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

general law relating handling time to prey size in birds
of different weight needs to be formulated.
Waders can take up to 40 to 160 small prey per
minute when ; ipidlv at one spot so thai they
hardly spend any time searching for prey. The minimum
time to take then a prey varies between 0.4 and
1.5 s. Film and video analyses showed that the time
taken io transport a small prey up the bill to the gape
amounts to 0.4 s for Knot taking a small piece of llesh
(Gerritsen 1988). 0.40 s for a Common Sandpiper
Actios hypoteucos taking a ehironomid larva (Blomert
& Zwarts unpubl.l. 0.46 s for a Sanderling taking a
small isopod (Myers et al. 1980), 0.70 s for a Blacktailed
Godwit Limosa limosa taking a ehironomid
larva and 0.79 s for a Black-tailed Godwit taking a rice
grain (Blomert & Zwarts unpubl.). In contrast to ducks
(e.g. de Leeuw & van Eerden 1992). waders have to
pick up and swallow each prey individually.
The total handling time must be longer than me
time taken to mandibulate and swallow, because the
prey must be (1) first recognized as edible. (2) grasped
and. if necessary, lifted from the substrate. (3) shaken.
or even washed, to clean it. (4) swallowed. (5) alter
which the bill can be lowered again to recommence
searching for. or handling, the next prey. Myers et al.
(1980) distinguished three components in the handling
tunc of Sanderlings and measured their duration: the
orientation time of 0.12 s preceded the swallowing
lime of 0.46 which was then followed by the time to
return the bill to the surface, the down time of 0.12 s.
The total handling lime was 0.69 s. thus 1.5 times the
swallowing time. Since Sanderlings took 0.86 s to
consume each prey when the prey density was high,
the average search and non-feeding time between
successive prey was only 0.86 minus 0.69. or 0.17 s.
per prey. A similar calculation can be made for Blacktailed
Godwits feeding on chironomids or rice grains.
On average, ehironomid larvae were taken every 1.5 s
(Blomert & Zwarts unpubl.>. exactly as found by
Dirksen et al. (1992) and Szekely & Bamberger
(1992). The handling time amounted to 1.0 s and was
thus 0.5 s shorter than the time required to consume
each prey. A quarter of this difference could be
attributed to time lost in handling prey mat were
subsequently rejected. Thus, less than 0.4 s per prey
was spent in non-handling lime. The swallowing lime
of a rice grain was 0.79 s and the total handling time
FOOD SUPPLY HARVESTABLE BY WADERS
0.97 s, or 1.13 s when the lime wasted in rejected rice
grains was included (Blomert & Zwarts unpubl.). A
Black-tailed Godwit needed at least 1.71 s lo find and
eai a prey, so the minimum search time was 0.6 s. The
three analyses show that the feeding rate actually
achieved was 20 to 35*31 below ihc maximum feeding
rate the birds would have attained if they only spent
lime in handling prey. Handling limes of small prey
were rarely measured, but feeding rates of waders
taking small prey have been determined in several
studies. Therefore we will compare handling times
estimated as ihe inverse ol ihe feeding rate and use the
term 'composite handling time' to distinguish it from
the 'true handling time'. 'Composite handling time"
overestimates handling time', but il probably gives a
more realistic description of the average minimum
lime needed to consume a prey since it includes the
wasted handling times and the time needed to move
the bill from one prey lo the next.
At first sight, it would seem obvious thai large
waders handle prey of all size classes faster than small
birds: for instance, their larger gape width would be
expected to enable them to swallow prey much more
easily. Surprisingly, the reverse trend has been found
for small prey: it takes a large bird more time than a
small one to eat tiny prey (Fig. 12A). The explanation
may be found in Fig. I2B where long-billed birds are
shown to take more time to handle a small prey than
short-billed birds. Body weight and bill length are
highly correlated in the sample of bird species used in
Fig. 12 (r = 0.88). A mulliple regression analysis i sec-
Fig. I2B) revealed that the effect of body weight on
composite handling time disappeared completely
when bill length was taken into account while the
influence of bill length on composite handling time
became even more pronounced than in the simple
regression.
As Fig. 12 includes the Black-headed Gull Lams
ridibitndus and Glossy Ibis Plegadis falcinellus and
their composite handling times do not deviate from the
trends found in the wader species, the relationships in
Fig. 12 may apply to all birds taking one prev at a time.
How to explain that the composite handling lime depends
on bill length ' Waders transport a prey from the
bill lip to the gape by a series of "catch and throw
movements' (Gerritsen 1988). Video analysis showed
that the duration of the swallowing time, the major