waders and their estuarine food supplies - Vlaams Instituut voor de ...

PREY PROFITABILITY AND INTAKE RATE
CAUSES OF VARIATION IN PREY PROFITABILITY AND ITS
CONSEQUENCES FOR THE INTAKE RATE OF THE
OYSTERCATCHER HAEMATOPUS OSTRALEGUS
Prev species have different morphological and behavioural adaptations to escape their predators. In this
paper we review how these prey defenses affect prey profitability and intake rate for one predator, the
Oystercatcher. Four rules govern profitability. First, within each species large prey are more profitable
than small prey, because flesh content increases more steeply with prev si/e than handling time. Second,
soft-bodied prey, such as worms and Leatherjackets, which can be swallowed whole, are much more
profitable than armoured prey, such as bivalves, which Oystercatchers have to open before the flesh can
be extracted from the shell. Third, heavily armoured surface-dwelling prey, like Mussels and Cockles,
are the least profitable prey of all. even if the armour is bypassed through stabbing the bill between the
valves. Fourth, within the burying prey species, the profitability of prey decreases wilh depth. Hence
burying bivalve species that bury in winter at larger depth than in summer, are in winter, if not out of
reach of the bill, anyway less profitable.
Despite the large differences between the profitabilities of the various prey species, the iniake rates do
not differ much when the prey species are compared, presumably because prey 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, vv iilun each species, ihc intake rate goes up if larger, more profitable prey are taken. Thus,
if the birds have to feed on smaller prey specimens, ihey fail to fully compensate for the low profitability
by an increase in the rate at which these prey are found. Although the profitability of prey differs seasonally
due to the variation in the prey condition, only a small seasonal variation in the intake rate was
found. Because burying bivalves and soft-bodied worms bury deeper and are less active in winter. Oystercatchers
necessarily rely on bivalves living at. or just beneath, the surface at that time of year.
Introduction
Oystercatchers Haematopus ostralegus feed on at least
15 different prey species along the NW. European
coast .("ramp & Simmons 1983. Hulscher 1996). On
intertidal sand and mudflats, iheir food consists of
Cockles Cerastoderma edule and Mussels Mytilus
ediilis. and lo a lesser degree also of other bivalves,
such as Macoma balthica. Scrobicularia plana and
Mya arenaria. worm species, such as Arenicola marina
and Nereis diversicolor. and the Shore Crab
('mi inns maenas. On rocky shores. Oystercatchers
take Limpets Patella aspara and P. vulgaia. Periwinkles
Liliorina liiiorea and Dogwelks Nucellu lapillus.
whereas in grassland they select mainly differeni
earthworm species (Lumbricidae) and Leatherjackets
175
(larvae of the crane fly Tipula paludosa).
Except for the worms and Leatherjackets, all
species selected by Oystercatchers are armoured with
a calcified exoskelelon. Nonetheless. Oystercatchers
never swallow the hard skeletal parts. Insiead. they eat
ihe llesh from bivalves, snails or crabs after prizing, or
stabbing, the bill between the valves or hammering a
hole in the shell or carapace Oystercatchers may easily
find prey lying at the surface but they have to spend
time breaking in and extracting the llesh. It lakes more
lime for an Oystercatcher lo locate benthic bivalves
living hidden in the substrate, since these prev are usually
found by randomly probing the bill into the mud
(Hulscher 1976. 19X2. Wanink & Zwarts 1985). Moreov
er. since these buried prey are also armoured against
predation. Oystercatchers. must, as with surface prey,