Saturday, May 20, 2017

Gray-cheeked Thrush or Bicknell's Thrush?

I spent most of the day on May 20, 2017, birding at New York Botanical Garden, in Bronx County, NY.

In addition to a good mix of warblers, I encountered four species of thrush. I was able to identify three of the species, Swainson's Thrush, Wood Thrush, and American Robin. I cannot identify the fourth because the potential species are reportedly not distinguishable by sight alone, per David Allen Sibley and Cornell Lab of Ornithology.

When submitting the bird to eBird, the designation as "Gray-cheeked/Bicknell's Thrush Catharus minimus/bicknelli" was flagged as rare for this date and location.



I assume if I just selected Gray-cheeked Thrush or Catharus sp. (which are both listed but for this I had to use the +Add Species button), it would not be flagged. I feel I am deferring to expert opinion stating that these species are not reliably distinguished by sight alone.

After ruling out other thrush species, consulting with the below resources, I am listing this as Gray-cheeked/Bicknell's Thrush, Catharus minimus/bicknelli. I am not selecting Catharus sp. as I think the other Catharus species can be ruled out. This bird was clearly not a Veery. It did not have the buffy spectacles of Swainson’s Thrush. I ruled out Hermit Thrush based on the lack of a complete eye-ring and not enough rufous in either wings/tail. The two remaining possible species in range are Bicknell’s and Gray-cheeked Thrush. As this location is in the migration route of Bicknell’s (according to maps in Sibley, All About Birds, and Audubon Field Guides), I believe I need to rule out Bicknell’s to in order to select Gray-cheeked (and vice versa).

I did not observe vocalizations of either Gray-cheeked or Bicknell’s (I was in this forest most of the day. I heard Swainson’s Thrush, Wood Thrush, and American Robins, but was not lucky with any other thrush song). I certainly could have missed it earlier, but I did not hear it vocalize directly before or after last seeing it.



Without vocalizations, I have my field observations (sight only) and photographs to rely on. I observed this bird for about 10 minutes total as it moved around a section of the Bridge Trail. The bird had a spotted breast. It had some- but very slight- rufous in the primaries and tail, but was mostly dull brown on the back, wings and tail. The bird had a grayish face, which was also something I could see in the field and in the picture. It appears in the pictures that there is a partial eye-ring (on the back of the eye).

  • From Cornell Lab of Ornithology’s “All About Birds,” “Gray-cheeked Thrush is very similar and probably not safely distinguished in the field.” and “Bicknell's Thrush very similar and probably not separable in the field.” See https://www.allaboutbirds.org/guide/Gray-cheeked_Thrush/id and https://www.allaboutbirds.org/guide/Bicknells_Thrush/id
  • Sibley Guides website (http://www.sibleyguides.com/bird-info/bicknells-thrush/) states, “The real problem is that the two species overlap in all visible features, including size and color. The study that led to the split (Ouellet, 1993) documented this overlap, and I noticed the same issue in my own studies. Song is the best way to distinguish the species.” In The Sibley Field Guide to Birds of Eastern North America, he writes that both species are nearly identical and only reliably distinguished by voice.
  • I found the article “Bicknell’s & Gray-cheeked Thushes – How to submit to eBird” from eBird Vermont (http://ebird.org/content/vt/news/bicknells-gray-cheeked-thushes-how-to-submit-to-ebird), which although being from a nearby state, has information from an expert on these two species, indicating that they receive inquiries from bird banders and struggle to secure ID unless they have 1) photographs of the head, back, and tail and 2) measurements,.



From the population estimates on Birdlife, it seems for every one Bicknell’s Thrush, there are at least 130 Gray-cheeked Thrushes. Gray-cheeked Thrush has a much larger migration range than Bicknell’s Thrush, so the ratio is probably not as drastic in Bicknell's pathway, but it is still a huge disparity. Based on that alone, it is very understandable that most people select Gray-cheeked Thrush.

The split is fairly recent (about 23 years), so there is probably some slow adoption of the change, especially since the species are difficult to tell apart.

Nonetheless, the experts seem to say that both Gray-cheeked Thrush and Bicknell’s are inseparable from each other by sight (while there are some supporting characteristics, from what I have read there is enough overlap in physical characteristics, that it rules out using individual field marks like extent of yellow on the lower mandible. Lighting plays a huge role in perceiving the extent of rufous on the wings and tail; lighting can mislead people). To me, that means if you are in the migration pathway, any given individual of this type needs to be identified by voice (elevation is apparently a clear divider on the breeding grounds). This seems similar to Traill’s type flycatchers, in that Willow and Alder need to be identified by voice (again, once on breeding grounds, outside of migration, habitat is a useful clue).

The prevailing approach seems to be that any given Catharus species (that is not a Veery, Hermit Thrush, or Swainson's) is a Gray-cheeked Thrush, unless it proves itself to be a Bicknell’s. The population data (separate from eBird) generally supports this. However, it would also probably distort the eBird data if every eBirder just said Gray-cheeked. It seems that Bicknell’s would be hidden by the assumption. The question is, in the absence of vocalizations, is submitting at the genus level (Catharus sp.) or as (Gray-cheeked/Bicknell's Thrush Catharus minimus/bicknelli) a more appropriate eBird entry?

eBird lists:
http://ebird.org/ebird/view/checklist/S37023169 (afternoon, with the thrush discussed above)
http://ebird.org/ebird/view/checklist/S37020373 (morning)

After reviewing the information and photographs I have, I am able to rule out other Catharus species, but not able to select which of two species this bird is should be called. If these pictures do indeed point to an answer at the species level, I would be happy to update the record.

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