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Small passerines forage for calciferous material on a daily basis during egg laying, but beyond this general observation, mechanisms of calcium uptake are poorly un- derstood. I investigated calcium uptake during egg laying in Zebra Finches (Taeniopygia gut- tata) by administering a 5-p.Ci dose of radioisotopic calcium (45Ca) by proventricular intu- bation exactly 1 h after oviposition. A nonlaying (control) female was dosed at the same time as each egg layer. Egg layers excreted less of the dose ( 1 p.Ci) over the entire ovulatory cycle. Egg layers incorporated more calcium into their skeletons than controls during the first 16 h post-dosing, but localization was similar to that of controls 16 to 24 h after the dosing period. Calcium was more than 60 times more abundant in the re- productive 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. The decline in skeletal incorporation of 45Ca 16 to 24 h after dosing may indicate mobilization of medullary-bone reserves to supply the calcium needed to complete shell secretion. Evidence from a number of avian species suggests that daily ingested calcium is essential for egg formation; my re- sults show in quantitative terms the fate of ingested calcium during egg formation in the

Uptake of Ingested Calcium during Egg Production in the Zebra Finch (Taeniopygia guttata)