Medullary bone is phylogenetically widespread and its skeletal distribution varies by taxon

Female birds produce medullary bone to offset heightened calcium demand during egg shelling. The ephemeral nature of medullary bone hampers its observation in wild birds; thus, its phylogenetic distribution and ecophysiological correlates remain controversial. To address this, I candled a phylogenetically broad sample of bird skeletons spanning a large range of body sizes. I hypothesized that the presence of medullary bone would reduce or eliminate light penetrance through long bones. Candling suggested medullary bone was present in many adult female specimens, which was confirmed visually or with digital X-rays. This survey yielded the first unambiguous evidence of medullary bone in several taxa, including hummingbirds, tinamous, megapodes, and kiwi. The phylogenetic distribution of these new occurrences confirms that medullary bone is phylogenetically widespread among birds, including palaeognaths and passerines. The rapid assessment of complete skeletons supports a relationship between medullary bone and hematopoietic marrow, and establishes that the skeletal distribution of medullary bone varies taxonomically. Because the presence of medullary bone confirms sex, maturity, and reproductive status, its detection in specimens of known collection date can improve our understanding of life history and reproductive timing in wild birds, even decades later.