Developmental changes in digestive physiology of nestling house sparrows, Passer domesticus

Six decades of studies have speculated that digestive capacity might limit avian growth rate or that developmental changes in the gut might determine developmental changes in digestive efficiency. However, there are no studie s on digestive enzymes during avian development, except for studies on main ly domestic birds that exhibit the precocial mode of development. We studie d alimentary organ masses, intestinal enzyme activities (sucrase, maltase, isomaltase, aminopeptidase-N), and pancreatic enzyme activities (amylase, t rypsin, chymotrypsin) during development of a wild passerine bird exhibitin g the altricial mode of development. Wild nestling house sparrows were stud ied immediately after removal from the nest (days 0, 3, 6 of age; day 0 = h atch), whereas captives were raised in the laboratory beginning day 3 on a formulated casein/starch-based diet until fledging age (after day 12). Dige stive biochemistry was dynamic. Tissue-specific activities of some digestiv e enzymes continued to increase through fledging, by > 10 times in some cas es (e.g., sucrase and maltase in midintestine). Total pancreatic amylase ac tivity increased 100 times between hatch and day 12 through a combination o f increases in tissue-specific activity and pancreas mass. House sparrows d iffer from poultry, in whom after about 2 wk of age the specific activity o f intestinal and pancreatic digestive enzymes is generally constant or decl ines during development. The data on intestinal and pancreatic enzymes help explain why digestive efficiency of nestling house sparrows improves with age, and the data seem consistent with the idea that digestive capacity mig ht limit feeding rate and hence growth rate.