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

Discussion
There is no difference in food consumption
between wild and captive birds
We did not find a difference in food consumption between
wild and captive birds. Since birds in ihe field
fly, and possihlv walk. more, than captive birds, some
difference between cage and field metabolism had
been expected, even for studies w here ihe captive birds
had to search and handle their own fcxxl. However, a
theoretical calculation shows that the expected magnitude
ol thi- difference is actually quite small. Free-living
Oystercatchers fly four times a day some 2 to 5 km
between roost and feeding area and occasionally make
short flights due to disturbance or interference. Assuming
that (1) for an Oystercatcher weighing 520 g.
ihe average energy expenditure is 7.S W at thermoneuiraliiy
(Fig. 3: 672 kJ day ' = 7.8 U S '). (2i captive
birds never flj and free-living birds llv for 10-30 mm a
.lav (authors' unpubl. data), and (3) Ihe Hying costs are
36 W (Pennycuick 1989; given thai the wingspan of an
Oystercatcher is 83 cm (Welham 1994)), the energy
budget of wild buds would only lie about 3-8'r above
thai measured in captive ones. As the three studies ol
free-living Oystercalchers refer to birds just before or
during the breeding season lhat foraged close lo their
nest or on a tidal Hat nearby, and Hew even less than
average (Zwarts & Blomert 19%. authors' unpubl.
data), it is perhaps nol surprising after all that their
daily consumption did not differ from thai of the captive
birds. Also the estimates of the daily energy expenditure
of adult breeding birds throughout ihe breeding
season (Kersten 1996: Table 8) confirm that (he life
of breeding Oystercatchers is not expensive.
Food requirements and body weight
Daily consumption increases with body weight in
Oystercatchers weighing between 420 and 520 g (Fig.
3). The bcxly weight of wild Oystercatchers varies.
however, over a higher range, between 500 in summer
and 620 g in midwinter or just before migration (Dare
1977, Goss-Custaid etal. 1982. Johnson 1985, Zwarts
etal. 1996b). We therefore seek lo identify the underlying
cause of the relationship before deciding if we
can saleiv extrapolate the regression line in Fig. 3 beyond
the weight range of the captive birds to the generally
higher weights of the free-living birds.
INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER
228
Oystercatchers with long wings (280 mm) weigh.
on average. 12% more than birds with short wings
(250 mm) (Zwarts et al. 1996c). Wing or bill lengths
have not been measured in mosl sludies summarized in
Table 1 and Fig. 3. Therefore, it is not possible to standardize
the body weights to birds of the same si/e.
Were this correction possible, the scalier along ihe
slope in Fig. 3 would possihlv have been smaller and
the slope less steep.
Variation in energy requirements due to intraspeeilic
variation in bcxly weight, were expected to scale
with the exponent 0.73, the exponent that applies
across species (Kersten & Piersma 1987). However,
we found the much higher exponent of 1.49. The exponent
is about 1.35 if basal metabolic rale (BMR) is
plotted against body weight in an other wader, the
Knot Calidris canutus (Piersma et al. 1995).
There are two explanations for this steep increase
of metabolism with intraspecific variation in total body
weight, as has also been found in other bird species
(Bryant & Tamer 1991. Tinbergen & Diet/ 1994 for
parent birds provisioning their chicks). First, the much
higher maintenance metabolism of heavy individuals
must be due to a change in the metabolic machinery.
Skin, leathers and the skeleton weigh the same for fat
and lean birds of the same si/e. but since these pails of
ihe body require less energy (Daan et al. 1990). the
costs of Itv ing are relatively low in lean birds.
Second, if birds increase the weight of their body,
they also increase the costs of living because walking
and flying become much more expensive. Taylor ei al.
(1980) showed that the oxygen consumption increased
in direct proportion to the added load. In other words.
the exponent is I if log(energy expenditure) is plotted
against loglbody weight). The costs of Hying increase
much more with body weight: the exponent against
body weight is 1.52 (Pennycuick 1989). Thus, the exponent
of the allometric relationship between body
weighi and maintenance metabolism depends on the
activities of the birds, i.e. how much ihey fly. Per 24 h.
Oystercatchers rest during more than half of the time,
they walk 30 lo 40'7 and fly during 1 to 2%. Hence.
the exponent of energy expenditure against body
weight should be closer to I than to 1.52.
In conclusion, the steep increase of maintenance
metabolism with body weight (exponent 1.49) may in
a small part be attributed to the unmeasured variation