Late Scarlet Tanager migration?

Scarlet Tanager

It's December, and as expected, bird migration is pretty much over. This is my last week of bird banding in Tortuguero, and I'm catching mostly residents now, plus the occasional winter visitor.

And... one other species that's neither a year-round resident nor a winter visitor here: Scarlet Tanager.

Apparently, this species is still migrating!

In fact, four out of five of my Scarlet Tanager captures here - and I've been banding from October 21 until today - have been in December. All five individuals were hatch year birds. There was a bird (male or female) on 12 November, followed by a male on December 3, then a female on December 6, and both a male and a female on December 7.

The BNA account for this species says that "most birds do not depart breeding grounds until mid- to late Sep; at Cape May, NJ, peak fall migration 10-20 Sep; Georgia late Sep - early Oct. Departure from Gulf coastal states by mid-Oct. Arrival dates at wintering grounds poorly documented, but the few available records suggest birds reach Colombia by mid-Oct, Ecuador by 1 Nov, and Bolivia by mid-Oct to mid-Nov" (Mowbray 1999).

Scarlet Tanager is a trans-Gulf migrant, and part of the route in fall is through Central America (including Costa Rica). See Fig 1 below.



Figure 1: Scarlet Tanager breeding range, migration routes and winter range.  
By Lincoln, Frederick C., Steven R. Peterson, and John L. Zimmerman. [Public domain], via Wikimedia Commons. Image courtesy of the U.S. Geological Survey.

Wondering if this little early December wave, apparently consisting entirely of first-year individuals, is something unusual or in fact annual, I consulted eBird. Here's an animated gif of all Scarlet Tanager 'fall' records in their database, ordered by month, starting with July, and running until December. See Fig 2 below:



Figure 2: Scarlet Tanager records in eBird by month, from July until December.  
Image provided by eBird (www.ebird.org) and created 8 December 2010.

Observer effort of course varies widely along the Scarlet Tanager's range, which explains why the wintering grounds don't all light up green as the breeding grounds are vacated. Still, we can see Costa Rican records light up in October and November, and then none for December!

How curious then that I caught zero Scarlet Tanagers during the second half of October, only one in November, and then four in the first week of December!

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 8, 2010).
Mowbray, Thomas B. 1999. Scarlet Tanager (Piranga olivacea), 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/479