Timing of Breeding in an Ecologically Trapped Bird

In human-modified environments, organisms may prefer to use habitats where their reproductive performance is lower compared to alternative options. Many such ecological traps occur in seasonally changing environments. Although the timing of breeding has been shown to impact reproductive performance in a variety of organisms, it has never been considered as a potential mechanism underlying ecological traps. We address this issue with a migratory bird, the red-backed shrike, breeding in a human-modified, farmland-forest landscape. Shrikes prefer breeding in forest clear-cuts where their reproductive performance is lower than in less attractive farmland. We compared brood size and quality of early (first broods) and delayed breeders (replacement broods) between the two habitats. We found a stronger seasonal decrease in reproductive performance in preferred forest clear-cuts than in farmland. Food resources were slightly more abundant in forest than in farmland at the beginning of the season but depleted more steeply in forest by the end of the breeding season. By contrast, the phenotypic quality of breeders did not decline over the course of the season in either habitat. This is the first report that the timing of breeding relative to the seasonal change in key resources may play a significant role in explaining low reproductive performance in ecological traps.