,000 birds. One site (R-88) was occupied by voltage drop in lithium batteries. Premature an intermediate-sized flock that peaked at 3,000- transmitter failure was assigned only when the 5,000 in late August. Two sites-(R-42 and R-97) bird had been visually sightedwith a nonworking were occupiedby major roostingflocks of 1O,OOO- transmitter. The unknown categoryincludes un- 35,000 birds.
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