Thursday, November 11, 2010

A very late Sulphur-bellied Flycatcher

Today the banding session here in Tortuguero, Costa Rica began with rain, which was light when I opened the nets but quickly turned to a downpour that lasted for a couple of hours, which had me close them. For a while, it didn't look like I was going to be able to reopen, but around 10 AM, the rain had stopped, so I opened again.

I did not catch very much, but there was a surprise in the form of this very late Sulphur-bellied Flycatcher. Late because this bird, which breeds from southeast Arizona and Mexico down to Costa Rica but winters in South America, is a common passage migrant in Costa Rica from early August to mid-October (Garrigues & Dean 2007, Stiles & Skutch 1989).

Precisely because it is so late, it occurred to me that other Myiodynastes flycatchers, perhaps even South American species, had to be carefully ruled out. Streaked Flycatcher is of course very similar to Sulphur-bellied, and does occur in Costa Rica, even winters here (mostly on the Pacific slope, rare on the Caribbean slope). Obviously we need to start with that bird.

The BNA account for Sulphur-bellied is not very encouraging in this respect, for it claims that Streaked Flycatcher is "often not readily distinguishable in the field". It goes on, however, to list distinguishing field marks: "Streaked is larger with a more robust bill [...], that has extensive pale area at base of lower mandible (dark only on distal half), a narrower dusky malar streak that rarely meets across the chin, which typically is whitish on Streaked and blackish on Sulphur-bellied. [Streaked also with] yellowish-tinged supercilium and mustachial stripes (unlike Sulphur-bellied). [...] Sulphur-bellied also has coarser streaking on underparts; (often) brighter yellow on belly; grayer, less tawny crown; and more whitish edgings on wing coverts (lacks rufous or buff edgings)" (Lowther & Stotz 1999).

With the very notable exception of the coloration of the lower mandible, everything here fits our bird.  While holding the bird in the hand, I noticed the light but extensive yellow on the belly, the dark malar stripe, and the whitish (not creamy) supercilium. For me, it was a Sulphur-bellied Flycatcher. At this point, I was still unaware of a reputed difference in the lower mandible.

But practically every field guide mentions it!

Howell & Webb (1995): "Sulphur-bellied Flycatcher has smaller bill [than Streaked Flycatcher] with little or no flesh below at base."

Stiles & Skutch (1989): "Sulphur-bellied Flycatcher has darker bill, chin and malar area than Streaked Flycatcher, with belly at least as yellow as breast."

Garrigues & Dean (2007): "Streaked Flycatcher very similar in appearance to Sulphur-bellied Flycatcher, but differs in having pink basal half of lower mandible, cream-colored superciliary, thin malar stripe, and whitish belly." (their emphasis)

Pyle (1997) mentions a measurable difference in bill size, which, had I known about it, I would have checked for on this bird. But I only found out after I let it go. He also mentions differences in the width of the streaks in the outer tail feathers, which are <3 mm wide in Sulphur-bellied and >3 mm wide in Streaked Flycatcher. He concludes with differences in malar stripe and chin color, and again with a color difference on the lower mandible: "lower mandible [of Sulphur-bellied Flycatcher] with an indistinct or no pale base (vs base of lower mandible extensively pale pinkish in Streaked Flycatcher)."

There are three more Myiodynastes flycatchers found in South America, but none of them look like this bird. Only Variegated Flycatcher shares some plumage characters, but that bird has a smaller head and a much smaller bill.

Sticking with Sulphur-bellied Flycatcher vs. Streaked Flycatcher, I believe this to be a Sulphur-bellied Flycatcher. All field marks are good for that species, except the coloration of the lower mandible.

As I was preparing this post, I consulted via Skype with Oliver Komar and Carlos Funes of SalvaNATURA. They looked at my photos and compared them with eight photos of Sulphur-bellied Flycatcher in the SalvaNATURA visual database. Six birds showed some pink at the base of the lower mandible, and two almost none. Clearly, this is a variable character, with my bird today likely at the other end of the spectrum.

12 November 2010 postscript:
Here are some photos from Arizona of Sulphur-bellied Flycatchers, including one photo of an individual with about as much light color on the lower mandible as my bird. 
Two photos of Streaked Flycatcher from Peru with a darker lower mandible can be seen here.

Clearly, this character is variable and should not be used to separate the two species.

Thanks to Oliver Komar and Carlos Funes for commenting on this bird, and for comparing photos of other Sulphur-bellied Flycatcher individuals.

Cited literature:
Garrigues, R. and R. Dean. 2007. The Birds of Costa Rica: a Field Guide. Cornell Univ. Press, Ithaca, NY.
Howell, Steve N.G. and Sophie Webb. 1995. A Guide to the Birds of Mexico and Northern Central America, Oxford University Press, Oxford, England. 
Lowther, Peter E. and Douglas F. Stotz. 1999. Sulphur-bellied Flycatcher (Myiodynastes luteiventris), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online:
Pyle, Peter. 1997. Identification Guide to North American Birds, Part 1 Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.
Stiles, F. G. and A. F. Skutch. 1989. A guide to the birds of Costa Rica. Cornell Univ. Press, Ithaca, NY.

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