Birds
appear to have an aversion to empty eggshells. Upon
discovery, shells are typically picked up with the bill,
flown from the nest, and dropped at some distance. Grebes
thrust their eggshells under the water, releasing them far
from the nest. Adult hawks usually eat the shells. Many
birds with precocial young desert both nest and eggshells,
herding their chicks elsewhere. These devices for distancing
chicks from the remains of the eggs attracted the attention
of the pioneer ethologist Niko Tinbergen, who studied the
shell-disposal behavior of Common Black-headed Gulls -- a
European species that is starting to colonize eastern North
America. Common Black-headed Gulls
normally fly away with the eggshell within a couple of hours
after a chick hatches; sometimes they carry it off within
minutes. Tinbergen hypothesized that the bright white lining
of the shell would make the nest easier to detect by
predators. But predators such as Herring Gulls and Carrion
Crows seemed to have little trouble locating blotched,
khaki-colored eggs that seem well camouflaged to the human
eye. Furthermore, there are risks involved in shell
disposal. When the gull leaves to dispose of the shell, the
chicks and any remaining eggs are exposed for up to ten
seconds, more than enough time for a winged predator to zoom
in, grab one of them, and depart. Tinbergen tested his
hypothesis in several ways. In an area patrolled by
predators, he distributed a mix of gull eggs, some
unmodified and some painted white. The results were
unambiguous: although both kinds of eggs were found and
eaten, the white ones were discovered more frequently. Then
he and his coworkers put out two sets of unmodified gull
eggs, some alone and some accompanied by empty eggshells
placed about four inches away. The eggs were covered with a
few grass straws to help camouflage them, and those with the
shells nearby were covered a little better than the lone
eggs. Again, the results were clear: even though they were
better camouflaged, eggs near shells were three times more
likely than lone eggs to be found and eaten by gulls and
crows. Further experiments showed
that the farther from an intact egg the eggshell was placed,
the safer the intact egg. And the gulls, when presented with
variously colored but otherwise identical eggshell "dummies"
(bent strips of metal), were most likely to remove from
their nests those resembling real eggshells. There was a
lesser tendency to dispose of dummies with very conspicuous
colors like red or blue, and no tendency to remove green
dummies that blended well with the surrounding grass. Color,
not shape, proved to be the crucial clue eliciting shell
disposal. These two experiments reinforced the idea that
eggshell removal improved protection from predators. The
experiment with dummies indicated, in addition, that
evolution had produced in the gulls a response that reduced
the conspicuousness of the nest not only through removal of
shells and other prominent objects, but through maintenance
of vegetation that might help camouflage eggs and
brood. Tinbergen went on to
discover a great deal about how the gulls differentiate
between an egg, a half-hatched chick, and an empty eggshell.
To determine whether the thin edge of a broken shell was the
main characteristic telling the adult that it was not an
egg, he did a series of tests using modified eggs -- blown
eggs with the shell intact but empty; blown eggs with
additional flanges of broken eggshell glued to them;
eggshells open and filled with either plaster or cotton
wool; and eggshells open and filled with lead weighing as
much as a chick. His results showed that it is the weight of
the chick that apparently prevents the gulls from disposing
of a hatching egg with a thin edge before the chick is free.
If a gull started to pick up a shell containing a lead
weight in it, it stopped immediately. Not a single
"chick-weighted" shell was removed from the nest. The gulls' shell-disposal
behavior seems to be grounded in both instinct and learning.
First-time breeders remove shells experimentally placed in
their nests even before they have laid their first egg,
presumably an act programmed into their genes. But birds
that have been given dummy eggs of unnatural colors
(including black) to incubate, preferentially remove dummy
shells of the same color. Such preferential association
seems to be a learned response "fine-tuning" their
instinctive egg-removal reaction. Tinbergen observed that
oystercatchers and Ringed Plovers removed eggshells from
their nests much more rapidly than did the gulls. He
concluded that the slowness of the gulls was related to
their colonial nesting habits. Some Common Black-headed
Gulls will gulp down their neighbors' pipped eggs or freshly
hatched chicks. Apparently it pays parent gulls to stay with
the chicks until they are dry and fluffy, in order to
prevent cannibal gulls from attacking them. Oystercatchers
and plovers, being solitary nesters, do not run the same
risk by leaving the nest early to dispose of
shells. SEE: Gull
Development;
Parental
Care. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.