Estrogen levels influence medullary bone quantity and density in female house finches and pine siskins.

Medullary bone, a non-structural osseous tissue, serves as a temporary storage site for calcium that is needed for eggshell production in a number of avian species. Previous research focusing primarily on domesticated species belonging to the Anseriformes, Galliformes, and Columbiformes has indicated that rising estrogen levels are a key signal stimulating medullary bone formation; Passeriformes (which constitute over half of extant bird species and are generally small) have received little attention. In the current study, we examined the influence of estrogen on medullary bone and cortical bone in two species of Passeriformes: the Pine Siskin (Spinus pinus) and the House Finch (Haemorhous mexicanus).