Towards an integrated view of escape decisions in birds: relation between flight initiation distance and distance fled

Rapid human population growth and increasing habitat fragmentation lead to more frequent direct encounters between humans and animals. Consequently, numerous habitats will become less suitable for some species due to an increase in perceived risk of predation. Studies show that different species vary greatly in their tolerance to human disturbance, but these findings are typically only based on flight initiation distance (FID, the distance at which animals flee when approached by a potential predator including a human). The aim of this study was to broaden the general view of escape behaviour by including distance fled (DF) in the analyses. We measured FID and DF in 699 birds belonging to 17 species in Estonian urban and rural settlements. We calculated the relationships between two types of escape decisions and behavioural, environmental and morphological parameters. There was a positive relationship between FID and DF for heavier species, but not for lighter species suggesting mass-dependent differences in the cost of escape. Flock size and starting distance in rural habitats were important predictors of FID while distance to refuge was only positively correlated with DF. Birds in rural habitats escaped earlier and further and exhibited a positive relationship between starting distance and FID, whereas no such trend was seen in urban birds, possibly due to a narrow zone of awareness. Our findings suggest that DF represents an independent and informative additional measure of antipredator behaviour that together with FID provides a more integrated view of the costs of escape. This, in turn, facilitates finding effective ways for mitigating effects of anthropogenic disturbance on wild animals.