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(Un-) Common Terns at Malibu Lagoon

October 29, 2015
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It's easy to see that the dark "carpal bar" is on the secondary coverts on the upperwing (Joyce Waterman 10/11/15)

It’s easy to see that the dark “carpal bar” is on the upperwing secondary coverts, between the shoulder and “wrist” (Joyce Waterman 10/11/15)

On the West Coast, Common Terns are not at all common, but recently at Malibu Lagoon, up to 14 birds were reported (Irwin Woldman), resting on the sand with the usual mélange of terns and gulls.

Worldwide, Sterna hirundo is one of the most widespread and commonly seen species of tern. In North America they breed in a roughly rectangular range from Great Slave Lake in Northwest Territories, south to west central Montana, across Canada between the Great Lakes and James Bay on the southern Hudson Bay, and eastward to central Labrador and the Maritime Provinces. When breeding season ends they migrate southward in a broad swath across the Great Plains and Mississippi Valley, down the east coast, and winter along both coasts from Texas to Patagonia, excepting most of the coast of Chile.

Carpal bar on the folded wing is horizontal (Joyce Waterman 10/11/15)

Carpal bar on the folded wing is horizontal (Joyce Waterman 10/11/15)

In Eurasia, they breed from the British Isles and Madeira and eastward in a broad swath to eastern Siberia. They winter around the coast of Africa from Senegal to South Africa to Ethiopia, the Indian Ocean coast from Iran to Australia, and throughout Indonesia and the tropical islands of the west Pacific.

A few southbound migrants wander westward from the western Canadian provinces, over the mountains and down to the Pacific coast, and continue southward, occasionally appearing at accommodating places like Malibu Lagoon.

At 15", the Willet is only 1/2" longer than the Common Tern (Joyce Waterman 10/11/15)

At 15″, the Willet is only 1/2″ longer than the Common Tern
(Joyce Waterman 10/11/15)

Unlike our Least Terns, they are not threatened. World population is 250,000 – 500,000 pairs. About 35,000 pairs nest in North America, 140,000 pairs in Europe, and the rest in the former USSR and adjoining nations.

Swedish scientist Linnaeus (Carl von Linné) described them in 1758, naming them Sterna hirundo. The genus Sterna derives from the Old English word for tern, stearn or stern. Species hirundo is Latin for the swallow, referring to the Common Tern’s buoyant, swallow-like flight and appearance with pointed wings and elongated outer tail feathers.

Black colors can appear at other parts of the wing (Joyce Waterman 10/11/15)

Black colors can appear on other parts of the wing (Joyce Waterman 10/11/15)

They eat mostly small fish, occasionally taking crustaceans, insects and parts of fish left by others. Opportunistic feeders, they readily switch to other prey when their preferred prey fails, perhaps one reason why they are so successful.

They breed primarily in colonies which can number into the many thousands, and lay eggs from April into June, depending primarily upon latitude. Nests are simple and average only 16″ apart. Normally three eggs are laid, but in poor food years they will lay two or sometimes

The amount of red on the bill is variable (Joyce Waterman 10/11/15)

The amount of red on the bill is variable
(Joyce Waterman 10/11/15)

only one. Incubation takes three to four weeks; chicks are altricial. By day six the chick recognizes the parents’ landing call; they fledge at 24-28 days. Adult breeding plumage appears by their third spring. Mortality of adults is 7-17% per year; they return to their birth area to breed, and can live up to 25 years.

SoCal birders can confuse them with their close – and, locally, far more common – relative, the Forster’s Tern. They both average 14.5″ long; wingspan for both species is 28.3″ – 32.7″. Plumage of both is a mixture of white, gray and black feathers. Eyes are dark, legs range pale pink to orange to red. Juvenile and 1st-year birds have dark bills, which in breeding become red with black tips. Adults of both species in breeding plumage have a complete black cap, from bill to bottom of nape, passing just below the eye. So far, very similar.

Forster's Tern - note pale gray at nape (Rob Hargraves, Bolsa Chica 10/8/11)

Forster’s Tern – note pale gray, not black, at nape
(Rob Hargraves, Bolsa Chica 10/8/11)

In non-breeding plumage, the best points to look at are the black areas on the head, and the presence or absence of a dark horizontal bar (“carpal bar”) on the folded wing. Both species have black around the eye: in the Common the black extents eye-to-eye around the nape; in the Forster’s the nape is a much paler gray. First-year Common Tern has a black carpal bar; first-year Forster’s might show slightly darker in this area, but it will be thinner and not as dark. Fortunately for SoCal birders, most Common Terns appearing on our shores are first-year birds. Unless, of course, we’re just not noticing non-breeding adults secreted among the Forster’s.

Black extents eye-to-eye around the back (nape) of the head (Joyce Waterman 10/11/15)

Black extents eye-to-eye around the back (nape) of the head (J. Waterman 10/11/15)

Many thanks to SMBAS member and frequent photographic contributor, Joyce Waterman, for taking these photos at Malibu Lagoon, and to Rob Hargraves whose photo of a Forster’s Tern, taken on our 2011 trip to Bolsa Chica, provided a nice comparison.   [Chuck Almdale]

Origins of bird names: The Dictionary of American Bird Names; Choate, Ernest A.; 1985; Harvard Common Press, Boston, MA.
Other data:
Handbook of Birds of the World; 1996; Lynx Edicions, Barcalona, Spain
Birds of North America: Dunn, Aldefer, Lehman; National Geographic Society, Washington, DC

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2 Comments
  1. ednalvarez permalink
    October 30, 2015 8:19 am

    Chuck – great piece! Thanks! Much to absorb. Appreciate your taking the time to put all of this data together in such a clear manner. And, of course, thanks to Joyce for her very clear photos.

    Like

  2. October 29, 2015 3:53 pm

    Chuck ~ I always enjoy the opportunity to provide photos, and I always enjoy seeing them on the blog, but MOSTLY I appreciate and look forward to your detailed, informationally rich text. I learn so much! Such a treat. Thank you!
    — Joyce Waterman

    Like

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