Terns of Alaska


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Tern Natural History

Terns are small seabirds related to gulls. In contrast to the languid gull, terns contend for the title of most superb aerialists in the avian realm. Terns are capable of hovering in the air like a hummingbird, flipping over backwards, and then slicing down into the water like a pelican. Although they may scavenge occasionally, terns prefer to hunt small fish which they spot from the air and plunge into the water to catch.

While their acrobatics are impressive, it is their endurance that is truly staggering. The Arctic Tern has been documented migrating up to 50,000 miles annually. In fact, the Arctic Tern’s annual migration from boreal and high Arctic breeding grounds to the Southern Ocean may be the longest seasonal movement of animal.  The migration patterns of the Aleutian Tern remain largely unknown, but they are suspected to be long-distance travelers as well. 

Alaska is just one stop on the long journey for terns, but it is an important one.  Terns come to Alaska to lay their eggs and raise their chicks. They nest on the ground on beaches, gravel bars, and lake shores, laying their eggs in shallow depressions called scrapes. The eggs are camouflaged to look like little rocks and are hard to see.

Terns are fun birds to observe, but they are sensitive to human disturbance so it is best to keep your distance. If you find yourself on a beach or a long gravely river bar with terns hovering nearby, you might be near a nest or chick. When they are disturbed, terns may abandon their nest or chicks, or chicks may become lost and separated from their parents and die. Aleutian Terns have been known to abandon their colonies when disturbed by people. To reduce risk to the breeding colony, keep back a reasonable distance to take pictures or observe, and keep your dog(s) leashed.  WATCH YOUR STEP; be careful not to step on a nest full of eggs! ATVs can also disturb individuals and crush nests or chicks, and are not advisable in or near tern breeding areas during the breeding season, approximately mid-May through mid-August.    

Terns breed in several areas around Yakutat, primarily in mixed Arctic and Aleutian Tern colonies, including the largest known colonies on Black Sand Spit and near the Italio river. The Aleutian tern colony on Black Sand Spit on the Yakutat Forelands is one of the largest in the world, historically supporting up to 3,000 Aleutian Terns; current population estimates are less than 1,000, but continue to represent a significant portion of the global population.  Aleutian terns were documented on the Spit as far back as 1923, and the colony appears to be stable despite apparent declining populations elsewhere within Alaska.  In part because of the large population of Aleutian Terns found there, Black Sand Spit was recently named as an Audubon Important Bird Area.

Three types of Terns are known to breed in Alaska, and these three species can intermingle in flocks or around breeding colonies.
 

Terns of Alaska

Latin Name

Frequency

Arctic Tern

Sterna paradesea

Common

Aleutian Tern

Onychoprion aleuticas

Uncommon

Caspian Tern

Sterna caspia

Rare

 


Arctic Tern

Arctic Tern

The Arctic Tern is Alaska's most common tern. Their breeding range is circumpolar, and their worldwide breeding population is thought to number over a million birds. Arctic Terns can be found nesting in single pairs or in large colonies, in both fresh and salt water. They are wide-spread across the landscape and can be found around in both fresh and salt water.

Adult Arctic Terns have a smooth black head without a white patch or crest. Their bill is bright red. Arctic Terns have a distinctive piercing "rhaaa" or "craaaw" call which they make constantly. They are the smallest of the Alaska terns but size is often difficult to judge in flying birds, especially over water.

Arctic Terns are known for their tenacious defense of their nests and chicks. If you find your self on a beach or gravel bar being enthusiastically dive bombed by a bird that matches this description, chances are it's an Arctic Tern. It is best to leave the area while being careful to watch the ground for nests.


Aleutian Tern

Aleutian Tern

Aleutian Terns are of special interest because we know very little about them and their breeding range is limited to Alaska and Eastern Siberia. They nest on the outer coast of Alaska from Glacier Bay to Barrow. They are by all accounts a rare bird, with a global population estimated at less than 20,000 individuals.  The breeding population estimate for Alaska is 9,500 birds. 

Aleutian terns have a distinct white triangular patch on their forehead just above the bill. They also have black feet, legs, and bill. Aleutian Terns are similar in size to Arctic Terns, but stouter with a more deeply forked tail. Perhaps the biggest contrast between Aleutian and other tern species is in their call. Instead of a harsh cry, Aleutian Terns emit a musical whistling sound similar to a shore bird.  Aleutian Terns are also much less aggressive in defending their nesting colonies; instead of dive bombing, they will generally hover high over the colony when disturbed by humans.

Besides limited colony counts, very little is known about the breeding ecology of this species, and essentially nothing is known about their winter distribution.  This species is designated as a species of concern by several agencies and conservation organizations, as well as a Forest Service Sensitive Species, primarily due to suspected population declines throughout their range.


Caspian Tern

Caspian Tern(photo by Nick Hatch/USFWS)

Caspian Terns are the largest terns in the world and the least common tern species found in Alaska.  While rare in Alaska, colonies of Caspian Terns are found throughout the world on every continent except South America and Antarctica, the North American (excluding Alaska) breeding population is the largest of the continental populations, estimated at about 34,000 pairs . There are currently no known breeding areas in the Yakutat area, but there are small colonies to the North in Icy Bay and the Copper River Delta, and they are occasionally seen migrating through the Yakutat area.  Some biologists believe that the Alaska population is a recent development and that these birds may be colonizing new habitat in Alaska.

Caspian terns are noticeably larger then both Arctic and Aleutian terns. More likely to be confused with a gull, they have an all black head and large orange bill. The feathers along the back of the head protrude out and form a bit of a crest at the back of the head. They have a deep call that sounds more like a croak. Like the Arctic Tern, they are aggressive defenders of their nesting colonies.


Current Research

The U.S. Forest Service has collaborated with other partners (University of Alaska Southeast, University of Hawaii, and Alaska Department of Fish and Game) since 2008 to gain a better understanding of Aleutian Terns.  In 2008 and 2009, researchers applied several population estimating methods for Aleutian Tern colonies in the Yakutat area.  Reliable population monitoring has proven to be challenging due to such natural history factors as movements between years and within breeding seasons, breeding area abandonment, failed breeding attempts, and re-nesting efforts.  Consequently, further research is necessary in order to determine a method for conducting censuses throughout the state and allow better population trend monitoring. 

Because the Aleutian tern’s wintering range is unknown, a high priority is to identify migration patterns, which may hold clues to the reasons for the species’ suspected population decline.  In 2010, researchers deployed tracking devices called “geolocators” on 113 Aleutian terns distributed throughout the Black Sand Spit and Italio River colonies.  Weighing as little as 1 gram, geolocators work by archiving data about day length that can be used to interpolate a physical location (latitude and longitude) on the earth’s surface, thus documenting the tagged birds’ migration route.  Although geolocators have been used on other tern species, this was the first deployment of geolocators on Aleutian Terns.  There are many challenges associated with geolocators; primarily that the individual bird has to be re-captured to obtain the data.  To date, 5 individual geolocator tagged birds have been recaptured. These birds showed a general pattern of migration to wintering areas in southeast Asia and Oceania.

In 2013, Forest Service biologists conducted population surveys of Aleutian Terns in the Yakutat area as part of a statewide census effort. Although populations appear relatively stable in the Yakutat area, recent data indicates a dramatic decline in the statewide population since 1960. Population monitoring and other research in Yakutat and other areas around the state is expected to continue as conservation concern for this species increases.

 


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