| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
|
|
|
|
|
Habitats Parenting Under Stress
This summer, on Brooks Island Regional Shoreline off Richmond Harbor in San Francisco Bay, I watch a Caspian tern fly in from the east with a 20-centimeter-long rainbow trout in its mouth. It probably picked the fish up in San Pablo Reservoir seven miles away and has carried it back to feed its chicks. If birds can feel discouragement, this one must be crestfallen now as a Western gull snatches the trout away and downs it whole. Life is hard for the Caspian terns (Sterna caspia) on Brooks Island. They face attacks and constant harassment from predators, nest-swamping high tides, and erosion of their beach, which is also vulnerable to invasive plants that render it untenable as nesting habitat. Although only authorized visitors are permitted on the island, the birds also suffer human botheration of several types, including alarming visits from curious bipeds like Oregon State University ornithologist Keith Larson and me. We have come to watch terns eat fish. There are a fair number of Caspian terns: probably about 100,000 nesting pairs worldwide with 15,000 here on the West Coast of North America. Their predicament defies the typical native versus alien cliche. Historically, they were not shore birds at all. Only with the destruction of their inland wetland habitat early last century did they come to the Pacific coast. Here they found islands isolated from predators and abundant fish to feed their chicks. Are they exotics here then? Not exactly. Natives? Not exactly. “Naturalized refugees” describes them pretty well...just like us. Caspian terns not only suffer from the habitats we take, they can also thrive in the ones we make. And that gets them in trouble, too. Until recently, the biggest North American nesting colony was on an artificial island made of dredge spoils in the Columbia River. So Caspian terns were chased from their native habitat and resettled on manmade lands—where they’ve caused enough trouble to be relocated again. Today, 10,000 pairs of Caspian terns nest on Oregon’s East Sand Island, about five miles from the mouth of the Columbia River. That’s about ten percent of the world’s total population. U.S. Fish and Wildlife Service (FWS) moved them there a few years ago from 16 miles upriver, where they had occupied a dredge mound called Rice Island. Unfortunately, all of the salmon stocks in the Columbia River are threatened or endangered, and all of their fry have to swim past Rice Island to reach the ocean. The terns—which took about 12 million young salmon each year—loved the arrangement, but the salmon and their human advocates didn’t like it at all. Caspian terns like to nest on low, open beaches, so fences were set up and wheat was planted on Rice Island to make the habitat less desirable. Meanwhile, sexy decoys were placed on East Sand Island down river and FWS broadcast the tern equivalent of love songs over loudspeakers. In two years, they succeeded in relocating the entire colony. But problems persist. The terns are taking only half as many salmon as they were upriver, but six million is still a lot of endangered fry. There’s another reason to relocate some of the birds nesting in the Columbia. Although Caspian terns are found on every continent but Antarctica, the 10,000 Columbia River nesters represent two-thirds of the Pacific Coast population. That’s a lot of one species’s eggs to have in one island basket. An oil spill or some other accident of man or nature might wipe out the entire colony. Which is one reason why the smaller nesting colonies scattered along the West Coast are so important, including the one on Brooks Island. The population on Brooks has bounced between 500 and 800 pairs over the past four years, according to biologist Steve Bobzien, who has been monitoring the nesting populations on the island for several years. Bobzien is the ecological services coordinator for the East Bay Regional Parks District, which oversees the island and the numerous research projects conducted there. He hopes to enhance the nesting habitat to draw more terns to the island.
The narrow, two-mile-long, 373-acre island is a 15-minute motorboat ride from Richmond, but feels a world away. It hosts one of the most diverse butterfly communities in the Bay Area, according to Bobzien. It’s not hard to count more than 100 different bird species here, including loons, hooded mergansers, black skimmers, peregrine falcons, merlins, and ruby-crowned kinglets. Three other terns also show up: elegant, Forster’s, and common. And at least two native grasses found on the island are not known to occur anywhere else on Earth. The island’s access-to-isolation ratio drew Bing Crosby, who had a private hunting club with his cronies here in the 1960s and 70s. They stocked the island with guinea fowl and pheasants. One lone pheasant still lives here. The island’s proximity to the shore also draws in other, potentially more dangerous, wildlife. A raccoon, for instance, swam the quarter-mile to the island not long ago. A single fox or feral cat could wreak havoc on these vulnerable ground-nesting birds. Caspian terns aren’t protected under the Endangered Species Act. However, because they migrate across international borders (they winter as far south as Guatemala), they are legally protected by the Migratory Bird Treaty Act. The vulnerability of their nesting colony in the Columbia River has caused concern. For safety’s sake, FWS wants to establish more evenly distributed nesting sites throughout the West, including San Francisco Bay. When Bobzien got a call from FWS asking if he knew of good tern nesting sites in the Bay, he told them about Brooks, where he’d been observing a growing colony for years. The current effort to evaluate the habitat, its constraints, and its future potential was born. Larson leads me along the sand spit that hosts both nesting terns and California gulls. He and his colleagues, working on contract for the FWS, are documenting the ingredients of a successful breeding colony, and determining what factors might limit the reproductive success of the Brooks Island birds. We’re walking quietly and quickly to minimize disturbance. Among other things, this study attempts to document its own impact on the birds. As with a sub-atomic particle, the act of looking at a nesting colony of terns alters it. “The effect is minor, I think,” Larson says. But the birds don’t appear to think so. As we cross some invisible trip line, the alarm goes up and the sky comes alive with gulls and terns making an unbelievable racket. The hostile gulls deploy their attack pilots; one swoops down and whacks me on the head. While the terns nest on the north side of the sand spit, their nemeses, the California gulls, nest on the south. The blind we duck into—about the size of a Mini Cooper on stilts but with less comfortable seats and smaller windows—sits between the two colonies. In some places, the species are only a few feet from each other. From the blind, Larson and his assistants spend five days a week watching the nesting terns, documenting every food-related encounter they see. “Fish watch,” they call the assignment. “Kleptoparasitism,” Larson whispers as he notes another theft by a gull of a tern’s hard-won fish. The gulls aren’t pro thieves yet, though. For their day jobs, the gulls scavenge in the nearby dumps of San Quentin and Richmond. One of Larson’s concerns is that over time the gulls may learn that they can make a living doing what they love—feeding off the terns. For the terns, that could be terminal kleptoparasitism. Fortunately, for now most tern meals are reaching their chirping, open-mouthed destinations. Larson also wants to know if the terns on Brooks are eating significant numbers of fish species that have survival problems of their own. So far it looks like 75 percent of the tern’s diet is composed of northern anchovy, Pacific herring, and shiner perch—all abundant in San Francisco Bay. The remaining 25 percent include some surprises. Larson and his crew are the first to document Caspian terns catching baby sharks, both leopard and brown smoothhound. They also eat midshipmen, the strange bottom-dwellers famous for their romantic humming. And I saw them bring in at least one endangered Chinook salmon fry, plucked out of the water on its way out to sea. Salmon makes up less than one percent of their diet, Larson says. More troubling are the human disturbances on Brooks Island. For instance, 150 meters off the beach a huge dredge barge works to clear the channel for the Port of Richmond’s tankers. It roars all day and adds bright lights to the racket at night. It’s hard to quantify the impact, but it must be significant, says Roy Tedder. Tedder’s no biologist, but he knows firsthand how disturbing the dredge can be. He and his wife, Heather, live on the island as caretakers. It arrives each spring just at the most sensitive part of the nesting season. It may harm the birds in another way, too. The channel it clears draws sediment from the edges of the island, eroding tern habitat there. According to Larson, the primary reason there aren’t more Caspian terns here is the paucity of quality nest sites. Then there are the uninvited human guests. They come by sail, motorboat, kayak, jet ski, and, more recently, by parachute. Bobzien says he’s seen paragliders fly in low over the island, sending the adult terns into a nest-abandoning panic. The chicks jump into the water and gulls move right in, he says. The guy sailing overhead has no idea that he’s just killed a bunch of chicks, destroyed a bunch of eggs, and stressed to the limit a bunch of grown terns. Better “Keep Out” signs advertising fines would help, but they can only do so much. “All it would take would be a disturbance at the wrong time and the terns would take off and not come back,” says Larsen. “People don’t realize that there are no places left for these birds.” Brooks Island and other near-urban sites may not be perfect refugia for Caspian terns, but they appear to be the best of a limited bunch. Gordy Slack is a freelance writer and contributing editor for California Wild |
