Mystery Bird: Black Tern, Chlidonias niger

tags: , , , ,

[Mystery bird] Black Tern, Chlidonias niger, photographed at Galveston's East Beach, Texas. [I will identify this bird for you in 48 hours]

Image: Joseph Kennedy, 14 July 2009 [larger view].

Nikon D200 ,Kowa 883 telescope with TSN-PZ camera eyepiece 1/800s f/8.0 at 1000.0mm iso400.

Please name at least one field mark that supports your identification.

Review all mystery birds to date.

[Mystery bird] Black Tern, Chlidonias niger, photographed at Galveston's East Beach, Texas. [I will identify this bird for you in 48 hours]

Image: Joseph Kennedy, 14 July 2009 [larger view].

More like this

Wrong tern?

Actually, I think this is a juvenile Black Tern. The brown secondary coverts, the darker area on the leading edge of the wing, and the way the gray on the wing linings extends out onto the primaries all suggest juvenile. Also, I can't see any evidence of molt in the primaries or secondaries. Finally, the photo was taken in mid-July, and the black terns seen around here last week were still largely in adult plumage.

Sorry, I hit "Post" before I was finished.

It's not a juvenile because the juvenal plumage has all feathers of the same generation (1st full plumage). Here you can see molt limits in both the primaries and the secondaries where older feathers are being replaced by new ones, thus there are two generations of feathers visible.

Primaries are molted seqentially from the inside to the outside. Here you can see the inner primries are fresh and gray while the outer primaries are abruptly darker. That's because they are old and the powder bloom of the fresh feathers has worn off making them look dark.

Secondaries are molted in the ooposite direction, from the outside inwards. That's exactly what we are seeing on this bird with fresh gray outer secondaries contrasting with old worn inner secondaries.

Adult Black Terns molts flight feathers only once a year for their prebasic molt. That molt takes place in the summer/fall and that's what we're seeing. This bird is molting from alternate (breeding) to basic (winter).

Juvenal plumage is acquire by a complete molt from natal down and all feathers are (by definition) of the same generation unlike this bird in which there are two generations of feathers visible. Also the mantle of juv. Black Tern is normally brown, not gray as in adults and as in this individual.

Hope this helps.

Two questions -- if this bird is indeed molting its remiges, why aren't we seeing any gaps in the trailing edge? I know when gulls or hawks are actively molting you expect to see a spot where the shorter feathers that are actively growing meet up with the fully grown ones.

Second, is it possible that the posted date is wrong? I didn't bother to check the range map before, but a Black Tern on the Texas coast is either a migrant or a non-breeding wanderer. July 14 seems an odd date for that, as well as a bird this far advanced in the body molt.

@ Tax Man:

We're both wrong. This bird isn't a juvenile, it's not an adult, and it doesn't appear to be in active molt. This is a first-summer plumage. In Common-type terns, we call this plumage portlandica, and you rarely see it in the states, since most first summer terns stay on the winter grounds. This explains the molt limit you noted, and the juvenile-type aspects to the wing plumage and lack of missing feathers on the wing that I noted. It also makes sense of the date and location.

I've personally seen maybe 5 portlandica Common or Forster's Terns up here near Chicago -- out of thousands of each. Never even thought about it in Black Terns before -- we no longer get enough to bother with this sort of thing.