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.