Rank-related response in foraging site selection and vigilance behaviour of a small passerine to different winter weather conditions

Abstract

The foraging patterns and time spent on vigilance behaviour by Willow Tits (Poecile montanus) were studied under varying weather conditions in a subalpine forest in central Norway during the winters of 2009-2013. The birds adjusted their microhabitat use in pines and their vigilance levels in response to perceived risk of predation and starvation. At low temperatures, the dominating adults foraged mainly in the upper and most predation safe areas in pines and the subordinate juveniles in the more predator risky lower halves. When the outer canopies of pines were covered by snow, the adults tended to restrict their foraging activities to the inner parts of the lower halves of the trees and thus overlapped considerably with the juveniles. In windy weather, the age classes differed in site use, mostly because juveniles foraged more in the outer part of the branches. Juveniles were significantly More vigilant than adults under all the weather conditions, and they spent about 60% of their time seeking food whereas adults spent about 85%. It is suggested that the high level of vigilance by the subordinate juveniles is costly and may be the main cause of the higher winter mortality of juveniles compared to that of adults.