Endangered Birds Lose Ground to Invaders : Nature: Aggressive avian species are competing for food with least terns in a preserve. A study will help determine if they can coexist.

They were weekly fixtures throughout the summer, during the sea bird mating season at the Bolsa Chica Ecological Preserve.

Wearing rubber boots and sporting binoculars, the team of graduate students tromped through the marshes, looking for bits of half-eaten fish discarded by the birds.

Jacqueline Wilson said she was willing to do it for science and to make a contribution to the local ecology. She and her colleague from Cal State Fullerton are helping to conduct a study funded by the U.S. Fish and Wildlife Service.

Its purpose: to determine whether the Bolsa Chica wetlands’ fragile colony of endangered California least terns is further threatened by the recent arrival of some rather aggressive feathered cousins.

“It’s a natural invasion,” said Michael Horn, the Cal State Fullerton professor and biologist who is conducting the study. “Nobody wants to scuttle the new birds. What we want to do is find out if they can peacefully coexist.”

According to Horn, the trouble started about 10 years ago when the area’s long-established least tern colony began being eclipsed by the arrival of more aggressive subspecies: elegant terns, Caspian terns, Forster’s terns and black skimmers.

A small bird with a black cap and yellow bill, the California least tern numbers about 2,000 mating pairs statewide and is on the state and federal endangered species lists. After spending its winters in South America, biologists say, the bird migrates north in mid-April to various California locations where it nests in the sand and lays eggs that hatch a month or two later.

To aid in that process, state biologists in the mid-1970s constructed two nesting islands at Bolsa Chica, complete with open stretches of sand and tile shelters for older chicks, conveniences that eventually began attracting an average of 200 mating pairs--about 10% of the state’s population.

But by 1991, the endangered birds on one of the islands had been completely displaced by the four larger and more aggressive competitors. “There’s only a limited amount of nesting space,” Horn said, “and the least terns just couldn’t hold up under this kind of onslaught. When you have an endangered species, every loss of a nesting site hurts.”

Eventually the competition for nesting space cooled, with the endangered birds remaining on the southernmost island and the new arrivals claiming the island to the north. Still undetermined, however, is the long-range effect of the competition for food--specifically, biologists say, whether their diets are similar enough to allow the larger and more aggressive new arrivals to eventually starve out the least terns.

To find out, Horn began his study in 1992. Each week during the nesting season, generally from early May to late September, the professor and his students don rubber boots, wade or row over to Bolsa Chica’s northernmost nesting island and spend the day collecting fish fragments that were either regurgitated, discarded half-eaten or dropped untouched by the 4,000 to 5,000 birds believed to be nesting there.

The students also spend hours using binoculars to watch the birds’ activities from observation points. And occasionally they board boats to follow the foraging animals as far as six miles out to sea to document where their prey comes from.

Last year, Horn said, the teams collected about 1,000 bags of fish fragments, all of which they are now painstakingly analyzing in a university laboratory.

Among the more than 50 types of fish identified so far, according to Horn, are the northern anchovy, Pacific sardine, California grunion, top smelt, jacksmelt, California killifish, long jaw mudsucker, Mozambique tilapia, largemouth bass and bluegill.

The study is supported by a $9,000 grant from the U.S. Fish and Wildlife Service, and Horn expects to publish the findings in a paper in June. So far, he said, it does not appear that the feathered newcomers of Bolsa Chica are competing with the endangered old-timers for food.

While all the birds share a pronounced taste for anchovies and sardines, the biologist said, the least terns appear to go after fish too small to be bothered by the more aggressive birds, which also have a much broader diet.

“What that probably means is that coexistence is possible,” Horn said.

Competition for nesting space, on the other hand, appears to be fierce, which probably will prompt Horn to recommend that new nesting islands be built.