Development of the Superaltricial Monk Parakeet (Aves, Psittaciformes): Embryo Staging, Growth, and Heterochronies

Knowledge about the embryonic stages of birds is important in answering many questions about development and evolution. We give the first description of 41 embryological stages of the monk parakeet (Myiopsitta monachus) on the basis of external morphology and comparison with the chicken. We also provide measurements of some external morphological characters (i.e. body mass, crown-rump, beak, forelimb, and third toe lengths) and perform comparisons with other precocial and altricial birds with the aim of identifying heterochronous developmental features. The following differences in the development of characters in the monk parakeet when compared with other birds were found: (1) delay of the feathers primordia, (2) wing buds initially greater than leg buds, (3) forelimbs and hindlimbs with similar relative size, (4) retroversion of the toe IV, (5) ventral curvature of the upper jaw, (6) positive regressions between stages and beak length with acceleration and higher values and III toe lengths with deceleration and lower values in the monk parakeet compared to the chicken. The growth pattern of the monk paraket Myiopsitta monachus could be influenced by some heterochronic processes like post-displacement, acceleration and/or deceleration. Results of this research allow the standard identification of stages in different species of parrots, recognize similarities and differences between precocial (the chicken) and altricial species (Myiopsitta), and provide planning data for future studies.