Individual Variation in Avian Muscles and its Significance for the Reconstruction of Phylogeny

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Abstract
Little is known about individual variation in the muscles of birds. We dissected the hindlimb muscles bilaterally in a large series of two species of passerine birds to assess the kinds and frequencies of variation in their structure and to evaluate the effect of such variation on the reconstruction of phylogeny. Variation was rare. Three of 33 muscles varied in Cardinalis cardinalis, four of 32 muscles varied in Hylocichla mustelina, and no individual possessed more than three variants. We found no correlation of variation with sex and no significant tendency for multiple variants to occur in an individual. However, there was a significant tendency for variation to occur in specific muscles. Muscular variation was divided into several categories for which descriptive terms are introduced. Two of these categories, minor variants and incongruous variants, are unlikely to cause problems in phylogenetic studies. In contrast, mimicking variants and singular variants do pose a danger of introducing error into phylogenetic analysis. A special case of rare occurrence is explosive variation, in which multiple variants occur at high frequencies, and which may also cause problems in reconstructing phylogeny. We found the error rate in identifying species-typical conditions to average 2.0% and to range from 0.5 to 3.3% in passerine birds. We propose a research strategy that maximizes the probability of avoiding errors while minimizing the number of specimens dissected. We conclude that this strategy yields highly accurate assessments for phylogenetic analysis, and that individual variation is probably a minor source of error in phylogenetic reconstructions based on myological data for birds.