Morphologic Stability versus Adaptive Variation in the Hammond's Flycatcher

Abstract

Geographic variation in size in Hammond''s flycatcher (Empidonax hammondii), a sp. that nests in boreal forests from alasks to New Mexico. The analysis is based on 545 specimens of summer residents which were divided for treatment into 13 geographic populations, were segregated by sex and age (first-year and adult categories), and were measured for the length of the 10th primary, tail length, bill length, bill depth, bill width, tarsus length, middle toe length, and body weight. This sp. is remarkable uniform in size throughout its range; no pattern of variation exists which merits subspecific treatment. In certain sex-age categories minor significant differences for certain characters occur between populations. For the most part these differences seem to be random in distribution. In British Columbia, however, there is evidence that certain populations of the Hammond''s flycatcher have diverged in bill size, apparently in relation to the presence or absence of congeneric spp. For example, birds from the population on Vancouver Island, in the absence or rarity of the Western flycatcher (Empidonax difficilis), a large-billed sp. with a similar ecologic niche, are significantly longer-billed than those from the adjacent mainland where Western flycatcher are abundant. Birds from the population of the Hammond''s flycatcher in south-central British Columbia, some of which are sympatric with individuals of the closely related least flycatcher (Empidonax minimus), have significantly narrower bills than adjacent populations of the sp. This may be an instance of character displacement and needs further detailed study. There is no morphologic evidence in E hammondii from the area of sympatry that suggests the occurrence of interbreeding with E minimus. Intrapopulational comparisons of degrees of sexual dimorphism in several measurements of the bill strongly indicate divergence in bill shape between the sexes, with the females usually having shorter but wider bills than the males. Such differences may function to reduce intersexual competition for food between the members of a pair on their territory. Reduction or absence of geographic variation, while possible resulting from lack of Ortstreue, seems best attributed to strong genetic homeostasis for the standardized "Empidonax phenotype," homeostasis which masks the extensive genetic variability present. Sexual monomorphism in plumage may be an additional point of evidence for the presence of strong homeostatic forces. Limited ecologic tolerance in the Hammond''s flycatcher, a feature shared by many other spp. of the same genus, is also probably related to the existence of relatively slight geographic variation.