CAUSES OF VARIATION IN PREY PROFITABILITY AND ITS CONSEQUENCES FOR THE INTAKE RATE OF THE OYSTERCATCHER HAEMATOPUS-OSTRALEGUS

Prey species have different morphological and behavioural adaptations to escape their predators. In this paper we review how these prey defe nses affect prey profitability and intake rate for one predator, the O ystercatcher. Four rules govern profitability. First, within each spec ies large prey are more profitable than small prey, because flesh cont ent increases more steeply with prey size than handling time. Second, soft-bodied prey, such as worms and leatherjackets, which can be swall owed whole, are much more profitable than armoured prey, such as bival ves, which Oystercatchers have to open before the flesh can be extract ed from the shell. Third, heavily armoured surface-dwelling prey, like Mussels and Cockles, are the least profitable prey of all, even if th e armour is bypassed through stabbing the bill between the valves. Fou rth, within the burying prey species, the profitability of prey decrea ses with depth. Hence burying bivalve species that bury in winter at l arger depth than in summer, are in winter, if not our of reach of the bill, anyway less profitable. Despite the large differences between th e profitabilities of the various prey species, the intake rates do not differ much when the prey species are com pared, presumably because p rey with a low profitability are only exploited if the search time is relatively short, i.e. if the density of harvestable prey is high. On the other hand, within each species, the intake rate goes up if larger , more profitable prey are taken. Thus, if the birds have to feed on s maller prey specimens, they fail to fully compensate for the low profi tability by an increase in the rate at which these prey are found. Alt hough the profitability of prey differs seasonally due to the variatio n in the prey condition, only a small seasonal variation in the intake rate was found. Because burying bivalves and soft-bodied worms bury d eeper and are less active in winter, Oystercatchers necessarily rely o n bivalves living at, or just beneath, the surface at that time of yea r.