Anthropogenically-Mediated Density Dependence in a Declining Farmland Bird

Abstract

Land management intrinsically influences the distribution of animals and can consequently alter the potential for density-dependent processes to act within populations. For declining species, high densities of breeding territories are typically considered to represent productive populations. However, as density-dependent effects of food limitation or predator pressure may occur (especially when species are dependent upon separate nesting and foraging habitats), high territory density may limit per-capita productivity. Here, we use a declining but widespread European farmland bird, the yellowhammer Emberiza citrinella L., as a model system to test whether higher territory densities result in lower fledging success, parental provisioning rates or nestling growth rates compared to lower densities. Organic landscapes held higher territory densities, but nests on organic farms fledged fewer nestlings, translating to a 5 times higher rate of population shrinkage on organic farms compared to conventional. In addition, when parental provisioning behaviour was not restricted by predation risk (i.e., at times of low corvid activity), nestling provisioning rates were higher at lower territory densities, resulting in a much greater increase in nestling mass in low density areas, suggesting that food limitation occurred at high densities. These findings in turn suggest an ecological trap, whereby preferred nesting habitat does not provide sufficient food for rearing nestlings at high population density, creating a population sink. Habitat management for farmland birds should focus not simply on creating a high nesting density, but also on ensuring heterogeneous habitats to provide food resources in close proximity to nesting birds, even if this occurs through potentially restricting overall nest density but increasing population-level breeding success.

Post-fledging habitat selection in a rapidly declining farmland bird, the European Turtle Dove Streptopelia turtur

“…Overall, our data highlight the importance of breeding habitat in close proximity to good foraging habitat (Dunn et al 2015b). This may be especially important early in the season when adults re-nest rapidly after nestlings fledge, sometimes starting to build the next nest while feeding nestlings in the first nest (Dunn et al unpubl.…”

On the relative importance of space and environment in farmland bird community assembly

“…Moreover, since open land species appear particularly sensitive to crop composition, and since there is a higher risk of potential mismatch between habitat preferences and breeding success in unpredictable landscapes, one may expect a positive effect of crop heterogeneity. Indeed a diversified landscape will more likely provide a suitable habitat and stable food resource at the territory scale [ 72 , 73 ]. However, our study only provided circumstantial evidence for this hypothesis, since results were based on spatial patterns.…”

The significance of seed food in chick development re‐evaluated by tracking day‐to‐day dietary variation in the nestlings of a granivorous passerine

“…, Dunn et al . ). Thus, our Yellowhammer population most likely formed a good test population for examining the seed component in the diet.…”