Sustainability of poultry farming relies on the development of more efficient and autonomous production systems in terms of feed supply. This implies a better integration of adaptive traits in breeding programs, including digestive efficiency, to favor the use of a wider variety of feedstuffs. The objective of the study was to better characterize the kinetics of development of the digestive tract in broilers, in relationship with digestive efficiency by measuring various digestive parameters as well as serum color. Absolute and relative growth of gastrointestinal tract organs were compared between 2 divergent chicken lines selected on digestive efficiency (AMEn) during 7 wk of development. We show that as early as 7 d of age, these 2 lines differs for several organs developments and that these differences remain visible later on. In addition, the allometry of the gizzard and intestine segments is different between the 2 lines, with efficient birds putting more effort in the upper part of the digestive tract during postnatal development and lessefficient birds putting more effort in the lower part of the gastrointestinal tract. Interestingly, we also showed that differences in serum pigmentation, which is a good biomarker for digestive capacity, could be a convenient diagnostic tool to discriminate between chickens with high or low digestive efficiency at early stages of development. In conclusion, this study showed that selection of chickens for AMEn had large impacts in gastrointestinal development including at early stages and is a valuable resource for further studies on the genetic and physiological control of the response of the animal to feed variations.