Influence of temperature and other factors on the daily roosting times ofblack-billed magpies

Abstract

On 102 occasions from September 1984 to April 1985, the time of morning departure and evening arrival of a population of black-billed magpies (Pica pica) was recorded at a winter roost near Edmonton, central Alberta. The goal was to determine whether an effect of cold on daily roosting time could be observed and distinguished from that of other environmental factors. Multiple linear regression analysis on the data revealed that, all other things being equal, magpies left the roost later relative to sunrise, and returned to it earlier relative to sunset on colder days, and also (i) on longer days, (ii) on darker, cloudier days, (iii) in the second half of winter, when compared with the first half, and (iv) when fewer birds were using the roost. These results held true only before the occurrence of overt territorial behaviour in March, which coincided with a notable shift towards earlier departures and later arrivals. The larger amount of time spent in the roost by magpies when it was cold may be related to an increased predisposition to inactivity induced by cold temperatures, or to possible microclimatological advantages offered by the roost. The amplitude of this temperature effect may, however, be dependent on the birds' level of hunger. Indeed, the effect of temperature on roosting times was much weaker in the case of departure, i.e., after a long night of food deprivation, than it was in the case of arrival.