Friday, March 18, 2016

Fri-D: How to Find a March Black-headed Gull

[Black-headed Gull , center, on a sand spit north of Miami Avenue, Villas, NJ, on Delaware Bay, Friday, March 18, 2016. The other two birds are Bonaparte's Gulls. Click to enlarge all photos. Best tide here is falling, two hours after high.]

March is perhaps the best gull month (February is the only competition in the mid-Atlantic.) The locally wintering gulls are still here, and are joined by gulls rare (like Iceland) and common (like Laughing) that wintered farther south.

Yesterday morning I was lucky enough to detect a Black-headed Gull on the bayshore a couple miles south of my home. This is an event I kind of have taken for granted the past few years, but this is a rare bird; I think it is the only Black-headed Gull being reported in NJ right now.

Right, so how do you find a Black-headed Gull in March? The standard i.d. competitor is Bonaparte's, and there are solid field marks differentiating the two. Black-headed's bill is red, Bonaparte's is black. Black-headed's legs are coral red, Bonaparte's pinkish. Easy-peasy, or so you would think.

But what if you are sifting a flock of 450 Bonaparte's floating on the water 300 yards offshore. Can't see the legs on a floating gull. And that red bill of Black-headed can be tough to see, because it can be a dull red.  Now what?

Well, there's size. Size should be the first go-to for any bird i.d. . . . But. Black-headed is in theory clearly bigger that Bonaparte's, with Black-headed averaging 16" total length (measured as birds in specimen trays from tip of bill to tip of tail), while Bonaparte's are 2.5" smaller at 13.5".

But. They look surprisingly similar in size. So again, what about those 300 yard floating birds?

In March, two things. First, I look for white or near-white Bonaparte's, because a near-white bonie is a Black-headed.

Second, Black-headed Gulls molt earlier, and develop a spring hood earlier than Bonaparte's. Thus, if you get a bird in a flock of Bonies in March with a lot of hood, it is probably not a Bonie.

A sleeper field mark is this: a Bonaparte's Gull that is not next to another Bonaparte's Gull is probably not a Bonaparte's Gull. Black-headeds often associate with Ring-billed, Laughing, or even Herring Gull.

Finally, you can wait for your bird to lift a wing:

[This is a shot from Delaware Bay from March 26, 2010. Black-headed Gull left, Bonaparte's right. Note the difference in the outer underwing, and that the Black-headed has nearly a full hood.]

[Typical winter/early spring Delbay scene: Forster's Tern (center, short legs, ear patch and orange bill base); Ring-billed Gull, Dunlin, Black-bellied Plovers, and Bonaparte's Gull (far right).]

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