Individual variation in intake rate: the relative importance of foraging efficiency and dominance

1. Individual variation in the intake rate of foraging animals arises large ly from variation in two characteristics: their intrinsic ability to forage in the absence of competitors (foraging efficiency) and the detrimental ef fect of competitors on their intake rate (susceptibility to interference). We use a model to explore the relative importance of foraging efficiency an d dominance, which influences susceptibility to interference, in determinin g the intake rate of foraging animals at a range of competitor densities. 2. The model is parameterized and tested for a shorebird-prey system, oyste rcatchers Haematopus ostralegus L. feeding on mussels Mytilus edulis L., in which interference occurs because dominant individuals steal prey from sub dominant ones. In both oystercatchers which open mussels by stabbing their bill between the shell valves (stabbers) and those which hammer a hole thro ugh the shell (hammerers), foraging efficiency is predicted to be the major determinant of intake rate at competitor densities below 100-150 birds ha( -1) and dominance the major determinant at higher densities. In reality, a very similar relationship is found in stabbers, but in hammerers foraging e fficiency remains the most important determinant of intake rate across the full range of observed competitor densities, in contrast to the model's pre diction. Oystercatchers typically forage at densities in the range 100-250 birds ha(-1), in the region in which foraging efficiency and dominance are of approximately equal importance to stabbers, while foraging efficiency is most important to hammerers. We suggest reasons for the difference in the model's predictive power for stabbers and hammerers. 3. We use the model to make the general predictions that the relative impor tance of foraging efficiency is higher when (i) it varies more between indi viduals, (ii) prey encounter rate is high, (iii) handling time is short, (i v) prey stealing (kleptoparasitic) attacks occur over short distances, (v) the probability of stealing prey is low, and (vi) movement speed while fora ging is low. In the oystercatcher-mussel system, prey encounter rate is low er and handling time longer than in most other shorebird-prey systems. This suggests that the importance of foraging efficiency in determining intake rate is likely to be greater in many other shorebirds. 4. Most studies of between individual variation in intake rate have focused on the importance of dominance rather than variation in foraging efficienc y. Both the model predictions and field data suggest that variation in fora ging efficiency may be a more important source of variation in intake rate than its previous depth of study would suggest.