Saturday, July 10, 2010

Dowitcher Challenge - Verdict! - 11 Jul 2010

I originally posted this blog on 25 June 2010 with the attempt at using published literature references to help me ID the two dowitchers shown below. After analyzing such features as 'color', 'shape', 'barring', 'feathering', and 'bill profiles' I concluded that one of the birds could be a Long-billed Dowitcher. Under the advice of Jean Iron and Ron Pittaway I posted a request to ID-Frontiers for opinions regarding the two birds.

I received some responses supporting my identification, and others calling both birds Short-billed Dowitchers. I then received a very detailed response from Kevin Karlson, co-author of The Shorebird Guide and one of the top shorebird experts in the country. Because of the extremely valuable information contained in his response I wanted to re-post this blog with his responses (and his permission) in their entirety. I'm breaking up the responses and posting them after my observations (which in the end proved to be the classic mistake of trying to see characteristics in an unknown bird that may/may not be there). In short, he believes both birds are Short-billed Dowitchers, and he prefaces his writeup with the following statement:

"Many of the points that you made concerning the two birds were proposed at one time or another as relevant to dowitcher ID, but are actually irrelevant to the ID of either species."

I should've listened to Paul, The Octopus - he thought they were both SBDO's... At the end of this post I have posted some new images of hendersoni Short-billed Dowitcher that I'm used to seeing (taken 10 Jul 2010). My sincerest appreciation to Kevin Karlson and all who have contributed to this post and have helped me learn a bit more about dowitcher identification (and you, Bob!).

This morning I took a quick ride out to Cell 3 of the Banana Unit at Pt. Mouillee SGA in Monroe Co., MI. Strong storms moving across the area that last few days got me wondering if any odd birds might show up, so I made the trip before heading into work. The exposed mudflats held several Dunlin, a half-dozen Semipalmated Sandpipers, and these two Limnodromus dowitchers. At first glance the one on the left appears to be a Long-billed Dowitcher (L. scolopaceus). It showed a brick-red coloration with heavy barring on the flanks,

Kevin: To me, the coloration is deep orange, similar to some hendersonis, especially in mid-summer. At this time of year, there is not much difference between LBDO and hendersoni in underparts color tone, and thus not pertinent to our ID.

Jerry: I mentioned brick-red coloration of the left bird. I don't believe that my photos showed the coloration as well as I'd hope, but understand your statements regarding the similarities in coloration that hendersoni and LBDO's can have this time of year. Is it typical to see both brick-red hendersoni and lighter-orange hendersoni during spring migration and early summer? We see only a few LBDO's in spring at Pt. Mouillee and they seem to stand out from the SBDO's in terms of 'redness' and barring. And we don't usually see LBDO's till August when they've started their molts into basic plumage.

Kevin: I don't see brick-red hendersoni ever in the spring, when Long-bills truly stand out in direct comparison with their deeper red color and much darker overall upperparts patterning. However, I do see a wide variation of pale to deep orange hendersoni in Texas in spring, where griseus basically does not occur. Some of these deep orange hendersoni contrast noticeably with paler birds, and this is probably a result of varying sex hormone levels creating a richer coloration to underparts. Young birds in their first life year also don' typically show the deep orange color, or at least the ones like your bird that is noticeably lacking in breeding feathers and shows retained juvenile tertials, some wing coverts and primaries. Dowitchers, like Black-bellied Plovers, don't follow a strict molt schedule with respect to age, with some birds attaining breeding plumage in their first life year, and others showing full nonbreeding plumage at the same age. Steve Howell published a great paper documenting the disproportionate molt of same aged Black-bellies, with some birds attaining almost complete breeding plumage and travelling to tundra nest sites to breed, while others show a total lack of breeding molt in spring, and replace older juvenile feathers with nonbreeding ones in spring. This must parallel the uneven molt that I see in many dowitchers, inlcluding deeper orange color in some, and paler coloration in others. On the breeding grounds in Churchill, however, only full breeding birds are present, and most are consistently a rich orange color in June, like your left bird. Birds that don't breed for one reason or another, regardless of their plumage, typically head south much earlier than breeding birds, and many birders report a southbound migration of breeders when they are actually nonbreeders who only made a partial journey north, and then turned around to head south at an earlier date. I see this with many SBDOs that are obviously nonbreeders from early to late June, with true arctic breeders heading south around the early part of July.

a slightly 'heavier' appearance,

Kevin: As for the slightly heavier appearance of the left bird, that feature is also not so relevant to the ID issue, since some large female SBDOs can appear heavier in body structure than some small male LBDOs, although the overall body shape and weight proportions are still different. It is hard to evaluate the body proportions on your birds when the bills are fully inserted into the water, with a relaxed posture with bill raised above the water much better to evaluate the weight distribution to the front half of the body and the neck thickness.

Both birds, however, look to have even weight distribution in front of and behind the legs, with LBDO typically showing a front-heavy weight distribution with a thick neck and bulky upper chest. Both birds also show a fairly straight undercarriage line, with LBDO having a more distended, or egg-shaped, undercarriage shape on relaxed, feeding birds. The combination of an egg-shaped undercarriage and rounded upper back creates a noticeable oval-shape to LBDOs overall body shape, while SBDO can show a distinctly rounded back, like your bird, but lacks the complimentary, distended, egg-shaped undercarriage.

This is a feature that I personally use to evaluate the many individuals of both species that I study together each year on the Upper Texas Coast in April. In fact, the first photo suggests to me that the bright left bird is actually more slender than the right bird, which would never be the case when directly comparing a definite SBDO like the right bird with a questionable bird such as the left bird. Any LBDO, even small males, would show a noticeably rounder, more oval-shaped body carriage when directly adjacent to a know SBDO.


and lack of wing projection.

Kevin: One of these statements is irrelevant to the ID of either dowitcher species, and that is the primary projection past the tail. This feature is variable between species, and although more SBDOs seem to show a longer wing projection on average, both species can show overlap in this feature.

The reason for the longer primary projection in these photos is that the right hand bird has retained its nonbreeding primaries from the previous winter, and they are consequently proportionally longer than the fresh primaries on the left bird, which were replaced several months later. The one long nonbreeding feather on the left bird is a tertial feather, and you can see how much longer it is than the surrounding breeding tertials. This is a point that most birders overlook. The fact that older, retained primaries and tertials will be substantially longer than newer ones, and will put an entirely different slant on the wing point formulas used to evaluate shorebirds.


The one on the right a Short-billed Dowitcher (L. griseus), possibly the interior race L. g. hendersoni, although I could not rule out a possible L. g. griseus.

Kevin: The right bird is clearly not griseus, which is the most common SBDO that we see in NJ in spring, but one that the rest of the country seems to have little or no knowledge of with respect to its strong plumage variability. The lack of heavy barring to the flanks, even on this transition-plumaged bird, rules out griseus, as does the clean, pale nonbreeding wing coverts that lack a noticeable dark central shaft. These pale, clean nonbreeding coverts are consistent with hendersoni, with griseus showing darker nonbreeding upperpart feathers overall with dark central shafts that approach LBDO in patterning. Hendersoni is easy to separate from LBDO in nonbreeding plumage because its nonbreeeding upperparts are so pale and typically white fringed when fresh, unlike LBDOs overall darker feathers with very dark central shafts, especially the tertials, which can show very heavy dark shading next to the central shaft.

Note the lighter, cinnamon coloration that extends into the belly region, lighter spotting on the flanks, slightly slimmer appearance, and wings that project beyond the tail. Unfortunately the birds made no sound, so identification had to be made entirely by sight.

Kevin:. The lighter coloration is an effect of low sexual hormones, or sustenance-challenged birds in spring who do not molt fully into breeding plumage. This has nothing to do with subspecies differentiation, although hendersoni will typically have a bit richer orange tone to its overall underparts coloration compared to griseus, which has a rich orange color to 2/3 of its underparts, and then shows a whitish to mixed white/orange vent.
There is so much variability in these underparts coloration, especially within the griseus subspecies, that I don't place any credibility on underparts color tone to separate the two subspecies, unless it conforms fully to the extreme ranges of plumage condition.

First life year birds also tend to show a slimmer appearance in spring and summer, possibly due to less of an urgency to fatten up for the long migration to the breeding areas. I have noticed this especially with first spring LBDOs in Texas, where they appear very much slimmer than adult birds. Also, females tend to be heavier, bulkier and larger overall than males of both species.


The issue here is that Long-billed Dowitchers 'shouldn't' be here this time of year. Am I looking at a breeding hendersoni SBDO? The two birds feeding in proximity to each other provided a wonderful opportunity to address LBDO and SBDO field identification challenges, especially here in SE Michigan, where LBDO are seldom seen in the May-July months. So I set out to photodocument both birds with the hopes of generating a detailed view into their differences (and to hopefully ID them both).

I digiscoped these birds between 7 - 8 am on a clear morning from about 60-70' using the Zeiss 85T*Fl Spotting Scope, 20-60X Vario Zoom eyepiece, Nikon Coolpix P6000 and homemade digiscoping adaptor. The eyepiece was set to 20X and the Coolpix was zoomed to ~1.5-2X to remove vignetting. Images were cropped, and only slightly adjusted (noise reduction applied to background with birds untouched). The following images are presented in order based on time taken, and are discussed with respect to published references I found concerning Limnodromus identification.

With their backsides exposed you can see the reddish coloration and barring on the underside of the LBDO (left). The SBDO (right) shows only light spotting on the flanks and white tail undersides.

Note the black covert feathers with white fringes on the back of the SBDO, which appears to be in worn alternate plumage. Lee and Birch (1) describe the V-shaped pattern of the covert feathers to be consistent with Short-billed Dowitchers (see Fig. 7 of their article). Spotting is also visible along the flanks and undertail. Cinnamon coloration is also detectable, which supports L. g. hendersoni and appears to eliminate L. g. griseus.

Spotting is visible on the neck and throat of the Long-billed Dowitcher (left). Jean Iron (2) describes this as a good field mark for separating LBDO from hendersoni, where spotting is weak-to-absent.

Kevin: The barring on the upper and mid-flanks of the bright bird (lacking on the lower flanks, where LBDO would show heavy barring), is moderate and not inconsistent with some hendersoni, though fully consistent with griseus. Intergrades between griseus and hendersoni are more common than most people realize, and many SBDOs that I see in NJ each May show plumage traits consistent with both subspecies. This bird seems to be a clean hendersoni, which can show the moderate barring on the flanks, but lacks the heavy spotting to the upper central breast of LBDO, which your bird clearly does.
In the head-on shot, the colorful bird shows a clean, orange upper breast, which is consistent with hendersoni and not LBDO. Also lacking at this early summer date is the pale white fringes that border the upper flank barring on LBDO in fresh breeding condition, and that are lacking in SBDO. However, these fringes wear off fast, and could be absent at this date, but the fresh plumage condition of the barring suggests that they should still be present to some extent.


Jerry: You also mentioned that you thought the left bird lacked the heavy spotting on the chest and neck. Do hendersoni show any spotting? because my bird does show spots on the neck, but I'm not experienced enough to know what is considered heavy vs. light spotting. One of the difficulties of posting digiscoped images is the amount of chroma noise inherent in these P&S cameras. I had applied Noiseware to the birds to remove chroma noise, and I think, in the process, may have faded the neck spots. My bird did not have the spotting that can be seen in Jean Iron's pics that I referenced, but they are there (but not as heavy), so I'm curious if hendersoni's can show any.

Kevin: As for the spotting on the neck and upper breast, Long-billed typically shows a much greater amount than your bird, with hendersoni showing little to moderate spotting in this area, but it is typically not as concentrated nor as complete as shown in spring and early summer Long-bills. Flank spotting and barring is another feature that hendersoni can show, with some birds having moderate barring and spotting, and others showing mostly spotting on the lower flanks. Virtually all Long-bills show fairly heavy barring on the lower flanks, which your bird lacks. Being a photographer that is well versed in Photoshop and Noise Reduction software, there is no way that any noise reduction would eliminate the spotting on the upper breast to the extent that it would almost disappear. Your bright bird seems totally consistent with the upper breast plumage of hendersoni, and nowhere near what I would expect on LBDO, except from mid-August onwards, when Long-bills may lose all the spotting on the upper breast and even the barring on the flanks. In fact, a great field mark on mid to late August hendersoni verus Long-billed is that if there is fading spotting and barring on both species at this late date, hendersoni typically shows consistent markings from the sides of the upper breast to the lower flanks, while Long-billed may shows a few bars and spots on the upper and lower flanks, but typically loses the markings on the mid-flanks below the wing. Michael O'Brien pointed this field trait to me several years ago, and I find it consistent on birds that fit this transition molt condition.

With the SBDO directly in front of the LBDO we see evidence of the 'longer' legs of the LBDO. The SBDO has its leg joint below the surface, while the leg joint on the LBDO is visible above water. Note, however, that this is an unreliable field mark. Numerous images I took of the two birds together showed the LBDO to be in deeper water despite the two birds next to each other. The next image is an example where using this characteristic can cause confusion.

Kevin: Peter (Adriaen) put forth a few excellent points about dowitcher ID, especially his astute observation that there seemed to be no noticeable difference in body shape and structure between the obvious partial breeding plumaged Short-billed Dowitcher on the right and the bright bird on the left. Even small male Long-billeds show a subtly different body shape, with a more egg-shaped versus straight undercarriage on SBDO, and a noticeable front heavy weight distribution with bulkier chest and neck on Long-billed. Short-billed has a more even weight distribution in front of and behind the legs, with a thinner neck and more evenly proportioned, slender upper breast. While sometimes tough to recognize with just one species present, these body shape and structural differences are more easily recognizable when both species are side by side. Long-billed will show an overall more rounded body shape in a relaxed feeding posture, with the egg-shaped undercarriage combining with a more rounded upper back to create this impression. Short-billed can show a similar rounded upper back when feeding, but the undercarriage is straighter and shorter than Long-billed, which translates into a subtly different and shorter body shape. Both birds seem very similar in size and shape to me, with the left bird even appearing a bit more slender than the right one.

Apparently the two birds are next to each other, but the SBDO (left) shows a longer-legged appearance next to the LBDO (right).

Kevin: Leg length is also very similar on these two birds, with Long-billed typically showing a longer tibia and consequently longer legs in direct comparsion to SBDO. Of course, the size and body proportion differences between large female and small males can affect this leg length difference, but both birds appear to be similar in size, and thus a LBDO would show proportionally longer legs. Differences between larger female SBDO and smaller males can easily support the slight differences in leg length that you mention. The plumage on the bright bird is also consistent with hendersoni SBDO, as is the probable first life year bird in transition plumage.

This image shows the color difference between the two birds. The LBDO (rear) has a richer-red coloration that is lacking in the SBDO (front). Although the brick-red coloration of LBDO's is evident in early spring during migration through this region, there are instances where SBDO's can take on this coloration, as well. Also, juvenile birds can appear 'browner' and darker than worn alternate adults.

Kevin:While the bright bird shows a moderate amount of barring on the upper flanks, the lower flanks are not barred like a LBDO would be, and the extent of barring is not atypical for some hendersonis. Another plumage trait consistent with hendersoni and not LBDO is the lack of substantial spotting on the upper breast and throat. The head on shot shows a mostly clean to lightly spotted upper breast, where a LBDO at this early summer date would still show moderate to heavy dark spotting in this area. Peter also brought up another good observation about the boldness and brightness of the upperparts feather edging, which is typical of hendersoni and not LBDO, especially in late June/early July. LBDO at this early summer date shows very narrow white edges to upperpart feathers in late June, which I saw on numerous birds when I worked on Alaska's North Slope for four summers. The overall look to the back is one of mostly black with very narrow, pale feather edging. Hendersoni, on the other hand, shows bold orange to whitish upperpart feather edging from early summer to mid-late August, and consistently appears brighter and more scalloped overall to the upperparts with moderate black feather centers. This general patterning observation is more helpful than most people give credit to.

Note the slight downturn of the bill in the SBDO in the previous image. References suggest that LBDO can have a straighter looking bill, but the bird at left shows some evidence of a down-turn at the tip. Because their bills are prehensile, they can give the appearance of a downturn, even an upturn in bill. Sarvela (3) found that bill length (A) to head width (B) can give A/B ratios of 1.4 - 1.8 (mean = 1.6) in SBDO while A/B ratios in LBDO ranged from 1.7 - 2.2 (mean = 1.9) in LBDO as long as head angle was less than 15º.

Sarvela also described 'height of bill base' (i.e., the thickness of the bill above and below the nostril). If you're close enough to observe these birds then birds with a high bill base (1.5-1.7) are definitely SBDO. LBDO have small bill base (1.1). I performed these measurements on two of the images that showed the bird's bills, and the SBDO does show a distinct 'high bill base'. The LBDO appears to show a bill base measurement consistent w/ Sarvela's description.

Kevin: The one photo that shows the brighter bird with its bill out of the water was the convincing field mark for me, since the bill is deep based and broad-based, like a SBDO and unlike the evenly tapered, narrower based bill on LBDO. The bill also has the distinctive bulging tip that droops slightly, which is diagnostic for SBDO. Long-billed Dowitcher does not show a bulbous tip to the bill, but has a thin, flattened tip to a bill that is fairly uniform in thickness from base to tip, and does not vary much in thickness for its entire length. LBDOs bill also does not have the thick base that SBDO shows, but shows a more uniform thickness in a side view.

The LBDO preening shows the barring along the sides and flanks, rich-reddish neck w/ spotting. Underwing feathers (not shown) showed heavy barring - I'm uncertain whether this is the same for SBDO.

The upperwing coverts of the LBDO showed 'squared-off' fringes, as opposed to the V-shaped fringes seen in the SBDO. See Figure 7 of Lee and Birch's article (1). Normally obscured, the tail feather of this bird shows black barring that is thicker than white bars, also an indicator for LBDO (see slide 26 of Birch's powerpoint presentation) (4).

Tail feathers are visible in this image. This feature also shows variability, but generally, thick white bands relative to black bands on the tail feathers indicate SBDO, while black bands are thicker than white on LBDO. More detailed discussions can be found here (5) (6).

So, with the images I"ve provided, and the apparent supporting evidence provided by literature, my suspect bird could be a Long-billed Dowitcher. So, why am I not convinced? When I check the other shorebird guides (7)(8)(9) and (10) I see photos that suggest I could be looking at a breeding L. g. hendersoni. Then begs the question, is the second SBDO a hendersoni?

The above discussion is most useful when comparing birds in worn alternate plumage.

Kevin: Another misconception made in your blog is the statement that both of these birds are in alternate plumage. The right hand bird is probably a first summer, or hormonally challenged bird, that did not molt into full breeding plumage, but retained a full complement of nonbreeding wing coverts and tertials. These are not newly replaced feathers, but retained from the mid-winter molt of these feathers that were not replaced in spring due to immature status or low energy levels. Whatever the reason, this is not a full breeding plumaged bird, but one that never achieved the full adult breeding coloration or full complement of feathers.

This young age or low hormone levels is the reason for the lack of consistent coloration between the two birds and the whitish vent on the right bird, not because one is a LBDO and the other a SBDO, or different subspecies of SBDO. Most young birds of both species show similar washed-out plumage appearance, and lack of consistent color throughout the underparts.


There are additional features of pre-alternate and alternate/breeding plumage than can be useful, but are left for later discussion. The references all discuss these indicators, as well as those for juvenal and basic plumages. But hopefully, I haven't butchered the efforts of those who did all the field work. Bottom line - identifying dowitchers can be very frustrating, especially for those of us who see these bird only during a narrow window of migration.

Kevin: I hope this helps and does not confuse the issue, but these two species are a constant issue of study for me, and I feel that I have uncovered a number of unpublished field impressions and details that combine to help identify tough birds like yours. I always consider a number of field impressions and details to reach an ID on a troublesome bird, and sometimes one or two features contradict the ID conclusion. I feel, however, that if five out of seven ID criteria line up, I am confident that the ID conclusion is valid. Dowitchers are so variable, and we are still uncovering loads of pertinent field impressions and details that help us to make sense out of difficult birds.

Feel free to post my thoughts wherever you wish.


Keep those optics clean!

References:

1. Lee, C-T., and Birch, A., Advances in the Field Identification of North American Dowitchers, Birding, Sept/Oct 2006, pp. 34-42.

2. Jean Iron's pics of LB Dowitcher in April and May

3. Pekka Sarvela discusses bill length

4. Cin-Ty Lee and Andrew Birch powepoint

5. Wilds, C. Dowitchers, in Kaufman, K., Peterson Field Guide to Advanced Birding, 1990, pp. 68-75.

6. Kim R. Eckert's MBWbirds site

7. O'Brien, M., Crossley, R., Karlson, K., The Shorebird Guide, 2006., Houghton Mifflin Co., pp. 194-206.

8. Paulson, D., Shorebirds of North America, The Photographic Guide, 2005, Princeton University Press, pp. 308-320.

9. Hayman, P., Marchant, J., Prater, T., Shorebirds, An Identification Guide, 1986, Houghton Mifflin Co., pp. 183-184.

10. Sibley, D.A., The Sibley Guide to Birds, 2000, National Audubon Society, pp. 190-191.

These hendersoni Short-billed Dowitchers were digiscoped just yesterday (10 Jul 2010) at Pt. Mouillee.



3 comments:

Cathy Carroll said...

Jerry, this is a completely amazing review of the challenges of dowitcher ID. Thanks for taking the time to put this together for us. For me, I don't get to see dowitchers all that often, but for upcoming trips to Mouillee, this is going to be helpful.

forestal said...

wonderful and informative post on a tough subject.

dan

"Dr. Bob" said...

Wow! This has been a fascinating exchange of information among distinguished birders! Jerry, it was wonderful how you documented your original blog with your always great photos and your research on characteristics and offered it up for comments. You presented a great analysis!
Then, it became even better because you shared it among the birding community and Kevin Karlson and others added personal observations with basically unpublished characteristics. Wow!
I can only aspire that someday I can approach your level of photographic ability and observational powers to inspire such an informative exchange!
I will always recall this exchange as using the power of the internet to immediately update our birding compedium of knowledge!
Besides giving me something great on Dowitchers to print out, it further inspires me to reach your level of observational ability, and also to remember that "the books" only tend to summarize, not definitize.
Did I say "Wow"!

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