This paper introduces the comparison of altricial and precocial birds as a model system for the study of patterns of intestinal growth. It reports on comparative morphometric and cytokinetic studies of intestinal growth in altricial European starling (Sturnus vulgaris) and precocial Japanese quail (Coturnix coturnix japonica). Morphometric data at five gut segments of neonates and adults of both species are provided to evaluate the growth of the intestine. A double labelling technique combining 3H-thymidine autoradiography with 5-bromo-2'-deoxy-uridine immunohistochemistry gives labelling index, length of S-phase, and tissue turnover time for hatchlings and adults of both species. The results of morphometry show a similar pattern of intestinal growth in both species, a strong dependency of intestinal growth on body size, and a clear topographic separation of areas of cell proliferation from the functional mucosal epithelium. The results of the cytokinetic experiments indicate a constant length of the S-phase independent of species, development, and age. The 3H-thymidine labelling index is significantly higher in neonate quail than in neonate starling. Between adult animals of both species, the labelling index goes not differ significantly except for the duodenum which has a higher index in starling. The turnover time necessary to replace the mucosal epithelium is 3 days in hatchlings of both species and ranges between 10 and 17 days in adults. A diurnal pattern of cell proliferation, as previously reported in adult quail, is not present in neonate quail. Thus, differences in intestinal growth among altricial and precocial birds are primarily based on size differences of proliferation areas whilst cellular proliferation rates have only minor impact on intestinal growth. The short turnover time and the high rates of proliferation suggest that the intestine is a flexible organ that may be adjusted in size and function to the actual needs of the growing bird.