An influx of Chuck-will's-widows?

Chuck-will's-widow, female in pine-oak, Montecristo, El Salvador

Several recent posts have been about unusually late migration of various bird species. For example, I had a late Sulphur-bellied Flycatcher in November in Tortuguero, Costa Rica. I wrote about an early December wave of Scarlet Tanagers also in Costa Rica, and the previous post was about a late push of juvenile Broad-winged Hawks into El Salvador last weekend.

My friend Oliver Komar pointed out in a comment on that last post another exceptional sighting last weekend, i.e. that of a Chuck-will's-widow, a bird noted in small numbers in fall migration in El Salvador, but rarely encountered there after the second half of November. He speculated that the juvenile broadwings we saw migrating had been knocked down by a couple of severe cold fronts that passed through the southeastern US recently, bringing frosts to areas all along the northern Gulf Coast, and that perhaps the same thing was going on with Chuck-will's-widow.

Chuck-will's-widow, male in tropical dry forest, Deininger, El Salvador

Monday December 13, Roselvy, Carlos and I saw a Chuck-will's-widow on the road to the cloud forest in national park Montecristo, El Salvador. On Saturday December 18, the Partners in Flight - El Salvador field excursion participants found one roosting in national park Walter Deininger, El Salvador. And then yesterday, we stumbled upon another one, again in Montecristo, but at a lower elevation, in the pine-oak forest. According to Oliver Komar, Chuck-will's-widow is rarely recorded in El Salvador in December, and three sightings within a couple of weeks could be indicative of an influx of this species into the area.

Caprimulgids are usually more readily identified by call, and visual identification of the members of this family is not easy. It may be worth briefly considering the field identification of our birds.

The first clue should be its apparent size: Chuck-will's-widow is larger than any other nightjar, resident or winter visitor, in the region. The BNA account states rather confidently that "size, tail pattern, lack of white (or buff) in wings, and overall rich brown coloration serve to eliminate nearly all other Caprimulgidae" (Straight & Cooper, 2000), with the exception of Rufous Nightjar, a species found in Costa Rica and further south.

Given its highly cryptic plumage, its retiring daytime habits, and its similarity to other nightjars, this species can easily go undetected when present. In fact, the extent of its winter range is still rather poorly understood, and largely based on specimens in collections rather than reports of birds in the wild, of what is after all not a rare species. eBird, for example, still has precious few wintering grounds records. According to BNA, chucks winter from east-central Mexico south throughout Central America and into northern South America. It is also found as a winter visitor in the West Indies (Straight & Cooper, 2000).

So... did we accidentally stumble upon three individuals of a species that regularly winters in El Salvador but often goes undetected? Or is this a normally rare winter bird in El Salvador that somehow this early winter is much more common here than usual?

References:
eBird. 2010. eBird: An online database of bird distribution and abundance [web application]. Version 2. eBird, Ithaca, New York. Available: http://www.ebird.org. (Accessed: December 24, 2010).
Straight, Carrie A. and Robert J. Cooper. 2000. Chuck-will's-widow (Caprimulgus carolinensis), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/499