The influence of environmental cues and anthropogenic activity on roost departure times in the Northwestern Crow (Corvus caurinus)

While communal roosting is fairly common in animals, the costs, benefits, and proximal factors influencing this behavior are poorly understood. Moreover, many species with communal roosting that have been studied to date have a strong association with humans; however, there have been few formal tests of how human activity might influence roosting behaviors. We studied roosting activity in the Northwestern Crow (Corvus caurinus) within a highly urbanized setting. Specifically, we were interested in whether variation in anthropogenic noise influenced departure times of crows from their roosts. We tested this by comparing roosting activity between days that differed in the extent and timing of automobile noise. We also tested whether other environmental factors (i.e., sunrise time and precipitation) might also influence roost departure times. We found that morning departure was later on rainy mornings and was largely correlated with sunrise time, consistent with previous studies of other species. We also found departure times accelerated over the season, to the point where the first groups of crows were waking well before any perceptible light (i.e., prior to astronomical dawn). We found no evidence that variation in anthropogenic activity, particularly from automobile noise, had any effect on crows' roost departure time. These results suggest that, although many corvids are closely associated with urbanized landscapes, these crows appear to maintain natural circadian rhythms at least for roosting departures, likely by means of detecting cues from natural light and other environmental factors.