On Sunday 31 July 2016 I was checking a group of 8 terns on the rocks on the lower coast, Trepassey when I came across a tern that stopped me dead. On first glance it appeared to be a Common/Arctic Tern with a jet black bill, a full black cap and dark legs. In the following seconds I recalled an Alaskan form of the Common Tern with a black bill in adult plumage. Gull-billed Tern was easily ruled out by small bill, short legs etc. Roseate Tern was also swiftly ruled out because of the small bill, smaller than adjacent Common Terns. What was it? Some species of tern from another hemisphere?
After settling down to look more closely at the bird while snapping photos as I began to fixate on the idea that this was a sub adult Arctic Tern based on short legs, small bill and rounded head. There were small flecks of white in the forehead indicating the idea of a subadult. Brownish hints in wing coverts while at rest and more obvious brown markings across wing coverts that were revealed in flight proved its sub-adult age.
The (nearly) complete black cap with a jet black bill was unlike any other Arctic Tern I've seen anywhere. There is not a lot of information out there on subadult Arctic Terns. The 1st summer (12 month old) birds are relatively obvious with their clean white foreheads and black band wrapped around back of head. A few of these show up around Arctic Tern colonies on the Avalon every summer. At this point I am not sure what a typical 2nd summer (24 month old) should look like but I think the Trepassey bird was such.
The black-billed Arctic Tern with a normal adult Arctic Tern and in background an adult Common Tern.
Cropped versions of the above photograph.
Flight shots showing brownish washing on lesser, median and greater wing coverts.
Note the dark legs and the uniform jet black legs.
Below are three photos of a tern believed to be a 2nd summer (24 months) Arctic Tern taken 16 July 2007 at St. Vincents, Avalon Peninsula, Newfoundland. The forehead shows much more white than the Trepassey bird.
Arctic Terns 16 July 2007 at St. Vincents beach. Two 1st summer (12 month) birds on the left and the 2nd summer as shown in two above pictures.