Vigilance and the functional response of granivorous foragers

1. Functional response models that predict the relationship between feeding rate and food density often include only two behavioural parameters, handling time and searching rate. However, vigilance can occupy a large proportion of foraging time and, consequently, may affect the functional response. Previous functional response models of granivorous birds showed no effect of vigilance on predicted feeding rates; these models assumed that all of handling time is compatible with vigilance and, therefore, overestimated the potential time for cost free vigilance to occur. 2. Here we have derived a new functional response model that incorporates the proportion of time spent vigilant (v) and the proportion of handling time that is compatible with vigilance (p). This model allows for the relationship between vigilance and handling to vary from completely compatible to mutually exclusive, and degrees in between. 3. To determine whether vigilance can affect the functional response of a granivorous bird, grey partridge Perdix perdix L, we measured the functional response and associated behavioural parameters, and used the behavioural estimates to parameterize the model. Any deviation from the feeding rates predicted using a model without vigilance indicates that vigilance is affecting the predicted functional response. 4. We found that vigilance only affected the predicted functional response at very low food densities (< 3 seeds m-2). Simulations show how the potential for vigilance to affect feeding rate (i.e. the values of p given v) increases as v increases. We parameterized the model using data from chaffinches Fringilla coelebs, which were shown to spend > 50% of their foraging time vigilant, and found that even with a high value of p vigilance reduced feeding rates at higher seed densities. 5. This study shows that vigilance can affect the feeding rate of a granivorous bird when either the proportion of time spent vigilant is high or the proportion of compatible handling time is low. This may affect larger scale ecological processes, i.e. spatial distribution of foragers and patterns of resource depletion, as individuals try to mitigate the effects of vigilance by maximizing their feeding rate whilst minimizing their predation risk.