Effects of exotic habitat on nesting success, territory density, and settlement patterns in the Blackcap (Sylvia atricapilla)

Animals are expected to distribute themselves in a heterogeneous environment in such a way that they maximize their reproductive output. When the environment is profoundly changed by human pressure, however, cues used for habitat selection in the past may no longer provide reliable information about habitat quality. I monitored the nesting success of Blackcaps (Sylvia atricapilla) in two types of forest in southern Moravia in the Czech Republic. I assessed their breeding density and territory size using a territory mapping method and the minimum convex polygon method. I determined spring arrival through direct observations and measured vegetation characteristics and pattern of spring leafing of shrubs in both forests. I show that Blackcaps preferentially settled in a plantation of introduced black locust (Robinia pseudoacacia) upon their return from spring migration. In this plantation, they reached twice the density as that observed in a natural floodplain forest nearby. However, they had significantly lower nesting success (15.5%) than in the floodplain forest (59%). Returning migrant Blackcaps may be lured by early-leafing shrubs in the exotic plantation to settle earlier and at higher densities in the reproductively inferior habitat. My results show that (1) it is not possible to assess habitat quality based solely on breeding densities, (2) human-modified habitats can function as ecological traps by luring settling birds into unsuitable habitats, and (3) by replacing exotic plant species with native ones we can restore native communities and increase the breeding productivity of bird populations.