Biology Reference
In-Depth Information
(a)
(b)
80
65%
60
40
22%
20
no egg shell
nearby
with egg shell
nearby
Fig. 2.15
(a) Black-headed gull removing an eggshell. (b) Results of an experiment in
which single hen's eggs, painted to resemble black-headed gull eggs, were placed in the
dunes, near a nesting colony. Those with an empty eggshell next to them (5 cm away)
were more likely to be taken by predators (n
=
60 in each treatment). From Tinbergen
et al
. (1963).
once, it has to leave the newly hatched chick unattended (the second parent is away at
the feeding grounds fuelling up for its next stint at the nest). Tinbergen observed that
the new chick, with its plumage still wet and matted, is easily swallowed and therefore
makes a tempting meal for a cannibalistic neighbouring adult. However, when the
chick's down has dried out and become fluffy it is much harder to swallow, and is
therefore less vulnerable to attacks from neighbours. The parent's delay before removing
the shell, therefore, probably reflects a balance between the benefits of maintaining the
camouflage of the brood and the costs associated with leaving a newly hatched chick at
its most vulnerable moment.
When the balance between costs and benefits is changed, the length of the parent's
delay might also be expected to change. This is borne out by observations of the
oystercatcher (
Haematopus ostralegus
), another ground nesting bird with camouflaged
eggs and young. The oystercatcher is a solitary nester and cannibalism by neighbours
is therefore not a risk associated with leaving the newly hatched chicks. The parents
benefit by restoring camouflage of the nest as soon as possible after hatching and, as
expected, the parent removes broken eggshells more or less as soon as a chick has
hatched and before its down is dry.
Optimality models
Tinbergen's study of eggshell removal illustrates how experimental studies of costs and
benefits can be used to unravel behavioural adaptations, but it has an important
limitation. The hypothesis about the trade-off between camouflage and chick
Quantitative
models of costs
and benefits
