Caspian Tern Hydroprogne caspia
Discovery Park, (north of) Seattle, Washington. August.

As large and as bulky as many gulls, the Caspian Tern is the largest tern in the world. Its thick coral red bill and very ‘front-heavy’/short-tailed appearance makes an adult Caspian one of the most easily identified terns throughout its worldwide range (despite its extensive range, it is monotypic, with no subspecies accepted - an unusual situation for a species found in North America, Europe, Asia, and Australia). They feed mainly on fish, which they dive for, hovering high over the water (Caspian Terns often patrol over the water at 20m high or more) before plunging down. They also occasionally eat large insects and the young and eggs of other birds. They may fly up to 60 km from the breeding colony to catch fish.

Adult Caspian Tern in typical hunting flight

Worn adult or possibly a 1st summer bird (note worn wings, incomplete dark bill-tip, and a few dark-centred coverts). I’d be interested in comments on this individual.

Caspian Terns have increased in some parts of the range as they increasingly utilise man-made nesting sites (ie regenerated wetlands, dikes etc). As the following photos were taken in Washington the abstract below from The Condor, Volume 106, Issue 4 (November 2004) may be of interest: “Since 1980, population size [in Washington] has more than doubled to about 12900 pairs, yet the proportion of the population nesting at inland (18%) versus coastal sites (82%) has remained constant. Although the breeding range of the Pacific Coast population has expanded northward into Alaska and farther south in Mexico, there was no net latitudinal shift in the distribution of breeding pairs or new colonies. The distribution of breeding birds among areas changed dramatically, however, with 69% of breeding terns now nesting in Oregon (primarily in the Columbia River estuary) versus 4% during the late 1970s. During the past 20 years, there has continued to be a greater proportion of Caspian Terns breeding at anthropogenic sites compared to natural sites.” [Robert M. Suryan, David P. Craig, Daniel D. Roby, Nathan D. Chelgren, Ken Collis, W. David Shuford, and Donald E. Lyons]

Caspian Terns - like all terns - show a marked difference in plumage between juveniles and adults (this stops adult male aggression towards the young birds in the crowded and febrile atmosphere of a breeding colony who they might otherwise see as competitors). Juveniles are heavily scaled, with dark tips to many of the feathers which clearly delineates many of the feather groups (especially on flying birds). The plumage is usually held into September/October then replaced.

Caspian Terns also show differences between the breeding (alternate) and the non-breeding (basic) plumages - but where many tern species almost entirely moult their dark crests, Caspians show a more subtle change with more or less just the forehead and crown becoming “flecked” with pale feathers (something akin to the changes in the hair of a middle-aged man I might ruefully add).

Juvenile (left) with two adults
(note adult in the rear has “flecked” crown as it moves into non-breeding plumage)

Three juveniles with one adult (rear of the group). Note the orangey tone to the bills of the young birds

Juvenile (left) with adult

Juveniles are especially striking in flight when all those dark feather tips stand out strongly, and they can almost look like a different species to the predominantly silvery-grey adults (especially in August it’s highly likely that adult birds will be present with the juveniles should you be in any doubt though). Note in the photos below that even in views of birds flying away from the observer or in high contrast light the dark markings in the tail are enough to age the young birds quite easily (this is analagous to the situation in gulls of course - have a look at 2nd winter vs adult winter Ring-billed Gulls for a similar discussion).

Juvenile Caspian Terns

And just by way of ’something completely different’ here’s a photo looking west from the beach at Discovery Park across Puget Sound where I photographed these terns. if you look very closely between the two headlands you may just be able to make out the snowy slopes of the Olympic Mountains in the far distance.

For more about terns read Mike’s What is a tern? post.

Photographs copyright Charlie Moores