Timing of mineral sequestration in leg bones of White-tailed Ptarmigan

Birds are unique among vertebrates in that they protect their eggs with rig id, calcium-rich shells. Thus, for a short period of time during the annual reproductive cycle, birds experience extraordinarily high demands for calc ium. Two strategies appear to exist for meeting those temporally high deman ds. Some birds apparently seek out calcium-rich foods immediately prior to and during egg laying whereas others may store calcium in their skeletons o ver a much longer period of time, mobilizing those reserves only when they are needed for production of eggshells. In this study, we used dual energy, X-ray absorptiometry and inductively coupled plasma mass spectroscopy to m onitor annual shifts in bone mineral content in the legs of White-tailed Pt armigans (Lagopus leucurus). The study organisms were known to live on calc ium-poor soils. Despite an apparent shortage of calcium in their diets, the test subjects stored substantial amounts of calcium in their leg bones in months prior to reproduction. Those stores were subsequently depleted durin g the egg-laying period. We suggest ability to store calcium in the skeleto n may afford this species more flexibility in selecting suitable breeding h abitats than would be possible otherwise.