Quantifying the interaction between food density and habitat structure in determining patch selection

Foraging site selection is likely to be influenced by vegetation structure since this can affect both the potential energy gain and predation risk that animals associate with a patch. We investigated foraging site selection by chaffinches, Fringilla coelebs, between a 'safe' patch with short stubble allowing earlier detection of an approaching predator and a 'risky' patch with longer, more visually obstructive stubble. When initial seed densities in the patches were equal, individuals foraged more in the short stubble and had shorter vigilance periods than those that foraged in the long stubble. These differences in foraging and vigilance behaviour changed if the food abundance in each patch was unequal. We used these variations in measures of foraging and vigilance behaviour to predict the level of increased food abundance in the 'risky' patch necessary for it to be used more often. As the relative density of seeds in the long stubble compared to that in the short stubble increased, the proportion of time individuals spent in the short stubble declined. Patch switching occurred at a seed density ratio of approximately 1:2.5. Individuals made a higher proportion of pecks in the short stubble with less than two times as many seeds in the long stubble. With more than three times as many seeds in the long stubble, they made a higher proportion of pecks in the long stubble. The results show that compensation through vigilance for the increased predation risk manifest in a patch's physical characteristics may greatly reduce the profitability of a patch.