Similarity of growth among Great tits (Parus major) and Blue tits (P. caeruleus)

Differences in morphology among species are proximately caused by changes in the ontogeny of individuals. It is therefore of importance to analyse possible differences in growth parameters among closely related species in order to understand what parameters are most and least likely, respectively, to change in evolution. In this paper I analyse growth in two closely related sympatric species, namely Great tit, Parus major, and Blue tit, P. caeruleus. The former is considerably larger than the latter in all external traits. The growth rates of the two species were found to be very similar for all traits, thus excluding differences in growth rate as a potential cause of evolutionary size changes. Offset of growth occurred at relatively similar times in the two species, excluding this factor as a major cause of the final size differences. However, size differences at hatching were pronounced and remained so throughout ontogeny, pointing to initial size (egg size or hatching size) as the target of factors promoting change. Bivariate allometric relations of traits vs. body size (mass) were similar between the two species at all ontogenetic stages. There was a high correlation among traits especially at intermediate age stages (5 and 8 days), but these correlations became weaker at older age and approached the low pattern of integration found in adults. All this suggests the operation of a general growth factor affecting all parts of the phenotype simultaneously, which has its major influence at the time of maximal growth. If closely related species in general have highly similar growth patterns, strong evolutionary allometry as found in many avian taxa is to be expected.