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PREY SIZE SELECTION AND INTAKE RATE
PREY SIZE SELECTION AND INTAKE RATE
Oystercatchers ignore small bivalves. These prey have a lower probability of being encountered, but this
alone is not sufficient to explain why they are not taken. Small bivalves are not profitable given the time
needed to open and eat them, and consequently they allow only low rates of return of energy. The size
rejection threshold is not fixed, but varies according to the average intake rate during feeding. Oystercatchers
increase (heir intake rate in two ways: they reject unprofitable prey and they adjust their searching
behaviour in order to increase the encounter rate with ihe more profitable prey. The intake rate depends
predominantly on the prey size taken. When large prey are present, the intake rate is high (3 to 4
mg dry flesh s 1 ) and small and medium-si/cd bivalves are rejected, due to their lower profitability. When
only small prey are available, ihe intake rate decreases to 1 mg s '.The daily consumption varies between
36 and 50 g dry flesh. This means that even when the birds feed at the average year-round rate of 2 mg
s '. 5 h feeding a day is sufficient to meet their normal maximum daily energy expenditure. Ninety-two
percent of the intake rates observed in the field exceeded the rate at which the gut of Oystercatchers is
able to process food (0.66 mg s '). Since processing rate forces the bird to stop feeding when their
oesophagus is full (12 g). Oystercatchers must stop feeding for digestive pauses when their intake rate is
high. Due to this digestive bottleneck, they have to feed 10 hours a day if there is one feeding period, or
8 hours a day if there are two feeding periods, digestive pauses included, to reach the minimum dailv
consumption of 36 g. required at theriiioneutrality.
Introduction
Do predators take all the individual prey belonging to
one species that they encounter, and if not. to what degree
are some prey under-represented in the diet and
why? These questions are important in every study of
the interaction between predators and their fcxxl supply.
This chapter first analyses the degree to which the
selection of prey sizes within one prey species by Oystercatchers
can be understcxxl as a passive process in
which a random I v searching bird takes all the prev it
encounters; this will be referred to as 'passive si/e selection".
As we shall see. however, Oystercatchers reject
certain size classes of prey which they actually encounter,
and hence there is also "an active size
selection'. The rules that Oystercatchers obey when selecting
size classes actively, and whether these can be
derived from optimal foraging models, are (hen discussed.
Finally, ihe chapter discusses how Oystercatchers
maximize their intake rate and how the rate at which
food can be processed sets a limit to the food intake.
The analyses are restricted to five bivalve species:
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the Edible Mussel Mytilus editlis and the Edible
Cockle Cerastoderma edule and the three clams: Macoma
balihica. Scrobicularia plana and Mya armaria.
There are three main methods far obtaining data on
size selection by Oystercatchers eating these prey
species: recovering opened and emptied shells, direct
visual observations and size-specific depletion. Shell
recovery is particularly convenient and is possible because,
having eaten the flesh, the birds leave the
opened shell behind. These are recognizable as prey by
the damage done to the shell and/or by iis position in
ihe substraie. Recendy emptied bivalves also contain
some flesh, sometimes along the mantle edge and always
where the adductors are atiaehed on the valves.
The collection of these shells is therefore an easy, reliable
method of obtaining a frequency distribution of
all the size classes taken by Oystercatchers in a certain
area, although there may be a sampling bias in the case
of Mussels (Ens 1982. Cayford 1988). It is thus no coincidence
that there are more papers on prey size seleciion
in Oystercatchers than there are in all the other
wader species put together.