Friday, November 30, 2007

Origin of the annually occurring Cave Swallow (Petrochelidon fulva) in South Jersey

Recently a couple of Cave swallows were found deceased at a popular roost site in Cape May City. Having these specimens now allows us to help to put to rest some of the debate as to whether or not the Cave swallows which show up in Cape May every autumn are from the southwest or the Caribbean.

Michael O'Brien was kind enough to share this short write up in hopes that it will help to clarify why we have always thought that these these birds were from the southwest. Thanks to Michael for providing the text and photos.

Cave Swallow (Petrochelidon fulva) in South Jersey

It has long been debated where the Cave Swallows that appear each November in New Jersey are coming from. A growing body of evidence supports the idea that most of these birds are coming from Southwestern/Mexican populations rather than from Florida/West Indian populations. Plumage characters support this idea: Mexican birds (and birds seen in New Jersey) have tawny-buff rump patches that are just slightly darker than those of Cliff Swallow (Fig. 1), and have primarily grayish-brown breast-sides, flanks, and undertail coverts (Fig. 2); West Indian birds typically show a darker rufous rump and throat and a distinct rufous wash to the breast-sides, flanks, and undertail coverts. However, uncertainty about variation in these plumage characters leaves questions.

Fig. 1 Note the tawny coloration on the throat, rump and forehead as well as the distinctly capped appearance. Cliff Swallow would show a blackish throat, darker chestnut cheeks, and less of a capped appearance. Most Cliff Swallows would also show white foreheads though Southwestern birds (which have occurred in New Jersey in late fall) show chestnut foreheads.

Fig. 2 Note the relatively clean looking belly with grayish-brown breast-sides, flanks, and undertail coverts. A bird of Caribbean origin should exhibit some rufous to the breast-sides, flanks, and undertail coverts and have a darker rufous throat.

A more solid criterion is wing measurement. At least one previous Cave Swallow record involves a specimen that was determined to be of the Mexican subspecies, P. f.pelodoma. On November 27, Richard Crossley retrieved two dead Cave Swallows from the roost site at Congress Hall in Cape May. Both are first year birds (determined by a mix of old and new flight feathers). The wings of these two specimens measured at 107 mm and 111 mm (Fig. 3), right in line with the Mexican subspecies but too long for the West Indian subspecies (P. f.fulva). Another subspecies in Cuba barely overlaps the shorter-winged specimen but Cuban birds show the richest, darkest rufous colors of the species so should appear distinctly different from these specimens.

Fig. 3 Note the "wrist" of the bird is securely held at 0 mm so that an accurate measurement can be taken. This is called a "flattened wing" measurement which is a typical measurement taken on birds. On this Cave Swallow, the wing length is about 111 mm, indicating the Mexican subspecies P. f. pelodoma.

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