UPTAKE OF INGESTED CALCIUM DURING EGG-PRODUCTION IN THE ZEBRA FINCH (TAENIOPYGIA-GUTTATA)

Small passerines forage for calciferous material on a daily basis duri ng egg laying, but beyond this general observation, mechanisms of calc ium uptake are poorly understood. I investigated calcium uptake during egg laying in Zebra Finches (Taeniopygia guttata) by administering a 5-mu Ci dose of radioisotopic calcium (Ca-45) by proventricular intuba tion exactly 1 h after oviposition. A nonlaying (control) female was d osed at the same time as each egg layer. Egg layers excreted less of t he dose (<0.7 mu Ci) than controls (>1 mu Ci) over the entire ovulator y cycle. Egg layers incorporated more calcium into their skeletons tha n controls during the first 16 h post-dosing, but localization was sim ilar to that of controls 16 to 24 h after the dosing period. Calcium w as more than 60 times more abundant in the reproductive tissues of egg layers than in controls 8 to 16 h after the dosing period, suggesting that the majority of egg calcification occurred during this period. T he decline in skeletal incorporation of Ca-45 16 to 24 h after dosing may indicate mobilization of medullary-bone reserves to supply the cal cium needed to complete shell secretion. Evidence from a number of avi an species suggests that daily ingested calcium is essential for egg f ormation; my results show in quantitative terms the fate of ingested c alcium during egg formation in the Zebra Finch.