Foraging behavior of an urban bird species: Molt gaps, distance to shelter, and predation risk

Predation cost (P_c) is often regarded as a pivotal component determining foraging behavior. We hypothesized that variations in two of its major constituents, predation risk (μ) and the marginal value of energy (∂F_s∂e, where Fs is the survivor's fitness and e represents the amount of acquired energy), will translate into variations in patch use behavior of ground-foraging birds. We studied patch use behavior of House Sparrows (Passer domesticus), as affected by the proximity to shelter, in two large outdoor aviaries. Proximity to shelter should affect μ. We manipulated the birds' flight performance by clipping primary flight feathers from their wings to increase μ, but the clipping may also increase ∂F _s∂e. To help distinguish between the birds' response to these confounding effects, we further augmented food in the aviaries to reduce ∂F_s∂e. Patch use, as measured by giving-up densities (GUD, the amount of food left behind in a resource patch following exploitation) was affected by distance from shelter only slightly and mainly when the birds were feather-clipped and food was not augmented. Food augmentation had a homogenizing effect on foraging costs by increasing GUDs and washing out the effects of distance and feather clipping. We argue that μ increases with distance from shelter but that, for the highly urban House Sparrow, this increase is only slight. Feather clipping then increased μ further to the point at which patch use discernibly decreased with distance from shelter. Our experimental manipulation of feather clipping also acted to increase ∂F _s/∂e and resulted in an overall lowering of GUDs. The seed augmentation counteracted the effect of feather clipping on ∂F _s∂e, allowing the birds to reduce their foraging efforts and washing out the qualitative effect of u with, respect to distance from, shelter.