On May 25, 2015, I photographed a Least Flycatcher at Cape Forchu, Yarmouth County. Since it was silent, I had to scrutinize the images to confirm its identification. Confident identification of Empidonax
flycatchers that are not vocalizing is one of the toughest challenges for birders. A helpful article entitled Identifying Empidonax Flycatchers: A Ratio Approach
by Forrest Rowland illustrates important proportional differences between these very similar species. Kenn Kaufman's Field Guide to Advanced Birding
also provides useful descriptions and comparisons.
Why is it a Least?
This flycatcher is clearly an Empidonax
, and not an Eastern Wood-Pewee due to its relatively short primary projection and lack of dusky chest sides. It is not a Yellow-bellied Flycatcher since it is lacking a yellowish throat. The most likely candidates based on our location that we are left with are the Alder and Least Flycatchers.
This flycatcher was found in a habitat better suited for an Alder Flycatcher. Cape Forchu is predominantly covered by young coniferous trees with sections of alders. Leasts are typically found in mature deciduous forests.
A few key features separate this bird from the similar Alder Flycatcher. Compared to an Alder Flycatcher, the Least has a more conspicuous eye ring and a shorter primary projection (Alderfer & Dunn, 2014). Rowland (2009) describes the Least Flycatcher as a short-winged and long-tailed Empidonax
. This structure may be appreciated in Fig. 1. The wing chord of the Least appears shorter in comparison to the Alder Flycatcher.
The tail of an Alder Flycatcher looks broad compared to that of other smaller Empidonax
. Kaufmann (2011) describes the tail of the Least Flycatcher as narrow (see Fig. 1). The Alder's wing contrast is lessened by the dull wing bars and tertial edgings in comparison to the typically more white wing bars and tertial edgings of the Least (see Fig. 1). In fresh spring plumage, Alder shows olive upperparts with a contrastingly gray face. The Least Flycatcher has fairly uniform brownish gray upperparts with darker crown and forehead and is washed olive only on the back. This difference in colour between the head and back in Leasts is visible in Figure 1, Alders appear to have more similar coloured heads and backs. Leasts also appears to have a relatively larger head than Alders (Alderfer & Dunn, 2014).
How old is it?
|Figure 1. Least Flycatcher (L) versus Alder Flycatcher (R). Photos by Alix d'Entremont.|
After analysis of this Least Flycatcher, I conclude that it is a second-year bird, born in 2014. Below are supportive arguments.
A bird born in 2014 (typically in July) would have gone through a first prebasic molt from Jul-Oct 2014 on the breeding grounds. The extent of this molt is variable, but it typically involves the body plumage and occasionally the secondaries and a few upper wing coverts. The primaries, primary coverts and a variable number of secondaries and secondary coverts will not be replaced during this molt. (Tarof & Briskie, 2008)
The first prealternate molt would have occurred on the wintering grounds from Feb-May 2015 and involves the body plumage, some inner secondaries and their corresponding greater and middle coverts along with a variable number of rectrices. Again, no primaries, primary coverts and outer secondaries and secondary coverts are not replaced. (Tarof & Briskie, 2008)
Pyle (1997) describes Least Flycatchers in their second year (SY) during Apr-Sep as having marked contrast between the retained, outer coverts that are brown and abraded compared to the duskier and lemon-tipped, replaced inner coverts. Second year birds also show strong contrast between the tertials and the adjacent secondaries. These features are clearly visible in Figure 2. After second year (ASY) birds show less contrast.
The primaries of this Least Flycatcher also appear very worn and brown as compared to the relatively fresh primaries that would be seen in an ASY bird. The rectrices appear tapered, also indicating a SY bird. The primary coverts are worn, brown and have no green edge. An ASY would show broader green edging to fresher and duskier primary coverts.
|Figure 2. Least Flycatcher. Photos by Alix d'Entremont.|
SY birds can replace 1-3 tertials during their prealternate molt. This bird appears to have replaced the longest tertial. There is slight, but apparent differences between this longest tertial and two inner ones. The inner tertials show slight wear on the feather edges while the longest shows a smooth edge (Fig. 3).
|Figure 3. Least Flycatcher. Photos by Alix d'Entremont.|
The retained juvenile feathers that were not replaced during the prebasic (Jul-Oct 2014) or prealternate (Jan-May 2015) molts show strong contrast with the newly replaced feathers. The primary coverts show no green edging. The rectrices are apparently tapered. These features indicate that this Least Flycatcher is a second-year bird, born in 2014. The differences between a SY bird and an ASY bird in spring arise due to the partial first prebasic molt compared to the complete prebasic molt of an AHY (after hatch year) bird.
Alderfer, J. & J.L. Dunn. 2014. (Ed). Complete Birds of North America, 2nd Edition. National Geographic Society. Washington DC, USA.
Kaufman, K. 2011. Field Guide to Advanced Birding. Understanding What You See and Hear.Houghton Mifflin Harcourt, New York, NY.
Pyle, P., S.N.G. Howell, R.P. Yunick, and D.F. Desante. 1997. Identification guide to North American Birds, Part 1, Columbidae to Ploceidae. Slate Creek Press, Bolinas, California.
Rowland, F. 2009. Identifying Empidonax Flycatchers: the Ratio Approach. Birding 41(2): 30-38
Tarof, Scott and James V. Briskie. 2008. Least Flycatcher (Empidonax minimus), The Birds of North America Online (A. Poole, Ed.). Ithaca: Cornell Lab of Ornithology; Retrieved from the Birds of North America Online: http://bna.birds.cornell.edu/bna/species/099