We explored decay rate dynamics in the early decomposition process of Gallus gallus. Our initial hypothesis was that the variation of the mass decay rate is sensitive to the alterations experienced by the carcass during decomposition. In order to establish a framework for the different patterns of carcass alteration, a sample with successive ontogenetic stages (from embryo to adult), set in three environmental conditions (subaerial, burial, and water-submerged), was examined over an 18-day experimental period. Values of the average mass loss and decay rate, and a measure of the overall decomposition rate () for all individuals in the same group and environment are provided. Decay rate was greatest during gas production and expulsion (days 7 to 10) in all age classes, and, at this stage, individuals underwent critical body alterations, in special in subaerial and water environments. The decay process ended at the skeletonization stage, reached after 11 days in the subaerial setting, and after 28 days when specimens were buried. The analysis of decay rate in water does not provide complete information since decomposition was conditioned by the waterlogging of feathers and tissues. The correlation of age and the decay rate indicated that older individuals decayed faster during the first 6 days, before the onset of the gas phase. Also, during the early stages of the process, differences in decay rate were induced by age, while during the gas phase, variation was driven by individual factors. When skin was removed, corpses underwent rapid dehydration, which induced an exponential decrease of the decay rate. By comparison, skin tended to dampen and varied in decay rate.