When I arrived at the Basket Island causeway (at Hills Beach in Biddeford, ME) one afternoon last month, I scanned the rising waters and the shrinking sand bars, noting the usual assortment of late-August birds. In late summer, on the rising tide, the causeway often hosts flocks of gulls, sandpipers, plovers, and terns. And sometimes, sifting through the masses reveals unusual or rare birds.
Tern numbers were low that afternoon, but when this black-capped, red-billed Caspian Tern landed to join the mixed flock of gulls on the causeway, I penciled in one more bird code (CATE) to my observation notebook. Caspian Terns are rare visitors to Maine, typically seen only during spring/fall migration, and this was my first (and only) sighting in 2015. (I photographed the following duo in May of 2014 at Goose Rocks Beach.)
To balance the scales with a single Caspian Tern (1.4 lb) it would take five or six Common Terns (4.2 oz/ea) or almost fifteen Least Terns (1.5 oz/ea)*. As you might guess, they are the world's largest tern. Royal Terns (weighing about 1 lb) are the only other species likely to be mistaken for a Caspian. The two look most similar in spring, but by mid-summer, Royals lose their full black cap (see All About Birds for photos), making them easy to tell apart.
*Weight estimates from David Allen Sibley's The Sibley Guide to Birds (2014).