PATTERNS OF MOBILIZATION OF CALCIUM, MAGNESIUM, AND PHOSPHORUS BY EMBRYONIC YELLOW-HEADED BLACKBIRDS (XANTHOCEPHALUS-XANTHOCEPHALUS)

Embryonic blackbirds (Xanthocephalus xanthocephalus) obtain most of their calcium from the eggshell (85-90%), but all of their phosphorus comes from reserves in the yolk (80-85%) and albumen (15-20%). Approximately equal amounts of magnesium are supplied by the eggshell, the yolk, and the albumen. Yolk is depleted of magnesium and phosphorus during embryogenesis, but excess calcium absorbed from the eggshell is stored in the yolk. Consequently reserves of calcium in the yolk actually increase 8-fold during embryonic development. Our results reveal that altricial birds manifest patterns of mobilization and deposition of calcium and other elements similar to those described for precocial species. Evolution of altriciality from precocity evidently did not entail major changes in how embryonic birds meet the challenge of obtaining the calcium, magnesium, and phosphorus required for development.