Plumage, Morphometric, and Song Variation in Mourning (Oporornis philadelphia) and Macgillivray's (O. tolmiei) Warblers

-Mourning (Oporornis philadelphia) and MacGillivray's (O. tolmiei) warblers are currently recognized as distinct species (AOU 1983). The specific status of these taxa, however, has been questioned based on morphological similarity and reports of hybridization in central Alberta (Cox 1973). I investigated the distinctness of these taxa by comparing plumage, skeletal, and primary song characters from fresh collections and recent tape recordings from the allopatric portions of their breeding ranges and the potential contact areas. The plumage analyses revealed overlap in characters originally used to diagnose these taxa; however, the incidence of extreme specimens was low. Separate multivariate analyses of morphological and song characters showed that the taxa are essentially distinct in multivariate space with little or no overlap. The Mourning Warbler was larger for most skeletal characters and had lower song frequencies. Plumage characters originally used to diagnose these taxa separate a majority of specimens, but they are ineffective for distinguishing extreme variants or hybrids. Song type proved 100% reliable in discriminating between taxa. I found no hybrid contact between these taxa. Furthermore, specimens collected near the potential contact zones were as variable morphologically as specimens from the allopatric portions of the breeding ranges. I believe these taxa should continue to be considered distinct species. Received 10 May 1989, accepted 9 September 1989. MOURNING (Oporornis philadelphia) and MacGillivray's (0. tolmiei) warblers form an eastwest species complex, whose breeding ranges meet in the northern Great Plains (sensu Rising 1983). The Mourning Warbler is the eastern taxon; it breeds in boreal forest from Newfoundland to northeastern British Columbia and south to West Virginia, parts of Michigan, Wisconsin, Minnesota, and North Dakota. MacGillivray's Warbler breeds in riparian habitat and disturbed second growth in the Rocky Mountains from northern Arizona to Alaska. Both taxa were described as separate species by Baird (1858) and are still considered separate (AOU 1983). Difficulty in identifying intermediate specimens, however, has caused some ornithologists (Chapman i917, Phillips 1947, Hofslund 1962, Mengel 1964, Mayr and Short 1970) to speculate that these taxa are eastern and western subspecies. Accounts of hybrid contact (Cox 1973, Salt 1973) have cast doubt on their specific status. The doubts are based on the equivocal nature of plumage characters originally used to diagnose these taxa (Table 1). Characters used in diagnoses are presumed to be unique to a given taxon and provide 100% discrimination of specimens. Further examination of Mourning Warbler specimens from different parts of its breeding range revealed the presence of intermediate specimens that possessed either dark lores, eyearcs, absence of black bibs, or some combination of these (Chapman 1917, Hall 1979). Lanyon and Bull (1967) acknowledged the equivocal nature of plumage characters and used an external measurement (Wing minus Tail: W T) to separate 98% of Mourning and MacGillivray's warbler specimens. Their results, however, were based exclusively on samples from the allopatric portions of the breeding ranges. Kowalski (1983) found much more overlap in W T, using specimens near the potential contact zones. This result suggests hybrid contact between the taxa. Cox (1973) collected some Mourning Warblers with "MacGillivray's-like" characters in central Alberta and concluded that the taxa hybridized there. Taverner (1919) collected a suspected mixed pair (Mourning male and MacGillivray's female) in Nevis, Alberta; but, based on the W T measurement, this female falls into the range of the Mourning Warbler (Hall 1979). Hall (1979) was also not convinced 161 The Auk 107: 161-171. January 1990 This content downloaded from 157.55.39.185 on Thu, 26 May 2016 06:15:56 UTC All use subject to http://about.jstor.org/terms 162 JAY PITOCCHELLI [Auk, Vol. 107 TABLE 1. Plumage characters used to diagnose adult Mourning and MacGillivray's warblers. Character Mourning Warbler MacGillivray's Warbler Loresa Absent Dark in males Eye-arca Absent in adults, present in immatures Present in adults and immatures Black biba Concentration of black feathers on Diffuse black feathering on throat lower throat and upper breast, and upper breast of males, present in males only absent in females Wing-minus-tailb measure ?10 mm <11 mm I Baird 1858, Coues 1903, Hall 1979. bPhillips 1947, Lanyon and Bull 1967. that all intermediate specimens reported by Cox and others (Patti and Meyers 1976, Beimborn 1977) were hybrids because he found intermediate specimens of Mourning Warblers from the allopatric portions of its breeding range. These extreme variants from eastern Canada raise the question whether the intermediate nature of these specimens is due to hybridization or falls within the normal range of variability of either taxon. My goal was to clarify the specific limits of these warbler taxa based on the distinctness of each taxon. I concentrated my analysis on males for two reasons: intermediate males show "hybrid characters" better than females and their singing behavior can be recorded and compared. I collected and compared fresh specimens from the allopatric portions of the breeding ranges and from the hypothetical contact areas, and I evaluated the range of variability and overlap in traditional plumage characters used to diagnose these taxa. I added two new suites of skeletal and behavioral characters to the analysis, and determined if specimens of these taxa occupy different "morphological or song space" based on principal components analyses (PCA) of morphological and song characters. Finally, I evaluated song syllable sharing