The functional response of oystercatchers (Haematopus ostralegus) searching for cockles (Cerastoderma edule) by touch

Abstract

1. We describe the functional response of oystercatchers (Haematopus ostralegus L.) searching for cockles (Cerastoderma edule L.) by touch, using a multiple‐prey version of the disc equation developed by Charnov (1976). 2. The model includes probabilistic time costs associated with the successful and unsuccessful handling of prey, seasonal changes in the flesh content of prey, and factors that affect prey availability. The encounter rate with cockles is estimated using a simple random search model. This model calculates the likelihood that a bird would locate a cockle buried in the sand per unit of time spent searching, as a function of the touch area of buried cockles (the largest cross‐sectional area of the shell enlarged by the surface area of the bill‐tip) and the bird’s probing rate. 3. Prey profitability increased with cockle size, and the model predicted birds should preferentially take cockles > 15 mm in length to maximize their rate of energy intake. The birds showed a systematic seasonal departure from the model’s predictions. During late winter, the observed and predicted maximum intake rates for individual birds were comparable. However, the mean size of cockles observed being taken by the birds was consistent with the predicted mean size taken by a bird, including all size classes in its diet, rather than the predicted size for a bird taking only size classes > 15 mm (for a bird maximizing its intake rate). During early winter, observed intake rates were lower than predicted values for a bird feeding unselectively, suggesting birds either reduced their searching effort, preferentially ignored the larger, more profitable size classes, or experienced longer time costs. The lower than expected intake rates observed during early winter resulted from prey choice: birds preferentially ignored the larger, more profitable size classes. 4. Data presented in this paper suggest that birds attempting to maximize their intake rate incur significant costs, in addition to those associated with searching for and handling prey, which are included in the model. Such costs could include a risk of bill damage when attacking large cockles, an increased risk of parasitism associated with large cockles, or a mass‐dependent metabolic or predation cost incurred by birds storing any excess ingested food in the form of fat. 5. A simple graphical model is presented to show how such costs, together with the need to achieve a given intake rate to avoid starvation, might interact to determine optimal intake rates. Within this framework, the maximum intake rate predicted by Charnov’s model should be viewed as a means of describing the constraints on intake rates, rather than as an explicit optimality model. Depending on energy demands and the costs of maintaining a given intake rate, birds could experience a range of optimal intake rates, but only under certain conditions would the optimal intake rate be the maximum. This has implications for models of the dispersion of predators across a gradient of resource densities, and behaviour‐based models of population dynamics, both of which implicitly assume that a predator would maximize its fitness by maximizing its rate of energy intake.