Wednesday, December 24, 2014

Nova Scotia's 4th Dusky Flycatcher

Our Fourth Dusky
On Dec. 2, 2014, Bernard Forsythe found what was later identified as a Dusky Flycatcher in Greenwich, Kings County. The Greenwich Empid spent its time at field edges and in orchards owned by Harold Forsyth. It was observed by many Nova Scotia birders over the next few weeks.
Greenwich Empid on Dec. 13. 2014 - Photo by Keith Lowe
This bird is only the 4th confirmed Dusky in Nova Scotia. Information about the previous three records is found below.
Oct. 6, 2014 - banded on Bon Portage Island, Shel. Co. (NS-RBA post)
Oct. 12, 2012 - banded on Bon Portage Island, Shel. Co.  (NS-RBA post)
Nov. 26 - Dec. 27, 1996 - Near Port Williams, Kings Co.
Details of early observations of the Greenwich Empid can be found at NS-RBA and a blog post by Owl and Marmot.
Challenging Empids
Identification to species of members of the genus Empidonax can be troublesome. Our North American empids are all small, gray (tinged olive, brown or yellow) birds with wing bars and eye rings.
NS has a history of hosting western flycatchers in late fall and early winter. The overall structure and colour of the Greenwich Empid points to one local breeder (Least Flycatcher) and 2 western vagrants (Dusky and Gray Flycatchers) as potential candidates. As compared to the other empids, these three have relatively short primary projections, and similar wing length divided by tail length ratios. This wing length (wg) over tail length (tl) ratio (wg/tl) supplies us with a number that represents how long winged, and thus, how sleek or stubby the bird appears. Below are these ratios for our three candidates.

Wing over tail ratios (wg/tl)
1.17 - Least
1.12 - Dusky
1.17 - Gray
Other empids such as Alder Flycatchers are relatively longer winged and short tailed, giving them a wing over tail ratio of  1.26. These ratios were calculated using average measurements as given in Identifying Empidonax Flycatchers: The Ration Approach found in the March 2009 edition of Birding. This Birding article makes use of differences between wing and tail lengths. Ratios are more useful when measurements aren't available.

Some early participants in the discussion on identification of this bird have suggested Hammond's Flycatcher. Due, in part, to the quality of the initial photos and videos, Hammond's Flycatcher was also a contender. Bruce's Dec. 9 photos showed a bird with drooped wings which made assessment of primary projection difficult.
Not a Hammond's
Hammond's Flycatchers are described as a relatively colourful empid. All individuals (HY or AHY) would already have completed their body moults by now. This means that a Hammond's would be at its most colourful at this time of year. Plumage colour can be difficult to assess, but the photo below does show the Greenwich Empid as looking less vibrant in comparison to a Hammond's.

Hammond's Flycatchers have the longest primary projection of the 4 species in question. This is quite obvious in the following photo.
Greenwich Empid by Keith Lowe (Left) and Hammond's Flycatcher on Seal I. on Sept. 30, 2014 by David Bell (Right)
A Hammond's usually shows a steep forehead and un-curved crown. These features are shown by the Seal I. Hammond's in the photo above. The Greenwich Empid typically shows a continuous curve from the forehead through the crown.

The bill of a Hammond's is the shortest of the four species in question at 6.0-8.0 mm (tip to nares). Least, Dusky and Gray are 6.3-8.4, 6.5-8.9 and 7.6-10.4 mm respectively. Without measurements and from photos, bill length differences in millimeters or fractions of a millimeter are tough to estimate. According to an article in the March 2009 issue of Birding by Forrest Rowland called Identifying Empidonax Flycatchers: The Ratio Approach, even bill length differences of only two millimeters can translate into discernible differences in the field. Bill size contributes to the overall impression of a bird's head. A small bill makes the empid's head appear large and rounded.

I am of the opinion that this bird has a medium length bill. It does not give the impression of a large, rounded head caused by a very short bill.
Greenwich Empid on Dec. 13, 2014 - Photo by Keith Lowe
There is a relatively long gap between the 5th and 6th primaries of the Hammond's folded wing. A Dusky Flycatcher shows two long gaps of equal length between P4/P5 and P5/P6. This characteristic of wing morphology in favour of Dusky can be seen in the following image.
Greenwich Empid on Dec. 13. 2014 - Photo by Keith Lowe
On Dec. 13, 2014, I was able to record audio of the Greenwich Empid's call. The files were sent to Andrew Horn at the University of Dalhousie in Halifax for analysis. He concluded that my recordings best matched the calls of a Dusky Flycatcher.
Listeners in the field described it as sounding like "whit" - this looks like a simple upsweep on a sonogram. The Hammond's call is described as a "peep" or "pip", which is represented on a sonogram as an upside-down "v". We can conclude that the Greenwich Empid's call is dissimilar to that of a Hammond's.

Audio Spectrograph comparing the Greenwich Empid with a Hammond's Flycatcher
All audio spectrographs were created using the free software Audacity. The y-axis is frequency in Hz and the x-axis represents time in seconds. The Hammond's call was sourced from XenoCanto.

The strongest arguments against Hammond's are most likely the dissimilar calls and lack of a long primary projection.
Not a Gray
On Dec. 9, Bruce Stevens was able to capture excellent video of the empid in question. This video clearly shows that the bird's tail movements are not similar to that of a Gray. Gray Flycatchers show a very distinctive down flick of the tail. From rest, the tail is usually raised ever so slightly then lowered slowly. It is then, more quickly, brought back up to its rest position.

The video clearly shows the tail being flicked upwards from rest. Below are two stills taken from the Dec. 9 video. The left photo shows the tail at rest while the photo at right shows the tail at the very top of the upwards flick.
Stills from a Dec. 9, 2014 video by Bruce Stevens

Below is the Dec. 9, 2014 video recorded by Bruce Stevens.

It terms of colouration, Grays generally look paler than any other empid. In Identification guide to North American Birds, Pyle describes the upperparts of a Gray Flycatcher as uniformly pale grayish or brownish gray (adult) to grayish olive (juv-HY). Kaufman (2011) states that Grays have upperparts and face as medium gray, with faint olive wash on the back but not on the head. This can be compared to a Dusky, in which Pyle describes as having a usually grayish head contrasting with grayish-olive or grayish-brown back.
A Dec. 9 photo by Chris Peters below shows obvious contrast between the darker olive back and grayish nape and face. Photos on the internet show Grays as a much paler bird than our Greenwich Empid (see All About Birds).
Greenwich Empid on Dec. 20 - Photo by Chris Peters
A Gray will always show a distinctly defined dark tip. This feature is missing on the Greenwich Empid, as seen below.
Greenwich Empid on Dec. 13, 2014 - Photo by Keith Lowe
The photo below is of the Dusky Flycatcher that was banded on Oct. 6, 2014 on Bon Portage. It shows the same mandible colouration as the Greenwich Empid. It shows gradual fading to a dark tip on the lower edge.
Dusky Flycatcher on Bon Portage - Oct. 6, 2014 - Photo by Avery Bartels
Birds of North America Online describes the Gray's call as a dry "pit" or "wit", with less of the thick, initial "wh" quality in calls of Least and Dusky. The Greenwich Empid shows a gradual upsweep that is missing in Gray. The Gray's call was sourced from XenoCanto. The second line that is parallel to the primary call for the Gray is a harmonic at twice the frequency of the main call.
Audio Spectrograph comparing the Greenwich Empid with a Gray Flycatcher
Our best anti-Gray arguments are incorrect tail movement, lack of defined dusky tip and differences in calls. 
Not a Least
One difference between a Least Flycatcher and the three western empids (Hammond's, Gray and Dusky) is the wing contrast. A Least has relatively dark flight feathers that contrast more with the pale edging and wing bars than the western species. The composite photo below demonstrates how the Greenwich Empid has less wing contrast that the Least Flycatcher to the right. 
Greenwich Empid by Keith Lowe (Left) and Least Flycatcher on Aug. 30 by Ronnie d'Entremont (Right)
The Greenwich Empid also shows a grayish head that contrasts with a grayish olive back. In comparison, the Least is fairly uniform olive above (see above photo).
Hammond's, Dusky and Gray Flycatchers all have bills with fairly straight sides. Least Flycatchers have bills with curved sides. The photo below shows that the Greenwich Empid has a straight sided bill.
Greenwich Empid on Dec. 9, 2014 - Photo by Bruce Stevens
A Least Flycatcher's call is described as sounding like "pwit" or "pit" (or "whit" by Birds of North America Online). The sonogram for the Least's call below shows more than a simple upsweep. There is another element to the call that is not shown by the Greenwich Empid. The Birds of North America Online's sonogram shows the Least's call as an upwards sloped line followed by a shorter down sloped line. The Least's call below was sourced from XenoCanto.
Audio Spectrograph comparing the Greenwich Empid with a Least Flycatcher
Strong arguments again Least are the contrast between the back and head, a straight sided bill and differences in call. 

Definitely a Dusky
We are now left with Dusky Flycatcher as the best fit for this bird. Wonderful photos, audio and video of this bird's behavior, sound, structure and colour have enabled us to be confident in our identification. The composite image below shows the Greenwich Empid with a Dusky that was banded and measured on Bon Portage in 2014.
Greenwich Empid by  Keith Lowe (Left) and Dusky Flycatcher by Avery Bartels (Right)
The final supporting argument lies in the audio recording. Both the Greenwich Empid and Dusky Flycatcher calls below show a gradual upsweep. This upsweep is what produces a "whit" instead of a "wit". The Greenwich Empids sonogram matches the Dusky Flycatcher's call very nicely as seen below. Again, the second line that is parallel to the primary call and higher in frequency for the Dusky is a harmonic at twice the frequency of the call.
Audio Spectrograph comparing the Greenwich Empid with a Least Flycatcher
 Note: A robust sonogram analysis would require comparing a large sample of recordings. The limited analysis of sound in this post should be taken simply as further evidence and not diagnostic due to the possible variability in calls.
Rowland, F. 2009. Identifying Empidonax Flycatchers: The Ration Approach. Birding. March 2009.
Birds of North America. Online.
Pyle, P., S.N.G. Howell, R.P. Yunick, and D.F. Desante. Identification guide to North American Birds, Part 1, Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.
McLaren, I.A. 2012. All the Birds of Nova Scotia: status & critical identification. Gaspereau Press Ltd, Kentville, N.S., Canada
Sibley, D.A. 2014. The Sibley Guide to Birds 2nd Ed. Alfred A. Knopf, New York, N.Y.
Kaufman, K. 2011. Field Guide to Advanced Birding. Understanding What You See and Hear.Houghton Mifflin Harcourt, New York, NY.