Anim. Behav., 1992, 44, 597 613 Optimal foraging and timing processes in the starling, Sturnus vuigaris: effect of inter-capture interval DANI BRUNNER*~, ALEX KACELNIK~** & JOHN GIBBONw *Department of Experimental Psychology, University of Cambridge, Cambridge, U.K. ~King's College Research Centre, King's College, Cambridge, U.K. **New York State Psychiatric Institute and Columbia University, New York, U.S.A. (Received 26 July 1991; initial acceptance 18 October 1991; final aeceptance 10 December 1991,"MS. number."A6119) Abstract. Laboratory experiments with starlings, Sturnus vulgaris, were conducted to investigate the interaction between timing and cost-benefit considerations. The design simulated an environment in which food was distributed in patches. Patches contained a random number of food items (N=04) separated by a fixed inter-capture interval or fixed interval. All patches ended with sudden depletion. The time elapsed since the last prey capture was the only way to detect the depletion of the patch. Once the patch was depleted a new patch could be reached by travelling between two perches. Three measures of timing were taken: (1) rate of working for food as function of'waiting' time in a patch, (2) the time of the last response in a patch or 'giving-in' time, and (3) the time at which travel was initiated or 'moving-on' time. The fixed interval that characterized patches was varied between conditions. The mean time of the peak in working rate was consistently centred around the fixed interval, while the other two measures of timing kept a roughly linear relation to the fixed interval, with slope greater than one. In accordance with Scalar Expectancy Theory, variability in the three forms of timing was proportional to the magnitude of the fixed interval. The birds seemed to take account of this increase in variability as shown by the mean value of their giving-up criterion. These results imply that information-processing constraints are important for modelling behavioural optimality. A large class of foraging algorithms require measurements of the passage of time. For instance, Krebs et al. (1974) suggested that great tits, Parus major, might decide when to leave patches using a giving-up time, i.e. a threshold searching period. Redhead & Tyler (1988) studied rats in determin- istically depleting patches in the laboratory, and suggested that their behaviour was based on measuring the interval between two captures, so as to leave patches immediately after the first capture that followed an interval longer than a predeter- mined criterion. Taking into account stochastic variability of patch quality, Green (1984) suggested a patch-leaving rule, based on the predator's ability to assess the total number of captures and total time elapsed since arrival at each patch. In another theoretical paper, Iwasa et al. (1981) showed that tPresent address: New York State Psychiatric Institute, Box 50, 722 W 168 Street, New York, NY 10032, U.S.A. w address: Department of Zoology, University of Oxford, Oxford, U.K. the choice of rate-maximizing strategy depends on the type of (temporal) prey aggregation faced by the predator. For example, if most patches con- tained a predictable number of prey, it might pay to hunt by expectation, i.e. to count a number of rewards before leaving each patch (Gibb 1958). These studies, and many more, suggest solutions to foraging problems that assume the predator's unlimited ability to measure time, remember inter- vals and adjust its behaviour accordingly to the time elapsed. This ability, on the other hand, is in itself the topic of research in a variety of psychological con- texts (e.g. Triesman 1963; Gibbon 1977; Allan 1979; Church 1985; Gallistel 1990). Deviations from perfect timing are of interest to foraging theorists because timing mechanisms can affect the range of options open to predators, and lead to constrained rate-maximizing solutions that dif- fer substantially from naive, unconstrained treat- ments of the same problem (Gibbon et al. 1988; Shettteworth et al. 1988; Kacelnik et al. 1990). 0003-3472/92/100597 + 17 $08.00/0 91992 The Association for the Study of Animal Behaviour 597