The Relationships of the Wood Thrush (hylocichla Mustelina) : Some Indications from the Electrophoresis of Blood Proteins

The generic relationships of the Wood Thrush (Hylocichla mustelina) have been called into question by the works of Dilger (1956) and Bourns (1967). The four species of North American woodland thrushes presently referred (32nd Supplement to the A.O.U. Checklist 1973) to the genus Catharus i.e., Hermit Thrush ( Catharus @tutu) ; Gray-cheeked Thrush (C. minima); Swainson’ s Thrush (C. ustuluta); and Veery (C. fuscescens) were formerly placed in the genus Hylocichla (A.O.U. Check-list 1957) and were long believed to be the closest relatives of the Wood Thrush. Dilger (1956) has found the Wood Thrush “a misfit in the rather homogeneous CatharusHylocichla assemblage . . . ,” and recommended its retention as the sole member of the genus Hylocichlu although he believed further investigation would place it in the genus Turdus. Dilger’ s investigations into morphology and behavior received support from Bourns’ (1967) series of serological tests on muscle and blood proteins. Bourns found that the Wood Thrush not only showed a greater relationship to the American Robin ( Turdus migratorius) than to any of the four other woodland thrushes (i.e., Catharus), but that the Robin-Wood Thrush relationship was closer than that seen to exist between any two of the other four species. Dorst (1950) had come to a similar conclusion based on the apparent similarity between Hylocichla mustelina and the Song Thrush (Turdus musicus, 110~ Turdus philomelos). Others, most notably Ridgway (1907) and Ripley (1952), have interpreted this similarity as due to convergence. It would appear that there are three alternative schemes to deal with the systematic problems of the Wood Thrush: (1) the Wood Thrush is a member of the enlarged genus Cathurus; (2) the Wood Thrush is the sole member of the genus Hylocichla; or (3) the Wood Thrush is a member of the genus TUTdus. At the moment, one can find some evidence to support each of these positions. Arguments over interpretation of existing data seem less likely to aid in the selection of the most reasonable of these hypotheses than does the accumulation of new evidence. Consequently, we have examined the electrophoretic properties of several blood proteins from the involved species in an effort to clarify our understanding of relationships in this group of birds. The basic philosophy behind the use of electrophoretic data on proteins as a source of taxonomic information has been published elsewhere and seems well justified (Sibley 1960, 1962, 1970; Zuckerkandl and Pauling 1965 ) .