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

PREY SIZE SELECTION AND INTAKE RATE
14 16 18 0 2 4 6
time on feeding area (h)
8 10 12
Fiji. 15. Daily consiiiiipiion as function of ihe lime spent on ihe feeding area, when there is A. One feeding period per day or B. Two feeding
periods vvilli a roosting period of 6 hours in between. Ihe iniake rale determines how lone it takes before Ihe hial is forced lo pause or reduce
iis intake rale due to the digestive bottleneck, this being Set by ihe three parameters: 111 die oesophagus may eonlain 12 g ash-free dry llesh. (Jl
defecation starts0 5 li alter ihe beginning of the feeding period, and (3) ihe processing rale is 11.66 mg s ' taller Kersten iW. Visser 1996a).
oesophagus contains 80 g wet (or 12 g dry) flesh; I2I
the birds start to defecate 0.5 h after the beginning of
the feeding period; and (3) the processing rate is 4.4
mg wet (or 0.66 mg dry) flesh s'. The processing rale
is thus much slower than the range of intake rates observed
in the field (1 to 4 mg s '). Indeed. Oystercatchers
in captivity can achieve even higher intake rales.
The highest rate of 16 mg s~' was observed by
Hulscher (unpubl.) in captive birds eating mussel
flesh. At that rate, a digestive pause would be necessary
after only 13 minutes of feeding, which was indeed
the actual length of the feeding period in
Hulscher's birds.
The finding lhat the processing rate is usually
much lower than the intake rate during feeding has
several implications. At least in the long run, birds
must interrupt their feeding when their intake rate exceeds
their processing rate (Zwarts & Dirksen 1990).
The overriding effect of this digestive bottleneck on
the daily consumption of the Ov stercatcher is shown in
Fig. 15. The maximum intake during a feeding period
is set by the three parameters given by Kersten &
Visser (1996a). At the very low average iniake rate of
1 tugs', the gut fills and forces the bird to reduce iis
iniake rate to a level equal to. or below, the processing
rale after 530 min of continuously feeding. But with
171
intake rates of 2. 3 or 4 mg s '. the alimentary tract is
full after only 135. 77 or 54 min. respectively.
The lengih of time an Oystercatcher has m spend
on the feeding area, including digestive pauses, to
achieve a given food consumption, is also shown by
lig. 15. The only assumption here is that the bird Starts
to feed with an empty stomach and leaves the feeding
area with a full oesophagus. With one feeding period a
day. the bird would have to feed for 10 hours to achieve
the minimum daily energy requirements of 36 g (Fig.
15A). Remarkably, the average intake rate while feeding
does not matter, as long as it exceeds 1 mg s'. Furthermore,
if Oystercatchers leave ihe feeding area with
a full gut. il lakes just over 5 hours lo process all ihe
food, or nearly as long as ihe usual high water roosiing
period. Consequently, ihe total consumption is hardly
affected when both the feeding and roosting periods
are st\ hours long (Fig. I5B). The figure points to three
important conclusions: (D as long as the intake rate
during feeding exceeds 1.16 mg s ; . ihe gui processing
rate determines ihc overall consumption during a low
waler period; (2) Oystercatchers cannot eat more than
25.1 g dry flesh during a low water period of 6 hours;
so that (3) one low water feeding period of 6 hours is
not long enough to obtain their food requirements.
What do birds do on the feeding grounds when