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

Predation cost (P-c) is often regarded its a pivotal component determining foraging behavior. We hypothesized that variations in two of its major Constituents, predation risk (mu) and the marginal value of energy (partial derivative F-s/partial derivative e, where F-s 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 mu. We manipulated the birds' flight performance by clipping primary flight feathers from their wings to increase mu, but the clipping may also increase partial derivative F-s/partial derivative e. To help distinguish between the birds' response to these confouding effects, we further augmented food in the aviaries to reduce partial derivative F-s/partial derivative 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 front shelter only slightly and mainly when the birds were feather-clipped and food was not augmented. Food augmentation had it homogenizing effect oil foraging costs by increasing GUDs and washing out the effects of distance and feather clipping. We argue that mu increases with distance from shelter but that, for the highly urban House Sparrow, this increase is only slight. Feather clipping then increased It further to the point at which patch use discernibly decreased with distance front Shelter. Our experimental manipulation of feather clipping also acted to increase partial derivative F-s/partial derivative e and resulted in an overall on lowering of GUDs. The seed augmentation counteracted the effect of feather clipping partial derivative F-s/partial derivative e, allowing the birds to reduce their foraging efforts and washing out the qualitative effect of mu with respect to distance from shelter.