February 2016

Vol. 44, No. 1

At a Glance: December 2015 Revealed

Wayne R. Petersen


Photograph by David Clapp

We end the 43rd volume of Bird Observer with a long-legged wading bird as the mystery species. It is easy to establish that the featured species represents a long- legged wader—heron, bittern, egret, ibis, and so forth—and not a shorebird because shorebirds, which also have long legs, have pointed, falcon-shaped wings rather than the wide, rounded wings exhibited by the mystery bird. Sandhill Crane, an even longer- legged species, would also appear larger, lankier, and have a much longer neck, a thinner and more pointed bill, and would hold its neck straight in flight, not in a crook like that of the mystery bird.

Having established that the photo represents a heron-like bird, we can at once eliminate Glossy Ibis as a candidate by the bird’s straight, not curved, bill. Similarly, none of the white egret species—Great, Snowy, and Cattle, as well as juvenile Little Blue Heron—are possibilities. Great Blue Heron could be a candidate, except that a Great Blue Heron’s legs would be noticeably longer, particularly the tarsi, that portion of the legs visible below the knee joints. A Tricolored Heron would appear notably slimmer and would show striking white wing linings and more extensive, neatly defined white underparts, and an adult Little Blue Heron would be entirely dark underneath without any streaking on the neck or upper breast. The crow-sized Green Heron’s legs would not be nearly as visible beyond the tail tip as they are in the mystery heron, nor would the bill be as stout or the belly as white.

With many of the long-legged wading birds now eliminated, readers are left only with American and Least bittern and Black-crowned and Yellow-crowned night-heron as viable identification candidates. We can dismiss Least Bittern by its tiny size, more delicate proportions, and much shorter legs. On the other hand, the American Bittern is a serious contender, as are the juveniles of the two night-heron species. Adult night-herons of both species have unstreaked necks and underparts, so clearly the mystery wader is not an adult night-heron. An American Bittern would show longer and more pointed wings than either night-heron species. Its neck, head, and bill would be significantly longer and thinner and its legs would be unlikely to appear as long in flight than those of a night-heron. Although not visible in the photo, an American Bittern’s wings display prominent black primaries contrasting with paler, buff-colored wing coverts when seen from above, providing another useful means of differentiation from the night-herons.

Having narrowed the possibilities to one of two night-heron species, identification is now straightforward. Because the legs of the Yellow-crowned Night-Heron are so much longer than those of the Black-crowned Night-Heron, their feet and some of their tarsi are readily seen extending beyond their tail in flight, not just a portion of the feet and none of the tarsi as would be the case with a Black-crowned Night-Heron. Even in the soft light of dusk as night-herons leave their daytime roosts and are visible only as silhouettes, these differences in leg length may serve to distinguish the two species. The mystery heron is a juvenile Yellow-crowned Night-Heron (Nyctanassa violacea).

Yellow-crowned Night-Herons are rare spring and fairly uncommon fall migrants in Massachusetts, seen most often along the coast. As breeders they are rare and local, nesting only irregularly in the state. David Clapp photographed this juvenile Yellow- crowned Night-Heron as it left its Eastham roost in early fall 2015.

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