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Seasonal Patterns in Roosting Flocks of Starlings and Common Grackles

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Seasonal Patterns in Roosting Flocks of Starlings and Common Grackles Condor 85:474&31 G The Cooper Ornithological Society 1983 SEASONAL PATTERNS IN ROOSTING FLOCKS OF STARLINGS AND COMMON GRACKLES DONALD F. CACCAMISE LINDA A. LYON AND JOSEPH FISCHL ABSTRACT. -Communal roosting is often a regional phenomenon that involves wide-ranging and long-lasting relationships among associations.We examined roosting behavior on a scale sufficiently large to detect regional and seasonal patterns. For five roosting seasons(June-November), we studied the population dynamics of all roosting flocks of European Starlings(Sturnus vulgaris)and Com- mon Grackles (Quiscalus quiscula) located within a l,OOO-km2census area in central New Jersey. Roosts were active from 3-20 weeks and ranged in size from 2,000 to over 100,000 individuals. The total roostingpopulation (TRP) in “major” (>2,000 birds) flocks increased through early summer, generally achieving max- imum size in mid-August when the largest number of roosts was active. When TRP was largest, size of major roosts varied greatly (range 2,000-100,000 indi- viduals). Through late summer and early fall, size and number of major roosts and TRP declined. By late fall few major roosts were active, but those remaining were large (> 30,000). Movements of individual birds (radio-tagged) suggested that changesin size of TRP resulted largely from exchangeof the local population between small, “minor” roosts (largely undetected and not included in roost censuses)and major flocks. Current hypotheses concerning the functional basis of communal roosting do not adequately explain patterns of roosting behavior that we observed. - Communal roosting behavior occurs in birds Starlings (Sturnus vulgaris) and Common of diverse taxa and habitats. Important func- Grackles (Quiscalus quisculu)within a l,OOO- tional role(s) are suggestedby the often large km2 censusarea. We investigated patterns of flock sizes and long roosting seasons.Several activity at individual roosts and population explanationsfor communal roostinghave been dynamics on a regional basis over five con- offered (Lack 1968, Siegfried 197 1, Tast and secutive seasons.Our objectives were to quan- Rossi 1973, Ward and Zahavi 1973, Weath- tify the seasonalpatterns in the size of indi- erhead 198 3) and some tests have been per- vidual roosting flocks, to determine the formed (DeGroot 1980, Loman and Tamm interrelationships among these flocks, and to 1980, Fleming 198 l), but for most speciesthe analyze the spatial and temporal distribution functional role of this behavior remains poorly of the roosting population on a regional basis. understood. Our goal was to provide an adequate descrip- Few quantitative data are available for ex- tive basis for testing current hypotheses ex- amining this behavior on a scale(temporal and plaining communal roosting behavior. spatial) sufficiently large to detect regional and seasonalpatterns. Previous studieshave either STUDY AREA AND METHODS examined activity at individual roosting as- Field work was conducted during the local sociations (e.g., Hamilton and Gilbert 1969, roosting seasons(June-November) of 1977- Swingland 1976) determined the distribution 198 1. A region within the Piedmont and inner- of roosts for a given speciesduring only part coastal plain provinces (Robichaud and Buell of the roosting season (e.g., Marples 1934, 1973) of central New Jersey (Fig. 1) was rig- Eastwood et al. 1962, Hein and Haugen 1966), orously censused(one to three times per week) or provided few data (e.g., Brown 1946, Gadgil for all major blackbird roosts: i.e., those con- and Ali 1976). These investigations have gen- taining at least 2,000 birds. The minimum size erally neglected regional relationships among of “major roosts” (>2,000) represented the roosts or seasonal variations in roosting be- smallest aggregationregularly detected in our havior. surveys. We counted some smaller roosts, but We examined the post-breeding population only at sites where they had formerly num- dynamics of all roosting flocks of European bered over 2,000 birds. Roost sites were usu- [4741 PATTERNS IN ROOSTING FLOCKS 415 KEY 0 ROOST SITE IDENTIFICATION NUMBER URBAN AREAS !P 0 369 km FIGURE 1. Map of the census area indicating location of the roost sites. Numbers inside circles are roost site identification numbers. ally located during our surveys by following ulation estimates,as roosting activity was then flightlines of birds returning to roosts in the erratic. We developed our field methods in evening. Some “minor roosts” (~2,000 birds) 1977 (Lyon 1979). were found when radio-tagged birds went to Eighteen starlings and eight grackles were them, but the small size and inconspicuous- equipped with radio transmitters (AVM In- ness of minor roosts made them difficult to strument Company) in 1980-l 98 1. Birds were detect and we made no effort to find them. We captured at roost sites using either mist-nets did not include minor roosts in size estimates atop 15-m poles or decoy traps in foraging of the roosting population. areas. Five starlings and three grackles were The size of the censusarea (approx. 1,000 lost soon after release and two starlings and km*) was defined initially by the limits of our one grackle were found dead (one predatory ability to conduct thorough censuses.After es- loss, one poisoned, one accidental). The 14 tablishing the distribution of roosting flocks remaining birds were monitored three to five (1977), we confined our searchesto the area times weekly on foraging areas during the day bounded by lines connecting the seven out- and at roosts at night for periods ranging from ermost roost sites. 13-139 days (mean 82 days). Active roosts were visited every 7-14 days. In order to estimate roost size, one to four RESULTS observerswere stationed along flight lines near roosts. Birds were counted by speciesas they PATTERNS OF ROOSTING BEHAVIOR arrived in the evening or departed in the mom- Roost sites first became active in early June ing. The total roosting population (TRP) rep- (Fig. 2). The number of active sites increased resentsthe sum of all birds in the censusarea through mid-August, then declined until the using major roosts (>2,000 birds). TRP was end of the local roosting season(early Novem- calculated at five-day intervals by summing ber). Individual roosts were active for 3-20 sizes of individual flocks that were active at weeks (Fig. 2). Generally the same sites were the beginning of each period. Counts during used in successiveyears and their heaviest use inclement weather were not included in pop- occurred over similar dates. Some sites were 476 D. F. CACCAMISE, L. A. LYON AND J. FISCHI abandoned (sites 4, 12, 14, 22, 25) after their vegetation was cleared, but others (sites 2, 6, 18, 20, 33, 35, 36) were abandoned without apparent physical changesin the sites or their surroundings.The size of individual flocks was usually similar among years, but several of them grew over the years from intermediate to very large (sites 21, 23, 42). We witnessed the opposite trend as well (sites 4, 6, 21). All roosts were composed primarily of star- lings and grackles(Fig. 3). Starlingswere gen- erally more abundant, although either species could represent more than 80% of an individ- ual flock during the early growth or late decline phasesof the activity cycle. Only once did we encounter a flock in which one speciescom- posed 99% of the population: at the beginning of the 1980 roosting season,site 36 was com- posed mainly of juvenile starlings. We also counted 11 secondary species roosting com- munally at these sites, but their roles were small, either becauseof their low numbers or their relatively brief stay (Caccamiseand Fischl, unpubl. data). The ratio of starlingsto grackles(Fig. 3) var- ied seasonally and independently of TRP for all roosts in the censusarea. Values were high early in the season because starlings began roosting first, but grackles joined the roosts while the TRP was still relatively small. Star- lingsgenerally exceededgrackles through most of each season,although sometimes(especially in late fall) grackles became more abundant. The proportion of starlings was much higher in 1978 than in any other year. We were not able to determine whether the grackle popu- lation was actually depressedin 1978 or the censusarea did not include the major grackle roosts. The dynamics of total roost population were similar each year of the study with respect to both maximum size (2 1O,OOO-270,000 birds) and seasonalpattern of growth and decline (Fig. 3). In each year (including 1977, Lyon 1979) except 1979, the TRP was largest in mid-Au- gust. In 1979, TRP grew more slowly and the maximum size was reached much later (3 Oc- tober). Peaking so soon before migration, the population also declined more rapidly than in other years. The number of active roosts in the census area increasedfrom June through mid-August (Fig. 2). The increase in TRP through this pe- 10 20 30 10 20 30 9 19 29 9 19 29 9 19 29 7 JUNE JULY AUG. SEPT. OCT. NOV. tification number (seeFig. 1) is locatedat the date of largest flock size. Shapeof border around roost site identification FIGURE 2. Activity periods (solid lines) of individual number indicates roost size class: circle 12,000; square roost sites for each year of the study. Each roost site iden- 2,000-10,000; triangle lO,OOO-30,000;hexagon >30,000. PATTERNS IN ROOSTING FLOCKS 477 250 10.0 200 2.0 150 1.0 z 100 0.5 8 r 50 0.1 z X Y i 0 p 250 10.0 G 5 200 2.0 2 p ’ z B 150 1.0 5 $ 100 8 0.5 F 2 g 50 0.1 8 i? : < 250 10.0 g & + 200 2.0 B b 150 1.0 ci Ii F zi 100 0.5 ii 50 0.1 10.0 200 2.0 150 1.0 100 50 0.5 0.1 lb 20 30 10 20 30 9 li 2.9 a 10 28 0 13 28 7 JUNE JULY AUG SEPT OCT NOV FIGURE 3.
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