“It is important to appreciate that color depends on lighting conditions and an observer’s own perception. Except for extremes, too much emphasis on color is not recommended. Instead, color contrasts between different parts of the bird should be the focus.”

Lee and Birch go on to detail in this section the use of the following: behavior; age and molt; vocalizations; habitat preference; and range, seasonal status, and migration for the task of, at least, winnowing the options. Novel and quite useful is “The Field Mark Matrix” on page 43. While probably not directly useful in the field, it should prove a great pre- and post-birding resource. That said, I’ll have more to say about the limitations of the matrix.  The final two bits of the introductory material are the excellent visual aids of a “Holistic Approach” (minimalist sketches of shape and gross plumage features) and a Venn diagram presented in a “Visual Similarity Map.”

Crown shape. Some readers might be confused by the illustration having multiple species listed in multiple crown shapes, and this was not explained and might lead to readers questioning it. I would have liked to have seen the most common crown shape of individual species (if that was possible to determine) being highlighted by, perhaps, boldface type.

Lower mandible color. While the illustrations and text in various places present the lower mandible color as it is most frequently seen, a confounding aspect is that juveniles of the various small green flycatchers frequently differ in this feature from that of adults.  Under appropriate federal and state permits, I have captured and banded many individuals of most of the species covered here, with much of that experience during fall migration in Michigan, New Jersey, and Colorado. I have caught juvenile Eastern Wood-Pewees in Cape May, and photographed others there, with nearly entirely blackish lower mandibles and, conversely, juvenile Western Wood-Pewees with relatively extensive yellow on the lower mandible. I have seen Hammond’s Flycatcher nestlings that sport entirely bright orange bills, not just the lower mandible (see Pyle 1997). While the bills of these juveniles change color rapidly, fledglings and even some southbound migrants still show sometimes extensive amounts of orange.

Overall coloration. Pine Flycatcher is included twice in the list of species under the “gray” illustration. I do not know whether another species should have been listed instead.

Species accounts and temporal occurrence graphs. The introductory material mentions that the bar graphs illustrate eBird abundance, but it does not tell us from what years the data are derived. Since my most continuous and intensive birding efforts were while I was a Colorado resident, I will turn now to a case study of Colorado, for which the book presents bar graphs for four Empidonax species accounts: Willow, Hammond’s, Dusky, and Cordilleran.

Colorado case study
The first problem is presenting data from a state like Colorado as a whole, rather than differentiating between western and eastern Colorado, where virtually all the state’s mountains are in the west, while the plains dominate the east. Since many other occurrence bar graphs are presented for portions of states (such as California and Texas), Colorado could have been considered for such treatment. All four of these species breed in the western, montane portion of the state, but only Willow and Least breed on the plains—although at only a handful of sites, and most of the sites supporting Least are near the Front Range edge. This difference between virtual halves of the state would not be so bad in eBird data if it were not for the skewed human population size and resultant eBird data abundance between the two halves. Three (of 64) Colorado counties that account for nearly 35% of all Colorado eBird data (Larimer, Boulder, and Jefferson  counties) straddle the montane/plains interface, making analysis of eBird data by county more than a bit tricky. Adding more mud to an already murky situation, fall migration of passerines is generally more robust and diverse away from the montane/plains interface.

For example, compare the eBird abundance data for these four Empidonax species in 15 montane Colorado counties (tinyurl.com/15-montane) with those from 15 plains Colorado counties (tinyurl.com/15-plains). First, scroll down to the sample-size chart to note that the peak number of eBird checklists from the montane counties peaks in the fourth week of June at 8,365 eBird checklists, whereas the plains counties data have peaks in the fourth week of April at 13,496 eBird checklists. Click on the abundance tab so that we can make direct comparison to this guide’s graphs. Except for the Cordilleran Flycatcher, which nests on houses and cabins (something the other species do not do), the abundance graphs are roughly similar to those presented in this book. But a different story is presented by the eBird data for the counties on the plains.

In the data for the counties on the plains, the peak of abundance is 0.04 birds per checklist, with three of the four species having identical fall peaks, both in abundance value and in the same week (the first week of September). Note how different the abundance graphs are in the montane-counties data. Cordilleran Flycatcher abundance peaks in the last week of July at 0.42 birds per checklist, more than ten times the peak values of the various species on the plains. This is likely a sampling artifact due to the species’ habit of nesting on human-built structures, making them more available to most eBirders and therefore more identifiable by most eBirders. Huge swaths of western Colorado are cloaked in pinyon-juniper woodland and Gambel oak shrubland, both habitats in which Dusky Flycatcher can be amazingly abundant, but which do not get eBirded in anything like those habitats’ abundance relative to other habitats.

My final quibble with the bar graphs is that they misrepresent abundance. To be clear, they probably represent relatively faithfully detected abundance, but are misleading about actual abundance. All four graphs show a great reduction in abundance beginning in mid-July or early August. However, actual abundance is almost certainly greater in late summer, as populations of all species, not just these four, have been augmented by the summer’s production of juveniles. While Willow and Least flycatchers are already on the move in, at least, eastern Colorado after mid-July, the absolute numbers of individuals of both species found well away from breeding localities is quite small. The inaccuracy of the abundance data presented is further exacerbated by birding efforts in Colorado montane habitats peaking in June and early July and tapering off through August, a time during which most bird species, including small green flycatchers, are much less vocal, thus much less detectable.

I hasten to acknowledge that the authors themselves were not the source of the data presented in this book! eBird as a data source can be spellbinding, but getting reliable results out of it—that is to say, results that match landscape-level reality—requires incredible amounts of analysis and effort and probably some supercomputer time.

The most important aspects of the Field Guide to North American Flycatchers: Empidonax and Pewees, the text and the illustrations, are presented so well by Lee and Birch, that I recommend the book on those strengths. This guide is a must-have for birders wishing to improve their field skills, specifically with small green flycatchers—and indeed with bird identification in general.

Works Cited

Chesser, R. T., S. M. Billerman, K. J. Burns, C. Cicero, J. L. Dunn, B. E. Hernandez-Baños, R. A. Jiménez, A. W. Kratter, N. A. Mason, P. C. Rasmussen, J. V. Remsen, and K. Winker. 2023. Sixty-fourth supplement to the American Ornithological Society’s Check-list of North American Birds. Ornithology 140: 1–11.

Leukering, T. 2014. In the scope: Seasonal timing—small, green flycatchers. Colorado Birds 48: 249–254.

Leukering, T. 2021. Featured photo: Flycatcher fundamentals. Birding 53 (5): 54–60.

Pyle, P. 1997. Identification Guide to North American Birds. Slate Creek Press, Bolinas.

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Tony Leukering is a field ornithologist, based on the Great Plains, with strong interests in bird migration, distribution, and identification. He has worked for five different U.S. bird observatories and considers himself particularly adept at taking quiz photos—that is, bad pictures! Tony has served on bird records committees in Colorado, New Jersey, and Oklahoma.