When
a weasel, fox, or other predator enters a breeding colony of
gulls, numerous birds gather in the air above the intruder,
making it very conspicuous. Gulls come from a considerable
distance and circle or hover over the predator for quite a
while, sometimes even landing in its vicinity before
returning to their territories. With the exception of those
whose nests are immediately threatened, the gulls show
little inclination to attack. Instead they appear nervous
and ready to flee. Experiments using models of
predators show that breeding Herring and Lesser Black-backed
Gulls are more attracted to models that have a dead gull
placed close to them than they are to the models alone.
Furthermore, once gulls have seen a predator model with a
dead gull, they are more attracted to it if experimenters
place it within the colony again on the same day, even
without the dead gull. Indeed, there is some evidence that
the heightened reaction to the predator lasts at least a day
after it is seen with the dead bird. This heightened
reaction is specific to the predator model seen with the
corpse -- there is no increased reaction to a model of a
different predator subsequently presented in the same place.
After seeing a predator model with a dead gull, the live
gulls alight farther from the model on subsequent
encounters. They remain attracted, but are more
cautious. These results indicate that
the attraction of the gulls to their enemies is a method of
learning about them. Apparently they can generalize -- they
draw conclusions about the predator after another gull has
had a lethal encounter with it. This is a beneficial
reaction, since mammalian predators such as weasels and
foxes may engage in "surplus killing -- dispatching more
victims than they can consume. Also these hunters can
specialize for a period of time on one group of prey. An
animal that has killed one gull may be more likely to kill
others; individual foxes have been observed habitually
killing gulls in breeding colonies. It requires little
imagination, then, to see the potential adaptive advantage
for gulls of investigating predators. SEE: Natural
Selection;
Flock
Defense;
Coloniality;
Distraction
Displays. Copyright
® 1988 by Paul R. Ehrlich, David S. Dobkin, and Darryl
Wheye.