3D reconstruction of high-resolution, micro-computer tomography (CT) scans of the thigh bone (femur) from a female Japanese quail (Coturnix japonica). In this image the viewer looks into the marrow cavity towards the knee. The white outline is the cortical bone tube. The small brown granular structures on the inner surface are the remnants of the medullary bone that previously filled the entire marrow space during egg-laying. Average length of an intact quail femur is approximately 40 mm.
Like many other birds, quails control the timing of their reproduction through a physiological process called photoperiodism. Quails are highly sensitive to day length and use this information to ensure that reproduction occurs during the most favourable season for their offspring (i.e. in the summer). Increase in day length results in a series of hormonal changes leading to greater oestrogen production and fertility. In birds, these hormonal changes also result in the formation of extensive "medullary bone" within the marrow cavity. This medullary bone is essential to supply the large amount of calcium required to form the eggshell.