The California Least Tern: What's the Story?

description flight food habitat history predators management seeing terns

The Least Tern is the smallest member of the gull family, called Laridae. It is similar in appearance to the Little Tern found in Europe and Asia. Everywhere, worldwide, these tiny terns are endangered due to the encroachment of human development and disturbance, predation, and pollution.

In the United States there are three subspecies of the Least Tern; all are listed as endangered. One nests on the Atlantic Coast, another nests inland along the banks and drainage systems of large rivers such as the Mississippi and the Rio Grande, and yet another, the California least tern, nests along the Pacific Coast from Central California through Baja, Mexico.

Description

Length from tip of beak to end of tail - 9in (22-24cm). Wingspan - 19in (48-55cm). Plumages vary with age and season. In the Bay Area we are most likely to see adults in breeding plumage and later in the season adults with their youngsters. The adults have a black cap with a white triangle on their forehead. The gray backs and tops of wings extend all the way to the tail leaving no white rump. Outer wing edges are black. Bill is yellow to golden with a black tip. Legs are very short and yellowish. Under-parts are white. Young birds or fledglings (young of the year) are similar in size to the adults. The bills are dark, and their backs have brownish/gray, scaly appearance. Black head caps lack defined triangle, but rather blend dark gray into the white frontal area. Except when mating and while observing some behavior patterns, it is next to impossible to distinguish males from females. The Forster's tern is often confused with the least tern, but it is much larger. Check your field guide and get acquainted with the larger tern that is here year around. When the little guy arrives, you will notice the difference.

Chicks are semi-precocial, meaning they are hatched with eyes open, covered with down, and capable of walking soon after hatching. They are the size of a small thumb when hatched. Their color varies widely and is often similar to the substrate where their parents have nested. This evolutionary characteristic is quite interesting. Chicks can be from very pale blond to speckled gray and brown. Some are even reddish. Consider all the sand colors you've seen and how important it would be to have chicks the same color. Some families have chicks of two or three different colors.

Flight

Least terns, being small and light, are rather buoyant in flight, likened to a butterfly or swallow. They have a deeper than usual wing beat for gulls or other terns in light wind. At one time these small terns were called sea swallows.

Food

The least tern eats almost exclusively small fish. They have brought about 40 kinds of fish back to nesting areas. The "dropped fish" have been collected and identified later. Most fish at the nesting site are juvenile and elongate, such as young anchovy or top smelt. On rare occasions they have been seen feeding on larva of aquatic insects and possibly other soft invertebrates found in tide pools. Their prey are caught with the beak by diving into surface waters where the fish can be seen. They have also been seen skimming shallow tide pools (flying low with beak reaching into the water) along the mud flats or over shallow waters along the beach. But this is not common.

Habitat

California least terns nest on coastal beaches and estuaries near shallow waters that host good numbers of young, elongate fish. The terns prefer open areas where they have good visibility for long distances to see the approach of both ground and avian predators. The substrate is usually sand or fine gravel and can be mixed with shell fragments.

History in California

The sub-species was first described on the west coast in a brief note from a specimen collected at a breeding colony along the Tijuana Estuary in San Diego County (Mearns 1916. Proceedings of the Biological Society of Washington 24:71-71) The bird was seen and recognized by Europeans long before this official description. There are records of nesting colonies as early as the mid-1800s.

The historical breeding range of this subspecies was thought to be from Monterey County, CA at the north to San Jose del Cabo, Baja California, Sur. But records of birds seen during the breeding season north to San Francisco Bay date back to the 1920s, occurring with some regularity until breeding was officially confirmed in 1967 in Alameda.

Some biologists feel that pressure on least terns to the south forced their expansion to the San Francisco Bay Area during the middle of the twentieth century. Although not documented, nesting was suspected by others when clusters of terns were seen day after day along the Bay. Elsie Roemer observed least terns at the Oakland Airport in the 1950s. Because access was not possible, nesting couldn't be confirmed. Later they were found to be nesting at the airport.

Population History

Least tern numbers before recent times are not reliable, but partial records from biologists and bird watchers show that the species was abundant and well-distributed along the southern part of California coast beaches. In 1909 one colony was described at having 600 pairs of nesting birds (San Diego County). Other colonies were described as "good-sized". The number of colonies were numerous. It is likely many thousands, maybe hundreds of thousands, of tern pairs nested along California beaches 150 years ago.

The California Least Tern appears to have avoided the onslaught of losses due to the millinery demand in late 1800s that waged tragedy for the Eastern subspecies. Egg collecting, random shooting, and human disturbance were early causes of least tern losses. When the Pacific Coast Highway was built in the early 1900s, secluded beaches where terns nested were opened to recreation. Families built summer homes and came with their pets to play. The absence of regulation allowed human disturbance along once quiet beaches. Nesting attempts failed, year after year. In 1973 the entire California population was estimated at between 300 and 600 pairs, near the size of the single colony documented earlier in the century. In 1970 the Secretary of the Interior drew up its first list of endangered species and the least tern was on the list. In the same year the state of California placed the least tern on its charter list of endangered species. It took several years before enforcement was effective. Little was known about the bird's natural history or how best to protect it, but with knowledge gained through careful study protective efforts have paid off. Based on California Fish and Game records: 1973-1976: about 300-600 pairs. 1977-1983: 1264 pairs with protection and management. 1984-1987: 952 pairs following a major El Nino weather pattern (1982-1983) and warm waters. 1988-1994: 2793 pairs. Over 4500 pairs were recorded in 2001, but that number fell to 3500 in 2002 with the lowest rate of fledgling production since 1979 (less than 500 young). The population is very vulnerable to quick demise with seasonal fish losses created by El Ninos, disturbance, and predation.

Alameda Naval Air Station recorded 74 breeding pairs in 1981 and 108 breeding pairs in 1991. There has been almost steady growth due to intelligent management and good conditions. The Alameda Refuge hosted 287 breeding pairs (CA state total 3511-3626) in 2002 and produced between 104 and 372 (statistical procedures create these wide ranges) fledglings (CA state total 442-675). Only four out of forty-one California colonies hosted as many or more breeding pairs than Alameda. These are draft calculations and may have adjustments for the final report. Alameda hosted about 8% of the breeding pairs in California yet produced between 25% (minimum) to 55% (maximum) of all fledglings. No matter how one looks at it, the Alameda breeding colony is vitally important to the survival of the species.

Life History

The least tern is a migratory bird that spends its winters at southern latitudes. The precise winter locations are for the most part a mystery, but a few birds were seen wintering in Colima, Mexico and a single bird was recovered from Guatemala. Leg bands put on these birds by biologists helped to identify them as coming from California. In middle April we begin to see them along California coastal waters. Courtship is an essential prelude to nesting. First signs include males with a small fish in their beak flying and calling. The fish-bearing males are often chased, presumably by females looking for a good fishing mate. These spirited, vocal chases can be spectacular and include the birds weaving high in the sky followed by swift and thrilling descents in close unison with what biologists call aerial glides. Ground courtship adds to their prenuptial charm. The male approaches the female carrying a fish. He will strut around her in a dance. If she joins the parade, it is a sign that she has accepted his invitation to mate. These rituals are repeated many times to cement pair bonds. In May one to three eggs are laid in shallow nest scrapes made on the ground. Substrate is usually sand or small gravel. Shell or other small objects can "decorate" the nest and help birds locate their nest in a large colony. The incubation period is about 21 days and it takes another 20 to 30 days to teach the young to have the needed flying skills to make short journeys. These are times of enormous risk. The cryptic eggs and chicks escape the notice of people playing on the beach and drivers of off road vehicles, but experienced predators appear to know how to find the camouflaged delicacies for a small meal. Chicks are precocial, which means that they can run around soon after hatching. If predation is a threat, parents will lead flightless youngsters to safer ground, sometimes a mile distance on a natural beach or even at the refuge.

There is a second wave of nesting in June. Color-banding studies indicate that younger adults arrive and nest later than older adults, and birds that experience nesting failure often renest before the season is finished adding to this second wave.

When young birds are ready to leave the nesting colony, they go with their parents to protected areas where waters are calm and fishing is good. At these locations young birds learn fishing skills and put on weight for the long trip south in late summer when they are only two to four months old. These locations are called "post-breeding foraging areas" and are considered by some biologists to be as important to breeding success as the nesting colony. Bay Area post-breeding foraging takes place at several specific salt ponds in the South Bay, some protected within the San Francisco Bay National Wildlife Refuge.

By late August most of the least terns have completed their breeding efforts and have headed south with their young to spend the winter in warmer climates. Young birds are not likely to return to the Bay Area until they are ready to breed, two to three years later.

Least terns have can live long lives. Banding birds has given biologists tools to recognize birds year after year. One least tern was observed at a nesting colony in Southern California for 21 years. Another was seen for 17 years. But records show that most disappear after 6 to 10 years. It may be that older birds just hang out in the south with the sub-adults rather than make the long flight north. No one knows. These long lives offer some protection to the species if there is one or two bad breeding seasons. But they could not survive long periods with low production rates.

Predators

You name it! The least tern seems to be fodder for so many critters. Because the endangered bird has been observed carefully for the last three decades, biologists have identified many animals eating least tern chicks and eggs, even such unlikely suspects as ants and pigeons! Ants have been observed drilling holes into eggs and cleaning them out. They have also been observed nibbling the feet and legs of hatchlings. The musical meadow lark has made the predator list as well. In Alameda we have not experienced rogue ant, pigeon, or meadow lark behavior. Our primary predators are somewhat predictable. Birds of prey, owls, cats, fox, and raccoon are our common problems. Because the Alameda colony is so carefully guarded, it suffers almost zero human disturbance. Quick identification of predator problems, accounts for breeding success. Adult terns in good health are unlikely prey for any predator with exception of our prized, aerial speedster, the peregrine falcon. The peregrine is a local breeder and is known to take a couple of adult birds each season, but it has not posed a significant threat to the colony's success in recent years. Occasionally, a jack rabbit will run through the colony crushing an egg. The predators, together with fish shortages, severe weather conditions, and perhaps some pollutants are the hazards of life for the least tern.

Management

There are a number of management issues. Site preparation for the breeding season is important. The Alameda colony site has vegetation growth which in time would consume the open character of the habitat. Hand weeding by volunteers on a regular schedule through the winter clears vegetation and many seeds so terns come to a cleared breeding site in the spring. A solar powered electric fence provides a discouraging shock to mammals that try to enter the colony. It is effective most of the time, but some animals seem to tolerate the shock to get at an easy meal. Trapping animals (mammals and birds) and removing identified predators from the least tern nesting area is part of the management policy. The beauty of the Alameda colony is that good foraging for predators that surrounds the colony at some distance. These grasslands and wetlands lure potential least tern predators away from the concrete/gravel colony site. The diverse character of the refuge allows for many wild animals uncommon to urban parks to live in concert with the terns most of the time. It is hoped that in the future, habitat enhancement will enrich the refuge to allow for even more diversity. Eggs that never hatch and dead birds are analyzed for contaminants. So far we've not been told of conditions that are so hazardous as to impact least tern production or health. But we don't know a whole lot about what these birds can tolerate either.

Future

The tern's future depends a lot on us. They will require close monitoring and careful management to insure their safety and good productive breeding years. Watching terns and other wild animals gives us opportunities to protect them and observe the health of our living environment too. The refuge serves us in many different ways.

Seeing Terns

Mark your calendar so that you will remember to start looking for least terns in mid April. They grace the Alameda skies, waters, and mudflats most of the spring and summer. Check our volunteer page for opportunities to see how you can be a part of the refuge friends group. We hope soon to have a great opportunity to turn this airfield into a gem of a national wildlife refuge. The fun is on going year round.

-Leora Feeney, FAWR Chair & biologist

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