Geographic variation in the plumage coloration of willow flycatchers Empidonax traillii

Abstract

The ability to identify distinct taxonomic groups of birds (species, subspecies, geographic races) can advance ecological research efforts by determining connectivity between the non-breeding and breeding grounds for migrant species, identifying the origin of migrants, and helping to refine boundaries between subspecies or geographic races. Multiple methods are available to identify taxonomic groups (e.g., morphology, genetics), and one that has played an important role for avian taxonomists over the years is plumage coloration. With the advent of electronic devices that can quickly and accurately quantify plumage coloration, the potential of using coloration as an identifier for distinct taxonomic groups, even when differences are subtle, becomes possible. In this study, we evaluated the degree to which plumage coloration differs among the four subspecies of the willow flycatcher Empidonax traillii, evaluated sources of variation, and considered the utility of plumage coloration to assign subspecies membership for individuals of unknown origin. We used a colorimeter to measure plumage coloration of 374 adult willow flycatchers from 29 locations across their breeding range in 2004 and 2005. We found strong statistical differences among the mean plumage coloration values of the four subspecies; however, while individuals tended to group around their respective subspecies' mean color value, the dispersion of individuals around such means overlapped. Mean color values for each breeding site of the three western subspecies clustered together, but the eastern subspecies' color values were dispersed among the other subspecies, rather than distinctly clustered. Additionally, sites along boundaries showed evidence of intergradation and intermediate coloration patterns. We evaluated the predictive power of colorimeter measurements on flycatchers by constructing a canonical discriminant model to predict subspecies origin of migrants passing through the southwestern U.S. Considering only western subspecies, we found that individuals can be assigned with reasonable certainty. Applying the model to migrants sampled along the Colorado River in Mexico and the U.S. suggests different migration patterns for the three western subspecies. We believe that the use of plumage coloration, as measured by electronic devices, can provide a powerful tool to look at ecological questions in a wide range of avian species.

Assessing migratory connectivity for a long‐distance migratory bird using multiple intrinsic markers

“…Evolutionary biologists and taxonomists alike have long been interested in characterizing and explaining geographic variation in avian morphology across species' ranges including along latitudinal or altitudinal gradients (Mayr 1956, Hamilton 1961, Browning 1994, Arizaga et al 2006, Nudds and Oswald 2007, McKay 2008, Rising et al 2009, Paxton et al 2010, Greenberg et al 2012. Moreover, a substantial body of work has characterized how natural selection (Greenberg et al 1998, Lehtonen et al 2009) and sexual selection (Møller 1995, Dunn et al 2008, Potti and Canal 2011 have shaped phenotypic variation.…”

“…One source of geographic variation that has not been combined with stable isotopes or widely used in migratory connectivity studies is morphological traits that show predictable geographical variation (Go´mez-Dı´az and Gonza´lez-Solı´s 2007). Many species of migratory birds show geographic variation in morphology, including body size (Conklin et al 2011), wing length (Saracco et al 2009), bill size (Greenberg et al 2012), and plumage characteristics (Lehtonen et al 2009, Paxton et al 2010). These morphological traits represent a potentially valuable source of data for studying migratory connectivity in many species (Paxton et al 2010), but to date there have been no attempts to combine morphological traits with other data sources to probabilistically assign individuals to breeding locations.…”

Winter Distribution of Willow Flycatcher Subspecies

“…We found significant differences in L and a of both the head and back (the shifts appear linear and constant over the ages ofthe specimens), and we adjusted these values for each specimen as a function ofthe year collected (¿_crown year = -0.0068, a_crown year = -0.0083, ¿_back year = -0.011, a_ back year = -0.0091). While solar radiation and abrasion can cause plumage coloration to change over time (Paxton et al 2010), we did not address seasonal fading in this study because of uncertainty on how fading progresses through the nonbreeding season. However, seasonal fading is of greatest concern with adults in fall migration, which are excluded from this analysis, as Willow Flycatchers initiate molt when they arrive on their winter grounds.…”

“…Then we developed a second discriminant function model that considered just Willow Flycatchers to assign specimens to one ofthe four subspecies. Although distinguishing between the eastern (E. t. traillii) and western subspecies is not possible on the basis of color alone (Paxton et al, 2010), the addition of measurements allows for discrimination between the two groups. All analyses ofthe museum specimens were run in JMP 8,0 (SAS, Inc.).…”

Current Topics in Avian Conservation Genetics with Special Reference to the Southwestern Willow Flycatcher

“…(4). Plot of individual's scores on Discriminant Function 1 based on six date-corrected coloration characters from [38] showing no example of a subspecies in which 75% of the individuals are distinct from 99% of the other individuals.…”