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This paper discusses phenotypic plasticity of the avian intestine and presents data about the dynamics and mechanisms accounting for size variation of the avian intestine. Gut lengths of 56 bird species were compared for interspecific and intraspecific coefficients of variation. Intraspecific gut length variation relates to seasonal changes in quality and composition of food in some species. Such phenotypic plasticity is thought to adjust gut structure and function to seasonal differences in nutrition and to maintain optimized gut function under differing feeding regimes. Morphometry at five segments along the intestine provided basic measurements of the quail intestine. Whereas the duodenum can be separated from the rest of the intestine in almost all parameters, the small intestine, rectum, and caeca are rather uniform. Changes of circumference and length of the gut segments account for the major portion of size changes of the resorptive surface. BrdU/3H-TdR double labelling experiments were performed to study the dynamics of cellular turnover of the intestine. The experiments revealed a constant S-phase length of 6.5 hours along the intestine. The turnover time of the mucosa epithelium ranges between 9 and 17 days, depending on the circumference of the gut segment. Such a turnover time permits reconstruction of the intestine in short periods and its adjustment to seasonal differences in composition and quality of food. The study revealed significant differences between cell proliferation activity at day and at night. Animals labelled at night had almost twice the labelling index of those labelled during the day. These results were consistent in other tissues (liver, muscle, connective tissue, glandular stomach). Because high night-time proliferation activity coincides with low antigen levels and high free radical scavenger concentration (e.g. melatonin),it is suggested that the circadian patterns in labelling index represent a fine tuning of DNA-replication activity to ‘safer’periods of day.