Hybrid wood warblers, Dendroica striata x Dendroica castanea (Aves: Fringillidae: Tribe Parulini) and the diagnostic predictability of avian hybrid phenotypes

—The plumage pattern and color and external morphology of hybrid wood warblers (Dendroica striata x Dendroica castanea) are described. This hybrid combination constitutes the only known case of hybridization between two broadly sympatric species of the genus Dendroica (Fringillidae; Tribe Parulini), that is represented by both male and female specimens in definitive alternate plumage. The plumage of the hybrids exhibited a mosaic of character states that varied in the degree of intermediacy between those of the parental species. External measurements of the hybrids fell within the cumulative ranges of characters of the postulated parental species. I hypothesize that the diagnostic predictability of hybrid phenotypes decreases as an inverse function of genetic relatedness of the hybridizing species. . Plumage color patterns are remarkably diverse among the 24 species of Dendroica (Aves; Fringillidae; Tribe Parulini) of North America and the Caribbean (taxonomy of Sibley & Monroe 1990), in contrast to the similarity of their body plans and trophic appendages. Several pairs of species thought to have recently diverged (Mengel 1964. Mayr & Short 1970. Bermingham et al. l92) exhibit striking differences in male definitive plumage, suggesting that plumage pattern may evolve rapidly in response to sexual selection. Our understanding of the inheritance patterns of plumage color in Dendroica has been limited to evidence gleaned from hybridization at contact zones between para pa trie tax a (e.g., Dendroica occidentalis X Dendroica townsendi\ see Rohwer 1994) and rare instances of intcrgeneric hybridization (e.g., Parkes 1978, Graves 1993a). Brodkorb's (1934) description of the hybrid, Dendroica striata (Blackpoll Warbler) x Dendroica castanea (Bay-breasted Warbler), was the first report of hybridization between broadly sympatric species of Dendroica. This specimen (University of Michigan Museum of Zoology [UMMZ] No. 53692) represents one of the few known intrageneric Dendroica hybrids represented by a male in definitive alternate plumage. Because male D. striata and D. castanea differ dramatically in plumage pattern and color, this specimen assumes unusual significance for the investigation of phenotypic inheritance in avian hybrids. Recently, the existence of a female hybrid (UMMZ 216628) and an additional male hybrid (San Bernardino County Museum, uncataloged) identified as D. striata X D. castanea, was brought to my attention, respectively, by Janet Hinshaw and Kenneth Parkes. Here 1 perform a hybrid diagnosis on all three specimens based on plumage color and pattern and external morphology, following Graves (1990). Materials and Methods The male specimen reported by Brodkorh (1934) was collected on 19 May 1920. 374 PROCEEDINGS OF THE BIOLOGICAL SOCIETY OF WASHINGTON Fig. 1. Lateral view (from top) of maJes: Dendroica ccuttmea; hybrids, D, striata X D. castanea (UMMZ 53692 and SBCM); and D. striata (bottom). by Norman A. Wood al Warren Dunes, Berrien County, Michigan (Figs. 1-4). The female specimen was collected on 25 May 1970 at Long Point, Ontario, by Joseph G. Sirauch (Figs. 5-7). The second male specimen was collected by Lawrence Sansone, III, along the Rio Grande south of Brownsville. Cameron County, Texas, on 9 June 1969. For the purposes of the hybrid diagnosis, 1 considered all wood warblers that regularly breed north of the Mexican border as potential parental species (n = 53). The first two specimens have broadly tapered rectrices indicating that they were adults in their second year or older (ASY in banding terminology) (Pyle el al. 1987). Plumage, especially the rectrices, of the Texas specimen is moderately worn. Thus, I am uncertain of the bird's age (SY or ASY). Because the specimens appeared to be in definitive plumage, I compared them to large series of definitive-plumaged specimens in the collections of the National Museum of Natural History, Smithsonian Institution. For comparative purposes I measured the size of three species: D. striata, D. castanea, and D. pinus. Data for D. pinna were omitted from Table 1, after analyses showed that it was not involved in the parentage of the hybrids. Measurements of wing chord, tail length (from point of insertion of central rec trices to tip of longest rectrix), tarsus length, and bill length (from anterior edge of nostril), were made with digital calipers to the nearest 0.1 mm. I evaluated the color of selected areas of plumage with a Color Mate Colorimeter (Milton Roy), employing a 9.4 mm aperture. The data in Table 2 were compiled from the averages of three independent measurements (specimen removed from apVOLUME 109, NUMBER 2 375