Ecological traps and behavioural adjustments of urban songbirds to fine‐scale spatial variation in predator activity

Abstract

AbstractThe influence of predators on bird populations is controversial and poorly understood, especially in urban areas where predator densities can be particularly high. We assessed if fine‐scale spatial variation in predator activity and proximity have direct and indirect effects on urban songbird distributions and breeding success, by testing the hypotheses that (1) songbirds that are sensitive to nest predation select territories with reduced activity of nest predators; (2) blackbird Turdus merula, a species that experiences high nest predation rates, lays smaller clutches in territories located in areas with higher numbers of nest predators as predicted by Skutch's hypothesis; (3) songbirds that are sensitive to nest predation have higher nest predation rates in areas with greater predator activity. We tested these hypotheses using two sites in urban Sheffield, UK, and focus on nest predatory corvids and grey squirrels Sciurus carolinensis. We found no evidence that songbirds that are most sensitive to nest predation adjust their territory location in response to fine‐scale spatial variation in predator distributions. It thus seems unlikely that urban predators are indirectly regulating urban bird population size by restricting habitat availability. Blackbirds did not vary their clutch size in response to predator distributions. These findings generate the potential for an ecological trap in which prey species fail to avoid areas with the highest concentrations of nest predators, or to exhibit behavioural adjustments to reduce the risk of nest predation at such sites. We find some evidence for such ecological traps as, while fine‐scale variation in grey squirrel occurrence and activity were not associated with nest predation rates, nests located in closer proximity to corvids and in areas with higher indices of corvid activity experienced slightly higher nest predation rates.