Social flocking increases in harsh and challenging environments

Abstract

Grouping with others can provide enhanced information about resources and threats. A key hypothesis in social evolution proposes that individuals can benefit from social information in environments where it is challenging to meet energetic needs. Here, we test this hypothesis by examining the environmental drivers of conspecific flocking behaviour in a large archive of citizen science observations of two common North American birds, the dark-eyed junco (Junco hyemalis) and black-capped chickadee (Poecile atricapillus). To quantify flocking behaviour, we apply the index of dispersion, D, as a metric of clumpiness in each species spatiotemporal distribution. We show that juncos in winter are nearly always more clustered than a random expectation, whereas chickadees span a range from uniform to socially clustered distributions. In both species, the degree of social clustering strongly increases with abundance. We identify several key environmental variables that explain the extent of conspecific flocking in both species. Flocks are more socially clustered at higher latitudes, higher elevations, closer to midwinter, and at temperatures that are colder than average given the location and time of year. Together, these findings support the hypothesis that sociality is a key strategy for coping with harsh environments. HIGHLIGHTSO_LIGrouping with others can be an important source of information about resources C_LIO_LIWe analyzed how flocking behaviour changes throughout winter in two bird species C_LIO_LIWe used the index of dispersion to quantify social clustering at a broad scale C_LIO_LIIn both species, social clustering increases in response to climate challenges C_LI