[Illustration]
The sportsman out for quail or woodcock uses dogs to drive out the
birds for him. Starlings and cowbirds about Chicago use the same
principle in hunting grasshoppers. Instead of dogs they use cows,
though of course the cows are intent on something else and presumably
unconscious of the fact that they're helping the birds.
As the cow grazes slowly across a meadow, it scares up grasshoppers
close in front of it. The cowbirds and starlings take advantage of
this. Instead of covering the meadow on foot, constantly alert for a
sitting grasshopper, or to chase one they flush, the birds keep with a
grazing cow. They take up a position by the head, or a foot, and catch
the insects the cow disturbs. The cow is so much larger than the bird
that it is likely to flush more insects. The grasshoppers on the wing
are much easier to see than when at rest in the concealing grass, and
some fly directly toward the bird. Too, the grasshoppers fleeing a cow
are less likely to be alert to other dangers.
CONFIRMED BY OBSERVATION The advantages of this to the bird are
obvious. But we've just assumed they were, and we had no data on the
relative efficiency of the two methods of hunting. A few years ago,
however, while in El Salvador, I was able to get quantitative data
proving that using a cow as a beater was advantageous, as we suspected,
and showing how much more effective it was, something we did not know.
The bird concerned was not the starling, which does not occur there,
or a cowbird, which occurs but consorts little with cows, but was
the grove-billed ani, a black cuckoo about twelve inches long of
the tropics of Central and South America. Like our starling and our
cowbird, it kept with cows, catching the grasshoppers and other insects
that flew up. Both anis and cows were common in the grassy fields about
our headquarters in San Salvador. We decided, my son Stanley and I,
to watch anis with cows for a few hours, and then without cows for a
few hours; thus getting the average rate for each type of feeding. We
quickly found it wasn't as easy as that. Something always happened;
even on the levelest and most open fields the birds were constantly
disappearing behind a tuft of grass, or in a hollow, or, if nothing
else, behind the cow's head or feet. Then, too, the ani we elected
to watch wouldn't pay attention to the job in hand. It would wander
off, or go to sleep. And sometimes, when we were about to discontinue
watching a somnolent bird, it would snap up an insect. Perhaps it
had been watching all the time. Finally we found we had to record
observations of many short periods, of from three to fourteen minutes
each, and add them together.
By dint of much patient watching we got our data. In the dry season
when insects were scarce and the grass short, it took an ani, hunting
alone, two minutes on the average to find an insect. In the same length
of time hunting with a cow the catch averaged three insects. Thus
hunting with a cow as a beater was three times as effective as hunting
alone.
The effect of the change of the season in abundance of food for the ani
was very striking. In the wet season the grass began to grow fast, and
insects became common. Then the anis had an easy time. Without a cow
an ani averaged between three and four insects a minute, more than six
times as much as in the dry times. There was less incentive to use a
cow as a beater, with food so abundant, but when the ani did so, its
rate of finding insects was still higher: between four and five insects
per minute. In a table it looks like this:
_Average Number of Insects Per Minute Found by Ani Feeding_