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.

Nest Predation Deviates from Nest Predator Abundance in an Ecologically Trapped Bird.

In human-modified environments, ecological traps may result from a preference for low-quality habitat where survival or reproductive success is lower than in high-quality habitat. It has often been shown that low reproductive success for birds in preferred habitat types was due to higher nest predator abundance. However, between-habitat differences in nest predation may only weakly correlate with differences in nest predator abundance. An ecological trap is at work in a farmland bird (Lanius collurio) that recently expanded its breeding habitat into open areas in plantation forests. This passerine bird shows a strong preference for forest habitat, but it has a higher nest success in farmland. We tested whether higher abundance of nest predators in the preferred habitat or, alternatively, a decoupling of nest predator abundance and nest predation explained this observed pattern of maladaptive habitat selection. More than 90% of brood failures were attributed to nest predation. Nest predator abundance was more than 50% higher in farmland, but nest predation was 17% higher in forest. Differences between nest predation on actual shrike nests and on artificial nests suggested that parent shrikes may facilitate nest disclosure for predators in forest more than they do in farmland. The level of caution by parent shrikes when visiting their nest during a simulated nest predator intrusion was the same in the two habitats, but nest concealment was considerably lower in forest, which contributes to explaining the higher nest predation in this habitat. We conclude that a decoupling of nest predator abundance and nest predation may create ecological traps in human-modified environments.

Maladaptive habitat selection of a migratory passerine bird in a human-modified landscape.

In human-altered environments, organisms may preferentially settle in poor-quality habitats where fitness returns are lower relative to available higher-quality habitats. Such ecological trapping is due to a mismatch between the cues used during habitat selection and the habitat quality. Maladaptive settlement decisions may occur when organisms are time-constrained and have to rapidly evaluate habitat quality based on incomplete knowledge of the resources and conditions that will be available later in the season. During a three-year study, we examined settlement decision-making in the long-distance migratory, open-habitat bird, the Red-backed shrike (Lanius collurio), as a response to recent land-use changes. In Northwest Europe, the shrikes typically breed in open areas under a management regime of extensive farming. In recent decades, Spruce forests have been increasingly managed with large-size cutblocks in even-aged plantations, thereby producing early-successional vegetation areas that are also colonised by the species. Farmland and open areas in forests create mosaics of two different types of habitats that are now occupied by the shrikes. We examined redundant measures of habitat preference (order of settlement after migration and distribution of dominant individuals) and several reproductive performance parameters in both habitat types to investigate whether habitat preference is in line with habitat quality. Territorial males exhibited a clear preference for the recently created open areas in forests with higher-quality males settling in this habitat type earlier. Reproductive performance was, however, higher in farmland, with higher nest success, offspring quantity, and quality compared to open areas in forests. The results showed strong among-year consistency and we can therefore exclude a transient situation. This study demonstrates a case of maladaptive habitat selection in a farmland bird expanding its breeding range to human-created open habitats in plantations. We discuss the reasons that could explain this decision-making and the possible consequences for the population dynamics and persistence.