Flight initiation distance (FID), the distance at which individuals take flight when approached by a potential (human) predator, is a tool for understanding predator-prey interactions. Among the factors affecting FID, tests of effects of group size (i.e., number of potential prey) on FID have yielded contrasting results. Group size or flock size could either affect FID negatively (i.e., the dilution effect caused by the presence of many individuals) or positively (i.e., increased vigilance due to more eyes scanning for predators). These effects may be associated with gregarious species, because such species should be better adapted to exploiting information from other individuals in the group than nongregarious species. Sociality may explain why earlier findings on group size versus FID have yielded different conclusions. Here, we analyzed how flock size affected bird FID in eight European countries. A phylogenetic generalized least square regression model was used to investigate changes in escape behavior of bird species in relation to number of individuals in the flock, starting distance, diet, latitude, and type of habitat. Flock size of different bird species influenced how species responded to perceived threats. We found that gregarious birds reacted to a potential predator earlier (longer FID) when aggregated in large flocks. These results support a higher vigilance arising from many eyes scanning in birds, suggesting that sociality may be a key factor in the evolution of antipredator behavior both in urban and rural areas. Finally, future studies comparing FID must pay explicit attention to the number of individuals in flocks of gregarious species.

Behavioral and Physiological Ecology Group Centre for Ecological and Evolutionary Studies University of Groningen Groningen The Netherlands

Behavioural Ecology Group Department of Systematics Zoology and Ecology Eötvös Loránd University Budapest Hungary

Department of Applied Geoinformatics and Spatial Planning Faculty of Environmental Sciences Czech University of Life Sciences Prague Czech Republic

Department of Biogeography and Global Change Museo Nacional de Ciencias Naturales Madrid Spain

Department of Plant Pathology Szent István University Budapest Hungary

Department of Zoology and Laboratory of Ornithology Palacky University Olomouc Czech Republic

Department of Zoology Institute of Ecology and Earth Sciences University of Tartu Tartu Estonia

Ecologie Systématique Evolution Université Paris Sud CNRS AgroParisTech Université Saclay Orsay France

Ecology Research Group Hungarian Academy of Sciences Hungarian Natural History Museum Eötvös Loránd University Budapest Hungary

Institute of Zoology Poznań University of Life Sciences Poznan Poland

Nature Inventory and EIA services Arctic Centre University of Lapland Rovaniemi Finland

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Urban birds' tolerance towards humans was largely unaffected by COVID-19 shutdown-induced variation in human presence

Resident birds are more behaviourally plastic than migrants

Face mask-wear did not affect large-scale patterns in escape and alertness of urban and rural birds during the COVID-19 pandemic

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