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

INTAKE RATE AND PROCESSING RATE IN OYSTERCATCHER
Tabic 1. Daily ciiiiMiiiipiion (g AFDW) .incl body weighl
Oystercatchers feeding on Cockles (Or), Mussels {Myt), commercial
food pellets (pd), Si rnhii ularia i Sci i or larvae of Tipula (77/>).
Source numbers (used in Pig. 3) and sources are given in ihe lirsi
and last column. All birds were adults except one I -y ear and lour 3-
OM bud'- (See column 'age"). All birds were held in captivity,
bin studies marked wuh I m column 'tree' were free-living birds.
All dala were collected in iherinoneuiral conditions, except loin and
two birds held al an average air temperalure ol 6 5 and 6.3 (
column 'T'l. Body weight was constant in all studies over the periods
concerned, bin decreased in study I (18 g in 8 days I. 23 1511 g in
26 day si. 28 (19 gin 30 days), and increased in Stud) 26(38 g bl 26
daysi. 27 124 g in 28 da] s). 29(13 g in 30 daysi and 30134 g in 34
days); column'BW ' gives weight change (gdaj ') Change in bod)
weighl was unknown in field studies 36 and 38. but assumed to be
constant Average body weight tg. column "BW") and month of observation
are indicaled. Body weight was not known foi the days of
observation in field study 36 and 38. but assumed lo be equal to the
average weighl of the buds of the same sev. such as measured in
oilier birds in Ihe same lime ot Ihe year and the same site. Columns
'g', 'kJ' and 'kJQ' give total daily coiisiiniplioii in trams ot gross
\i i.ivv (gi. grossenergj ik.li and metabolized energy ik.li. respectively.
Kersten & Piersma 11987) found in pellets 22.8 kJ g : fresh
weighl being equivalent to 25.8 Id g"' AFDW. Goede (1993
25.1 kJ g ' AFDW Ibrdifferenl kind ol food pellets, BM>& lieimiilh
I unpubl. I 19.9 kJ in ihe pellets they used. Heppleston (1971) 22.56
U g' 1 AFDW in Mussels. Merck (1983)20.7 and 21.9Ug" 1 AFDW
in Cockles and Mussels, icspcelively. laken hv the buds studied by
Ens (unpubl.), Blomert tt /wan- itinpuhl.i 24.5 U in Leathcriackeis
in the same area where Ens collected his data (same month but
later years). Zwarts I unpubl. I 22.2 U for Scnihieularia laken b) the
bird studied by Blomen «•; al. (19831, and /warts & Blomert (1996)
22.9 kJ for Lealherjackels in April, ll is assumed that in the remaining
five Studies the average energy contenl of Mussels was 2' kJ and
kles 22 kJ g ' AFDW; column xkJ' gives ihe average energy
content (kJ g ' AFDW; printed in italics if estimated). Column 'Q'
gives the digestibility and 'kJQ ' the metabolized energy consumption
ikJQl corrected for weight change and thermoregulation costs
(see text). Each mcasurcincni concerns an individual bird, except
Kersten A: Piersma 11987) and Goede (1993) whose measurements
averaged 6 and 12 birds, respectively. Studies 32 lo 34 concern ihe
same siv individuals being weighed each week Alter a selection
was made of weeks with a temperature > 10 C and constant body
weights. Ihe average consumption was calculated separately for
three categories ol body weight
27;
(1996) observed some breeding pairs from sunrise un-
til sunset in April, in the week before egg-laying, and
measured prey fragments in the droppings to recon-
.i.n.i prey weights. Second, ii is possible to tstimate
nocturnal fcxxl consumption in nesting birds from
weight changes recorded on an clccinunc balance
placed under the nest (Kersten & Visser 1996b. Ens.
Dirksen. Nieuwenhuis & Smit unpubl. and Exo &
Scheiffarth unpubl.). The relation between weight
change and consumption was calibrated by comparing
weighl changes to measured food consumption during
ihc day.
Compared to these few field Studies, many studies
have measured daily consumption in captive buds
(Table 1). With daily, consumption expressed as
AFDW. the variation is large. The average gross con­
sumption is 323 g AFDW with a SD of 5.1 g, or
15.8% of the mean. This large variation is not due lo
daily variation in consumption, since the data in Table
1 for captive birds all refer to studies that averaged the
consumption over longer periods, and in some cases
over several individuals. We therefore first investigate
to what extent this variation was due to differences in
i I i energy contenl of ihe prey. (2) digesiihilnv ol ihc
prey, (3) costs of thermoregulation, (4) weight
changes, (5) body weight. (6) activity costs. (7) age
and (8) season. This will then allow us to assess
whether a difference in food consumption occurs be­
tween captive and free-living birds
(1) Energy content of the prey The captive Oyster­
catchers were fed artificial food pellets. Cockles or
Mussels. The four field studies refer to breeding birds
feeding mainly on Cockles (Ens et al. unpubl.) and
Lealherjackels iZwarts & Blomen 1996, Ens et al. un­
publ.) and to a non-breeding bird feeding on Scrobicu­
laria (Blomen et al. 1983). As different prey types
contain different amounts of energy, this diversity of
food types makes it likelv lhat the variation in daily
consumption would be less if it was expressed as gross
energy intake. Hulscher 11974) found thai three Oys-
letcatchers alternately offered Cockles and Mussels,
consumed, on average, per day 37.4 g AFDW if Cock
les were taken, but 33.2 g, or 11*36 less, if their food
was Mussels. The energy content of both prey was not
measured, but other studies have found that the energy
content of Mussels is 5-1095 higher than that of Cock­
les iChambcrs & Milne 1979. Merck 1983. Zwarts &