Every spring, when millions of small songbirds are returning from their southern wintering grounds to begin breeding, these birds are struggling to garner as many resources as possible to allow the earliest and most prolific reproduction they can manage. Although we have methods for assessing the protein and fat stores of swallows on living specimens, we have not previously been able to assess the calcium stores without sacrificing the bird. Schematic experimental arrangement for radiography measurements at D-line.
Calcium is likely to be important, because it is one of the principal constituents of egg shells.
The amount of calcium that must be shunted through the laying females' system and deposited in shells every day is far in excess of the amount that she could obtain from the environment. All birds must rely on stores of calcium that can be built up before the breeding season and rapidly depleted during laying. Previous research on pigeons and chickens has indicated that the principal site for calcium storage is in the hollow interiors of their leg bones. Analysis is being done after digitizing the recorded information. The spatial resolution obtained is about 15 mm.