Change in flight initiation distance between urban and rural habitats following a cold winter

Adaptation to proximity of humans should be particularly advantageous during adverse environmental conditions because the ability to withstand proximity to humans allows for exploitation of abundant food. We carried out flight initiation distance experiments in 207 bird species in 7 pairs of urban–rural areas from northern to southern Europe before and after the extremely cold winter of 2009–2010. Flight initiation distance in different species of birds on average became shorter after the cold winter, but only in resident urban populations (frequently exposed to humans) and not in migratory or rural populations of the same species. There were independent partial effects of human population density with an increase in flight initiation distance from before to after the cold winter of 2009–2010 at low population densities and a decrease at high population densities, and there was an independent effect of latitude with the flight initiation distance increasing from 2009 to 2010 at low latitudes and decreasing from 2009 to 2010 at high latitudes. The difference in flight initiation distance before and after the cold winter increased with severity of the winter across different study areas. Therefore, the difference in flight initiation distance between urban and rural habitats increased after the cold winter, especially in areas with the coldest winter weather. These findings are consistent with the hypothesis that a behavioral component of urbanization is due to reduced flight initiation distance in urban environments and suggest that weather conditions (i.e., cold winters) have played an important role in the process of urbanization.