Variation in the Nesting Phenology of two Positively Interacting Passerines

Abstract

A heightened risk of predation can promote the formation of protective nesting associations between avian species which are well-known for vigorously defending their broods. Thus, by nesting under the protective umbrella created by an aggressive neighbour, individuals can enhance their reproductive success. For positive interactions between different species to come into being, there must be spatial and temporal coincidence. The overlap in nesting dates may be beneficial to neighbouring individuals of these species because either can warn the other of an approaching predator and the broods can be communally defended. The main aim of the present study was to compare the nesting phenology in populations of two positively interacting passerines: the Barred Warbler ( Sylvia nisoria) and the Red-backed Shrike ( Lanius collurio). Even though both species arrived at their breeding grounds at the same time, warblers began laying eggs one week earlier than shrikes. Most Barred Warbler females attempted to lay eggs as early as possible in the first half of the breeding season. Egg-laying was spread out more evenly in time in the population of Red-backed Shrikes compared with that of the Barred Warblers. There was substantial overlap in the breeding dates in most nests of both species; nonetheless, in some seasons there was significant phenological shift. The spatially closest broods of the two species were better synchronised. Due to the narrower time window and decrease in reproductive success, the present data showed stronger selective pressure on late nesting warblers than shrikes.