Mark K. Sogge Chapter 6: Breeding Season Ecology The willow flycatcher (Empidonax traillii) breeds and includes comparisons with other populations across much of the conterminous United States and in and subspecies. portions of extreme southern Canada. As might be expected in such a wide-ranging species, willow fly- catchers in different portions of the range exhibit Breeding Range and Taxonomy _____ differences in appearance, song, and ecological char- The willow flycatcher is one of 11 Empidonax fly- acteristics. The intent of this chapter is to provide catchers that breed in North America. Although the information on the breeding-season ecology of the Empidonax flycatchers are considered a very difficult southwestern subspecies, E.t. extimus. However, most group to identify by sight alone, each has unique ecological studies to date have dealt with other willow morphological features, vocalizations, habitats, be- flycatcher subspecies. Relatively few studies have haviors, and/or other traits that have allowed taxono- been published on E.t. extimus, and much of what is mists and biologists to characterize each species. The currently known is presented in unpublished litera- willow flycatcher differs from most other Empidonax ture (e.g., agency and consulting firm reports); these in lacking a conspicuous eye-ring, and having both a sources are relied upon heavily in this chapter. This completely yellow lower mandible and a whitish throat chapter does not address habitat characteristics in that contrasts with a pale olive breast. While these depth, other than for nest sites (refer to A Survey of differences may be subtle, the willow flycatcher also Current Breeding Habitats for additional details). Al- has a distinctive song (often termed fitz-bew; see though southwestern willow flycatchers are frequent below) that clearly separates it from all other bird victims of nest parasitism by brown-headed cowbirds species. (Molothrus ater), this chapter will not address the The willow flycatcher was first described by J.J. topic of parasitism and its effect on breeding ecology Audubon, who collected a specimen in the woods along (refer to The Ecology of Brown-headed Cowbirds and the Arkansas River in the early 1800s (Audubon 1831) their Effects on Southwestern Willow Flycatchers for and named it Muscicapa traillii. Since that time, the details). Readers interested in more details of willow species has undergone a series of name changes and flycatcher biology and ecology are encouraged to read species/subspecies designations (see Aldrich 1951, McCabe’s (1991) treatise, which is based on over a Browning 1993). Prior to 1973, the willow flycatcher decade of willow flycatchers research in Wisconsin and alder flycatcher (Empidonax alnorum) were USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 57 treated together as the Traill’s Flycatcher (A.O.U. are lacking). Records of probable breeding southwest- 1957), but subsequent work proved that they do not ern willow flycatchers in Mexico are few and restricted interbreed (Stein 1958, 1963), have different vocal- to extreme northern Baja California del Norte and izations (Stein 1958), and are genetically distinct Sonora (Unitt 1987, Wilbur 1987). (Seutin and Simon 1988). The American The southwestern willow flycatcher is generally Ornithologist’s Union (1973) accepted the separation paler than other willow flycatcher subspecies (Unitt of willow and alder flycatchers in 1973. McCabe 1987), although this difference is indistinguishable (1991) reviews the many common names historically without considerable experience and training, and given to the willow flycatcher. without study skins as comparative reference mate- The southwestern subspecies was first described by rial. All three western subspecies differ from E.t. Phillips (1948). Unitt (1987) re-evaluated the sub- traillii in wing formula (Unitt 1987). Differentiation of species taxonomy of the willow flycatcher and recog- subspecies in the field is not reliable, due to the nized four subspecies, each with a distinct breeding subtlety of morphological differences, inconsistent range and differentiated primarily by subtle differ- conditions for comparisons, and the inability to re- ences in color and morphology. Browning (1993) per- peat or reassess the identifications of individual formed a similar evaluation and proposed five subspe- specimens (Hubbard 1999). cies, rather than four. Both authors, however, reconfirmed the validity of E.t. extimus, which has also been accepted by most authors (Aldrich 1951, Behle Vocalizations _____________________ and Higgins 1959, Phillips et al. 1964, Oberholser Willow flycatchers are suboscines, and their songs 1974, Monson and Phillips 1981, Schlorff 1990, appear to be innate, rather than learned (Kroodsma USFWS 1993). Based on Unitt (1987) and Browning 1984). In fact, even hatching-year flycatchers can sing (1993), the breeding range of the southwestern wil- (Kroodsma 1984, Sogge 1997). As with most birds, low flycatcher (Figure 6-1) includes southern Califor- singing behavior is regulated by hormone levels, nia (from the Santa Ynez River south), Arizona, New which in turn are influenced by a number of factors Mexico, southwestern Colorado, extreme southern including photoperiod, time of day, and auditory and portions of Nevada and Utah, and western Texas visual stimuli from other birds of the same species (although recent breeding records from west Texas (Kroodsma 1984, Catchpole and Slater 1995). Figure 6-1. Breeding range distribution of the subspecies of the willow flycatcher (Empidonax traillii). Adapted from Unitt (1987), Browning (1993) and Sogge et al. (1997a). 58 USDA Forest Service Gen. Tech. Rep. RMRS-GTR-60. 2000 The willow flycatcher has a distinct primary song, males, song rate may remain high throughout most of often referred to as fitz-bew, that distinguishes it from the breeding season. Males may sing up to 60 songs per all other Empidonax flycatchers, and from other birds minute (H. Yard and B. Brown unpublished data). in general (refer to Stein 1963 for a detailed discus- Unmated males and males with territories near other sion). This is the primary territory advertising song of willow flycatchers tend to vocalize more than males in male willow flycatchers, and all subspecies sing fitz- isolated territories (M. Sogge and M. Whitfield, un- bew. Singing bouts are usually comprised of a series of published data). fitz-bews, sometimes interspersed with creet notes, Being highly territorial, willow flycatchers readily lasting from less than a minute to over a half-hour. sing and/or call in response to broadcast tapes of Breeding males sing to advertise their territory to willow flycatcher song (Gorski 1969, Tibbitts et al. prospective mates and other nearby males. Males sing 1994), which they apparently perceive as an intruding from a series of song perches throughout their terri- flycatcher. This ready response to taped vocalizations tory, usually from tall perches but sometimes from forms the basis of standardized survey protocols cur- within dense vegetation. Weydemeyer (1973) and rently in use (Craig et al. 1992, Sogge et al. 1997a). In McCabe (1991) described willow flycatchers singing many cases, willow flycatchers that are not vocalizing during flight in the evenings, but this has been ob- when surveyors first arrive at a site begin singing in served only rarely in E.t. extimus. Migrant willow response to a broadcast taped song. Territorial breed- flycatchers often sing from tall song perches during ing males and females, migrants, and (perhaps rarely) spring migration, in much the way that territorial even recently fledged (6-8 week old) willow flycatchers birds do (Johnson and Sogge 1997). Sogge et al. (1997b) will respond to tape playback (Sogge 1997, Sogge et al. found that migrants accounted for up to 64% of the 1997a). However, much as with the general song spontaneously singing flycatchers found each year patterns, response to tape playback declines over the along the Colorado River in the Grand Canyon. This course of the breeding season, and breeding flycatch- makes it difficult if not impossible to separate territo- ers may not respond strongly after nesting has begun. rial birds from migrants based on singing behavior Another common vocalization used by flycatchers is alone. the whitt call, which is frequently given by both sexes. Female willow flycatchers also sing, at least in Whitts are given as an alarm call and during interac- some circumstances. Seutin (1987) reported female tions between flycatchers. Whitts are often the most willow flycatchers singing in response to tape-play- common vocalization used during mid- and late breed- back experiments in southern Canada. Although he ing season. Many other bird species have similar whitt clearly established that females sing, the extent of calls, so unlike the fitz-bew, the whitt is not generally female song under non-experimental conditions was considered unique to willow flycatchers. unknown. Until recently, it was generally assumed that females seldom sang, and/or that their songs were quieter and/or not given from song perches in the way Foraging and Food ________________ that males sing (Sedgwick and Knopf 1992). New Foraging Behavior research, much of it with banded individuals of known sex, has shown that female southwestern willow fly- The willow flycatcher, as the name implies, is prima- catchers regularly sing (though not nearly as often as rily an insectivore. It is an agile aerialist, capable of males).