Generalists versus specialists

ELISABETH M. BROWN

Thunk! Crunch! Early-morning hammering on the roof ends any

possibility of sleep.

It’s the neighborhood crows, who pluck acorns from nearby oaks and

crack them open on the roof. Getting the newspaper, we step around

acorns on the half shell on the front porch. Why do some animals

prosper and others fail when people alter wild habitats?

The crows know the answer.

My walk to the office takes me through a green corridor planted

almost exclusively with nonnative trees and shrubs. Recently, I

surprised some crows feeding busily on a bush covered with berries.

They moved up into a tree and found something to eat up there as

well. Neither shrub nor tree was a native; ditto the oaks, but the

crows don’t care.

Now put these observations together with the increasing population

of crows in Laguna. Crows are generalists. Not fussy about their

food, they take what they can get. In our urban neighborhoods, that

includes ornamental berries, garbage, dog food, insects, fruit and

seeds.

There’s another clue in those half-eaten acorns. Compared to

specialists like scrub jays and acorn woodpeckers, crows are not

optimized for eating acorns. Their beaks are too large to remove all

the nutmeats from the acorn shell; much is wasted. When food is

plentiful, however, there’s always another acorn nearby.

When food is short, the specialists have the advantage, because

they can extract more from each acorn. It works for all kinds of

seeds, according to 25 years of research on the famous finches of the

Galapagos Islands. During a drought, each type of finch falls back on

the seed it is especially good at opening.

But our farms and neighborhoods never run out of crow food; it’s

always boom time when you feast on human leftovers.

On the UC Irvine campus, huge flocks of crows congregate on lawns

and in trees. At dusk, they head off like bomber squadrons to their

communal roosts. The built environment must be providing whatever

they need.

What happens to the more choosy birds when development encroaches?

In dwindling natural habitat, a predictable sequence of birds begins

to disappear. At the top of the list is the California gnatcatcher, a

tiny songbird that gleans insects from coastal sage scrub. It’s the

ultimate scrub-dependent bird. When threatened, it doesn’t fly away,

it dives into a shrub -- not great when a bulldozer is coming.

Next to vanish is the roadrunner, followed by California quail,

California thrasher, spotted towhee, Bewick’s wren and wrentit. These

birds are not common at backyard bird feeders, even if the feeder is

right on the edge of a large open space park or preserve. They are

mostly specialists, with narrow food preferences, particular

requirements for nesting, or low population numbers.

Highly attuned to the native trees and shrubs where they live and

feed, they disappear when the habitat changes.

The green corridor where I walk is one block away from a coastal

sage scrub reserve. Living successfully over there are gnatcatchers

and other scrub-dependent birds. I see delightful songbirds on my

walks, but no gnatcatchers or other list birds. They don’t leave

their native habitat.

Maybe that’s why people use the expression “as the crow flies,” to

describe a distance, and not “as the gnatcatcher flies.”

* ELISABETH BROWN is a biologist and the president of Laguna

Greenbelt Inc.