Factors influencing roost use by Short-toed Treecreepers Certhia brachydactyla at urban sites

Although treecreepers Certhia spp. are known to roost communally, the factors associated with roost use have never been quantified. Here, we investigate the influence of environmental conditions on the roosting behaviour of Short-toed Treecreepers C. brachydactyla in an urban setting in The Netherlands. We performed transect-based nocturnal counts of 120 roosts on 487 days during a three-year period. We correlated total number of birds per roost per day to multiple relevant environmental factors (daily duration of precipitation, daily sum of precipitation, daily mean wind direction, daily mean wind speed, daily maximum temperature, daily minimum temperature, daily mean temperature, daily mean humidity and daylength). Numbers of roosting Treecreepers were positively associated with minimum daily temperature and northern winds and negatively with day length and maximum daily temperature. Our study shows that Treecreepers respond to short-term temperature fluctuations by altering their roosting strategy. Communal roosting in the species is thus likely a behavioural response to a combination of severe weather and short day length. Most roosts are generally vacant (86.8 ± 3.5% SD) or hold single roosting treecreepers (7.6 ± 3.9%) and a few apparently optimal sites hold consistently large numbers (6.6 ± 3.5%). Vacant roosts peak in number during the breeding season (89.5 ± 3.8% vs. 85.8 ± 2.9% in winter), whereas singleton roosts (8.0 ± 4.2% vs. 6.7 ± 2.5% in the breeding season) and communal roosts (7.3 ± 3.4% vs. 4.8 ± 3.3% in the breeding season) are most common during winter. Our study fills an important gap in our knowledge of roosting behaviour in passerine birds, but much remains to be learned.