Tuesday, November 22, 2016

ID of Dowitchers in Panama

Until some time ago, identifying wintering dowitchers in Panama was pretty straight forward... only Short-billed Dowitchers were expected in this country at any habitat since Long-billed Dowitcher is very rare and expected to winter south only to northern Central America; however, some records exist from Panama.  The problem is precisely our assumption that any dowitcher seen in Panama is a Short-billed Dowitcher, and so far the most important publications on birds of Panama have a cautionary note that both status and distribution of Long-billed Dowitcher in Panama are uncertain due to difficulty of identification.  Nonetheless, more and more Long-billed Dowitchers have been recorded in Panama, specially this fall with great numbers recorded already.  How good are you identifying dowitchers?  Try with this bird:
Dowitcher sp. Panama Viejo, Panama. November 13th, 2016
Dowitcher sp. Panama Viejo, Panama. November 13th, 2016
I took the above pictures in the coastal mudflats of Panama Viejo, in Panama City.  As usual, I assumed it was a silent Short-billed Dowitcher in almost complete basic plumage (some retained alternate feathers in the back and scapulars).  The best way to separate both species is by voice... but if your bird is silent (as it usually happens in Panama), what field marks are you going to look for? The first thing to do when you try to ID these birds is aging them, because it is easier to separate juveniles of both species than adults in basic (non-breeding) plumage.  With our country well into the usual winter range of Short-billed Dowitchers (and way to the south of that of Long-billed Dowitchers), we will certainly find juvenile birds molting into basic plumage.  It doesn't matter how advanced in molt they could be, since most of them retain at least some juvenile feathers until January (including the characteristics tertials feathers).  In general, juvenile Short-billed Dowitchers have dark-brown upperparts feathers with broad golden-buff fringes and variable internal markings, particularly in the tertials (given them the typical "tiger-striped" look), and dark brown crowns that contrast with the white superciliums.   Underparts whitish with buff-brown wash, slightly brown-streaked upper breast, sparse spotting and barring on flanks and undertail coverts.
Juvenile Short-billed Dowitcher. Juan Diaz, Panama. November 4th, 2010.  Dark crown, retained juvenile greater coverts, diagnostic retained "tiger-striped" tertial feather.
Juvenile Short-billed Dowitcher. Panama Viejo, Panama. November 12th, 2016.  Dark crown, retained juvenile greater coverts and scapulars, diagnostic retained "tiger-striped" tertial feather.  Also notice shape and structural differences (see text).
Juvenile Long-billed Dowitchers are darker above, with only thinner and duller buff fringes to scapular and tertial feathers, the latter almost lacking any internal mark.  Juvenal greater coverts are uniform gray.  Compared to Short-billed Dowitchers, Long-billeds' crown is grayer with slightly less contrast with the rest of the plumage.  The underparts are very similar to those of juvenile Short-billed Dowitcher.
Juvenile Long-billed Dowitchers. Finca Bayano, Panama. November 10th, 2016.  Gray crown, retained scapular feathers with just buff fringes (almost no internal markings) and retained tertial feathers with just thin buffy fringe.
Our mystery bird lack any retained juvenile-patterned feather, making it an adult bird.  Separating basic plumaged birds of these two species used to be considered an impossible task... a task that we have to deal with in Panama on a daily basis, specially now that we know that Long-billed Dowitcher can occur in great numbers as well.  Several papers on identification can be found on-line.  Most of them describe plumage differences, but differences in structure, shape and habitat are useful too, although structure and shape differences varies among populations, age and sex, even within a same species.   I'll illustrate some of these differences with the next two photos of an adult Long-billed Dowitcher and a juvenile Short-billed Dowitcher (same photo posted above):
Adult Long-billed Dowitchers. Finca Bayano, Panama. November 10th, 2016.  Straight, thin-based bill, slight indentation at the back, big-chested look with attenuated rear, wingtips doesn't reach the tail tip
Juvenile Short-billed Dowitcher. Panama Viejo, Panama. November 12th, 2016.  Notice bill shape, more oval-shaped bodywith no indentation at the back, rotund rear and wingtips reaching tip of the tail.
The length of the bill (as their name suggests) is useful to differentiate females Long-billeds and males Short-billeds.  But more important, notice the difference in shape, with Long-billed Dowitchers having very straight bills while Short-billed Dowitcher shows a slightly downcurved tip, that looks kinked, as if the bird had left the tip of the beak trapped when a door was closed.  Also, bill of Short-billed Dowitcher tend to be more wide-based and blunt-tipped than the thin-based and pointed bill of Long-billed Dowitcher.  Body shape also differ, with Long-billed Dowitcher having a deep-chested look and shorter wings, thus attenuating the rear of the bird that often look with cocked tail; different to the more oval-shaped and horizontally oriented body shape of Short-billed Dowitcher that have longer wings, with wingtips usually reaching the tail tip or beyond (thus looking with more rotund rear).  Long-billed Dowitchers are also longer legged, an useful field mark to detect lonely birds among a flock of Short-billed Dowitchers.  Again, all of these structural differences varies depending on sex, age and population, and not only on species, but can be useful when inspecting a distant flock of dowitchers or under bad light conditions to see plumage field marks.
Adult Long-billed Dowitchers (ID by voice). Las Macanas marsh, Herrera. January 28th, 2011.  Notice bill length and shape
One field mark that is proving to be quite reliable is the loral angle given a direct profile view.  This is the angle between an imaginary extension of the gape of the bill toward the back of the head and the line connecting the gape of the bill with the center of the bird's iris.  A diagram probably explains it better:
Close-ups of previously posted photos.  Long-billed Dowitcher, above, showing a more acute (smaller) loral angle than Short-billed Dowitcher (below)
This angle reflects differences in facial expression, forehead shape and relative position of the eye between these two species.  As you c an see, Long-billed Dowitchers have more acute (smaller) loral angles than Short-billed Dowitcher.  Ok.  I have to admit that all the differences mentioned so far are quite difficult to asses in the field, but take into consideration that you need several field marks, instead of an unique and definitive feature, to ID correctly basic-plumaged dowitchers in Panama.  Once you are used to these differences, then you can compare the plumages of these both species.  Starting with the most common, basic adult Short-billed Dowitcher tend to be paler than Long-billeds, with patched gray breast that doesn't contrast strongly with the white belly and setting a more conspicuous white throat, and spotted and chevroned (instead of barred) flanks that are paler than in Long-billed Dowitcher.
Adult Short-billed Dowitchers. Panama Viejo, Panama. November 4th, 2010.  Pale and patched gray breast making little contrast with belly, pale and plain backs, chevroned (not evidently barred) flanks.  Notice also body and bill shapes.
Adult Long-billed Dowitcher. Finca Bayano, Panama. November 10th, 2016. Dark and solid gray breast contrasting with belly, barred and contrasting dark flanks, inconspicuous white on throat.
Notice that the back feathers in Long-billed Dowitcher have dark centers explaining the uniformly scaled and darker look than in Short-billed Dowitchers (with plainer, paler backs).  Also notice the white-fringed coverts feathers in Short-billed Dowitcher (vs brown-fringed in Long-billed Dowitcher).  At flight, the lesser coverts in the underwing of Long-billed Dowitchers are white with no bars (barred in Short-billeds), a field mark seldom noticed.
Adult Long-billed Dowitcher. Finca Bayano, Panama. November1st, 2016. White lesser underwing coverts diagnostic. 
The tail pattern is partially useful in Panama where the three subspecies of Short-billed Dowitcher occur... both caurinus and hendersoni show variability in tail feather patterns.   Now, back to our mystery bird:
Dowitcher sp. Panama Viejo, Panama. November 13th, 2016
Dowitcher sp. Panama Viejo, Panama. November 13th, 2016
When I reviewed the photos, it was the tail feathers pattern that caught my attention.  It seemed that this bird had wider black bars than white ones, consistent with Long-billed Dowitcher; however, as I mentioned earlier, it doesn't seem to be useful in Panama as a unique field mark to ID this bird.  Other field marks are covert feathers fringed in white, patched gray breast, pale flanks and back (although it looks faintly scaled), white throat and open loral angle... all of them suggestive of Short-billed Dowitcher.  Other structural differences are not evident in these photos (except long wingtips).  Last clue is habitat.  This bird was recorded in coastal mudflats, an habitat where Long-billed Dowitchers have never been recorded in Panama; however, take into consideration that the fresh water/salt water habitat preferences described for Long-billed and Short-billed Dowitcher is vague... Short-billed is fairly common in fresh water habitats inland in Panama, while there is at least one record of Long-billed Dowitchers in coastal saltflats (as in Aguadulce, central Panama).
Adults Short-billed Dowitchers. Juan Diaz, Panama. October 8th, 2010
I'm not discussing differences in alternate plumage because it is seen for a short period of time in Panama, but I encourage you to study about them in order to adequately document the different populations that winters in Panama.  Take these alternate plumaged hendersoni Short-billed Dowitchers for example:
Alternate adults Short-billed Dowitchers (ssp. hendersoni). Aguadulce salinas, Cocle. August 5th, 2016.
Well, there is a lot to study and to reveal about these two species in Panama... is time to grab your bins and to get some dowitchers to ID!
Literature consulted:
1. Ridgely R, Gwynne J. A guide to the birds of Panama. Princeton University Press; 1989.
2. Chandler RJ. Dowitcher identification and ageing. A photographic review. Brit Birds 1998; 91: 93-106.
3. Lee C, Birch A. Advances in the field identification of North American dowitchers. Birding 2006; sept/oct: 34-42.
4. Angehr G, Dean R. The birds of Panama. A field guide. Zona Tropical; 2010.
4. Karlson K. kevin's id tips: Long-billed and Short-billed Dowitchers. Available at http://www.kevintkarlson.com/dowitchers 

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