Avian Communal Roosting: Implications of Diurnal Activity Centers

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Abstract
Most current explanations of avian communal roosting focus on benefits derived at the roost itself and assume that the roost is the individual''s base of operation. A very different view of communal roosting emerges when the birds are markedly individually. A recent radiotelemetric study of communally roosting European starlings (Sturnus vulgaris) revealed that individuals are far more faithful to their diurnal activity centers (DAC''s) than to their roosts (Morrison and Caccamise 1985). Studies of other communally roosting species have suggested that DAC''s are probably not unique to starlings. The finding that at least some communally roosting species have DAC''s has important implications for both foraging-based and predation-based interpretations of communal roosting. Our studies of starlings suggest that foraging factors were of primary importance in the evolution of both DAC''s and communal roosting. Foraging daily on a familiar DAC may have benefits similar to those of holding a territory. Furthermore, DAC-based birds should forage away from the DAC only when the food intake rate in the supplemental feeding area (SFA) more than compensates for the cost of commuting to that SFA. And finally, although DAC-based birds probably join small communal roosts near their DAC''s to reduce the risk of nocturnal predation, they join much larger, more distant roosting groups primarily to reduce commuting costs to supplemental feeding areas. We hypothesize that major communal roosts may simply be aggregations of individuals roosting as close as possible to especially rich feeding areas. Our patch-sitting hypothesis generates predictions that clearly distinguish it from other foraging-based hypotheses. We encourage researchers to look for DAC-based behavior in other species and to consider a DAC-based perspective when formulating and testing hypotheses about communal roosting.