Early diversification of the avian brain:body relationship

Scaling of avian brain:body mass throughout the diversification of the class was investigated by analysis of a large collection of adult brain and body masses. Linear regression model analysis of whole-class brain:body scaling resulted in scaling exponents ranging from 0.574 to 0.609, values which exclude several prior empirical and theoretical estimates. Taxonomic level-specific analysis of brain:body scaling was performed by major-axis regression of trait variances partitioned among levels of taxonomic distinction. Brain:body scaling exponents varied markedly among avian orders, but were not easily related to ecological differences among taxa. Avian brain:body scaling exhibited a partial taxon-level effect, in that scaling exponents vary with the taxonomic level of investigation. However, scaling exponents were greatest at the family level, a pattern not consistent with prior ontogenetic or genetic covariance models of trait diversification. Instead, it is suggested that initial diversification among birds was largely through body size diversification, while later diversification of families within orders contained a relatively greater degree of brain size diversification. Avian developmental mode, known to influence avian brain size at hatching, was associated with relatively little variance in adult brain mass. Avian brain:body diversification has occurred relatively uniformly in precocial taxa, while diversification within altricial taxa is marked by a relatively high degree of brain mass diversification among families within orders.