Do molting birds renovate their skeletons as well as their plumages? Osteoporosis during the annual molt in sparrows

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Abstract
Four decades ago two independent investigators reported a pronounced osteoporosis during the molt in several species of birds. With the exception of reports of statistically stable dry masses of bones in molting waterfowl this cyclic osteoporosis has received little additional attention. Using scanning electron microscopy and light microscopy we surveyed the tarsometatarsi and tibiotarsi of White-crowned Sparrows during winter and during the molt. A subtle change in bone metabolism appears coincident with the postnuptial molt in well-nourished White-crowned Sparrows. This is evident from (i) the appearance of the bone in cross section, (ii) decreases during the molt in bone density (g/mL), and (iii) increases during the molt in both the ratio of marrow-cavity area to total bone area and in the void area per unit area of cortical bone. Renovation of the bone during the molt in White-crowned Sparrows, however, appears either to proceed at a slower pace or to be less extensive than previously reported for other avian species.

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