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MEPS prepress abstract   -  DOI:

How much of foraging success is luck and how much tactic? The case of the Imperial shag

Rory P. Wilson*, Mark D. Holton, Andrew Neate, Monserrat Del’Caño, Flavio Quintana, Ken Yoda, Agustina Gómez-Laich

*Corresponding author:

ABSTRACT: It has been proposed that predators searching for prey acquire food according to a probabilistic framework, where success is based on ‘luck’ and the odds of success vary with prey abundance. If true, this has major ramifications for variation in the rates of energy acquisition within animal populations, which is particularly pertinent in offspring provisioning and breeding success because smaller animals (the young) cannot starve for as long as the adults. However, despite much general speculation about rates of food acquisition, no-one has measured whether food encounter is probabilistic in wild animals. We used animal-mounted cameras to document all prey captures by wild Imperial shags Leucocarbo atriceps as they hunted underwater and show that, although they mostly do not have inter-prey acquisition time distributions that accord with a ‘luck-based’ framework assuming a constant probability of finding prey over time, there is no difference in the amount of food predicted captured between models that use the empirical data or theoretical Poisson-based fits of the data. We also noted considerable inter-individual differences in foraging success that far exceeded any differences between empirical and theoretical inter-prey acquisition time distributions. The data were used in a probabilistic foraging model that made explicit the mechanistic link between random prey encounters and food-dependent breeding success. This indicated that our ‘less lucky’ individuals could not provision their brood at rates commensurate with normal growth while the ‘lucky’ birds could do so easily. Given the nature of food encounter in our cormorants, coupled with substantial inter-bird variation in foraging success, we suggest that more successful individuals are particularly choosey about when, how and where to forage, which results in them operating with higher odds of success.