GEOGRAPHIC-VARIATION IN ONTOGENY OF THE FOX SPARROW

Ontogeny of two morphologically differentiated populations of Fox Spar rows (Passerella iliaca) was compared to reveal the developmental base s of adult morphological differences. Growth of tarsus, wing, bill. an d tail length relative to body mass was compared between the two popul ations to determine if geographic variation in adults results from dif ferent relative growth rates or different relative timings of onset an d offset of growth. Differences in adult morphologies between populati ons result from both changes in allometric growth rates and changes in starting points along the same allometric trajectory. Sexual dimorphi sm in bill length is mostly the result of differences in growth rate o ccurring late in ontogeny. Differences between populations greatly exc eed those between sexes. Factors underlying growth are complex and pro bably result from many different genetic and environmental factors.