Patterns of Association of Secondary Species in Roosts of European Starlings and Common Grackles

This study examines population dynamics of secondary species associated with roosts composed primarily of European Starlings (Sturnus vulgaris) and Common Grackles (Quiscalus quiscula). Our goal was to determine patterns of association for secondary species and to evaluate the role of interspecific...

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Bibliographic Details
Published in:The Wilson bulletin (Wilson Ornithological Society) 1985-01, Vol.97 (2), p.173-182
Main Authors: Caccamise, Donald F., Fischl, Joseph
Format: Article
Language:English
Subjects:
Aggregation
Aves
Birds
Censuses
Foraging
Nighthawks
Population size
Predators
Quiscalus quiscula
Sparrows
Starlings
Sturnus vulgaris
Transgenetic organisms
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Summary:This study examines population dynamics of secondary species associated with roosts composed primarily of European Starlings (Sturnus vulgaris) and Common Grackles (Quiscalus quiscula). Our goal was to determine patterns of association for secondary species and to evaluate the role of interspecific relationships in roosting flocks. We estimated sizes of roosting populations over 2 seasons for all roosts within a large census area. From June to November roosts were active 3-20 weeks and varied in size from 2000 to over 100,000 individuals. We found 17 secondary species; numbers ranged from fewer than 10 individuals (Green-backed Heron, Butorides striatus) to over 20,000 (American Robin, Turdus migratorius). They were in nearly every roost, irrespective of season or roost size. Although each species had a unique pattern of association, their presence together resulted in considerable species heterogeneity. For those roosting species with distinct foraging requirements, interspecific exchange of foraging information is an unlikely reason to form mixed species roosting flocks. Also, species that form large (>100) roosting flocks apparently do not join mixed species flocks only for added predator protection, as these benefits are likely asymptotic at relatively small flock sizes. There are many factors that make roost sites attractive. By selecting occupied roosts, individuals can lower the risk of choosing poor sites, and be assured that, at least, minimum requirements will be met. This concept provides an explanation for why very large roosting flocks develop, and why they are often composed of species with little apparent basis of commonality.
ISSN:0043-5643