Alterations of Growth Trajectories in Rook (Corvus frugilegus) Ontogeny

The growth of birds is a subject of constant interest to researchers, and somewhat contradictory data on this issue have accumulated. It is known that the growth patterns of fore- and hind limbs differ during the embryonic and postnatal periods, whereas their increments change rather similarly. The question arises: how are these contradictory trends reconciled, and what role do they play in shaping body proportions? Our study was carried out on Rook embryos and nestlings. We studied the dynamics of changes in head, forelimb, and hind limb length in relation to body mass using regression analysis. Then, the dynamics of relative sizes were compared with the dynamics of relative increments of these traits that we studied earlier. We distinguished four periods in the dynamics of relative sizes, just as we identified earlier in the dynamics of relative increments, when the growth trajectories of traits undergo significant changes. The studied traits scaled mainly with positive allometry in the first period (from the 10th to 13th days of embryogenesis) and third period (from the 5th to 23rd days of the postnatal period), negative allometry in the second period (from the 14th day of embryonic to the 3rd day of postnatal development) and isometrical in the fourth period (from 25th to 30th days of the postnatal period). Despite the roughly consistent changes in an organism within the designated periods, variations in the regression exponents led to changes in proportions. The hind limb had the highest growth rate during embryogenesis, while the forelimb grew faster across most of the nesting period. We assume that changes in the dynamics of body parts' growth are associated with the preparation of organs for function and with the influence of factors that limit the growth of an entire organism.