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

INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER
WHY OYSTERCATCHERS HAEMATOPUS OSTRALEGUS
CANNOT MEET THEIR DAILY ENERGY REQUIREMENTS IN A
SINGLE LOW WATER PERIOD
Captive Oystercatchers consume daily 25-40 g dry flesh or 550-850 kJ. of which thej metabolize 450-
7(K) kJ. Free-living Oystercatchers eat more than captive birds but, contrary to expectation, this is not due
to greater activity costs but to a higher body weight. When body weights are equal, free-living and captive
Oystercatchers consume the same amount of food. The intake rate of Oystercatchers generally varies
between 1 and 3 mg dry flesh s ' feeding, but if non-feeding times are included, the crude intake rate
usually varies between 1 and 1.5 mg s '. Extremely high intake rates, above 4 mg s" 1 , are only observed
in birds feeding during a short bout after a long resting period. According to Kersten & Visser (1996a)
such high intake rates cannot be sustained for long, because a maximum of 80 g wet flesh, equivalent to
12 g dry tlesh. can be stored in the digestive tract and the processing rate does not exceed 4.4 mg wet
flesh s" 1 or 0.66 mg ash-free dry weight (AFDW) s '. Due to this digestive bottleneck, the birds are forced
to spend much time on the feeding area each day. Since the exposure lime of their intertidal feeding areas
is usually 5-6 h, Oystercatchers cannot meet their daily energy requirements in a single low water period,
which would often suffice if intake rate was the limiting factor. Fora given length of the feeding period,
the bottleneck model predicts the maximum crude intake intake, called CIRm.ix. that can be
achieved, i.e. the Ingliesi intake rate including the non-feeding time. When the birds are able to feed lor
less than 3 h. the achieved crude intake rate usually remains far below this maximum, suggesting dial ihc
rate at which prey are found and eaten determines the intake rate. The consumption is also usually less
than would be allowed by digestive constraint when the birds feed for 12 h or longer, because the birds
at thermoneutral conditions do not need more Uian 36 g a day. When the birds spend 3 to 12 h on the
feeding area, the average consumption is usually close to. or below the predicted maximum. However,
in a few cases, the maximum was clearly exceeded. These studies do not invalidate the bottleneck model,
because there is ample reason to believe that food consumption was overestimated. A detailed invesii
gaiion of the many sources of error indicates that food consumption is more likely to be overestimated
than underestimated in field studies.
Introduction
The present-day consensus is that birds may fail to collect
a sufficient amount of food in the time available, if
they fail to choose correctly the prey species to take or
the place to feed. Recently. Kersten & Visser (1996a)
challenged this view by suggesting that Oystercatchers
cannot process the food sufficiently fast during ihe
lime available for digestion, so that the digestion rate is
often a more important constratni on consumption than
the rate al which food is ingested. At first sight, this is
a remarkable view as the time available lor digestion
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will always exceed the time available for feeding.
Thus, internal processing of the food may commence
immediately after the first prev item has been ingested
and continue long after the incoming tide prevents
feeding. According to Kersten & Visser (1996a). food
is processed at a constant rate, so thai iniake rales can
only exceed this processing rate during periods when
the digestive tract has not yet been filled to capacity.
The digestive bottleneck hypothesis has important
implications, quite apart from challenging the traditional
emphasis placed on maximization of intake rale
in optimalitv models. It throws, for example, a differ-