The value of a mouthful: Flight initiation distance as an opportunity cost

Abstract

AbstractFlight initiation distance of animals when approached by a potential predator reflects the risk that an individual is willing to take when the individual has to gauge the value of staying put relative to the cost of flight. I predicted that this cost–benefit balance would depend on the opportunity cost of fleeing. This opportunity cost can be estimated as the difference in flight initiation distance (FID) between an individual engaged in eating rather than just loafing. I estimated FID of 55 species of birds when approached by a human whilst eating or loafing. There was highly significant variation in difference in FID between these two situations amongst species. Species eating mobile food that is difficult to catch showed little difference in FID between the two situations, whilst species eating immobile food such as seeds had longer FID when eating than when loafing. This difference was fully attributed to differences in relative eye size, because species that had longer FIDs when foraging rather than loafing had small eyes, whilst species with long FIDs when loafing rather than foraging had large eyes. Species with long FIDs, when foraging compared to loafing, had low adult annual survival rates and vice versa. This effect was independent of whether mobile or immobile food was consumed. These findings suggest that individuals of different species adjust their FID to the probability of adult survival and also that differences in visual acuity among species as reflected by eye size linked to differences in food mobility affect the opportunity cost of risk taking.