Ever-adapting cattle egrets and cowbirds
THE NATURAL VIEWPOINT
A cattle egret is a saucy white bird with a large yellow bill and
feet. Standing upright almost two feet tall, it makes its living
following large grazing animals -- catching insects, earthworms,
frogs and snakes stirred up by its host’s big feet.
You’ve seen them in African documentaries, riding the rumps of
elephants. Finding a similar way of life in cattle ranching, they’ve
migrated around the world, briefly appearing in Laguna Canyon in the
1980s. It was quite a thrill to see them ride the cattle around: the
African plains in our own back yard.
Cattle egrets don’t follow large grazers only; they also follow
farm equipment, like gulls after a fishing boat. Any disturbance that
turns up insects will do; I’ve seen them in Hawaii, dozens of birds
in large white clouds, trailing roadside grass mowers. They’ve simply
transferred their behavior to other hosts.
Another bird here has changed hosts -- the brown-headed cowbird.
Before human intervention, its impact on local song birds like the
California gnatcatcher was devastating.
A cowbird is a small blackbird, and the male has a brown head on
its glossy black body. Once known as a buffalo bird, it made its
living following the ever-moving herds of buffalo on the American
Great Plains, feeding on grasshoppers and other insects stirred up by
the big shaggy beasts.
But the cowbirds had a problem: how could they raise a family when
they never stayed in one place long enough?
Their solution was to find foster parents. Female cowbirds lay
their eggs in nests of other birds, take off, and never return. Often
the cowbird egg is larger than the host’s eggs, which the foster
parents don’t seem to notice.
The final trick is nasty: the cowbird chick hatches first and,
being larger than its nest mates, grabs more of the food brought to
the nest. The foster parents end up rearing cowbirds instead of their
own young.
As cattle replaced buffalo, the cowbirds switched hosts, following
cattle ranching all the way to California. Here they pursue a more
sedentary life, but they haven’t given up their lazy, parasitic ways.
In the Great Plains ecosystem, songbirds have had thousands of
years to adapt to unwelcome “presents” in their nests, altering their
behavior to minimize the damage. However, cattle ranching is only 150
years old in Southern California and our great loss of coastal sage
scrub habitat is very recent.
For already declining songbirds like the Least Bell’s Vireo and
the California Gnatcatcher, cowbirds made a precarious situation
desperate: a few years ago, 90% of the local gnatcatcher nests
contained cowbird eggs.
Our solution is to trap the cowbirds in those large wire-mesh
cages containing seed and water that you see in the park. The traps
are operated only during nesting season. Once a day an attendant
unlocks the door, releases sparrows, towhees and other non-target
birds and takes away the cowbirds.
It’s been a smashing success: nest parasitism is down in the
single digits and the local songbirds again have a future.
* ELISABETH BROWN is a biologist and the president of Laguna
Greenbelt Inc.
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