Food availability, energetic constraints, and reproductive development in a wild bird: Central Arizona Phoenix

For many animals, the seasonal timing (phenology) of gonad cycles is an important adaptation to local environmental conditions. The available evidence suggests that birds adjust to urban areas by advancing the phenology of vernal gonad growth. To test whether the habitat-related disparity in gonad growth phenology is due to energetic constraints, we measured energetic status (i.e., body condition and fat stores) and hypothalamic levels of neuropeptide Y (NPY, a neuropeptide that potentially links food abundance to reproductive endocrine activity) of urban and desert Abert’s Towhees, Melozone aberti. To shed light on the physiological mechanism underlying variation in gonad growth phenology, we compared the (neuro)endocrine activity of the reproductive system at the hypothalamic (gonadotropin -releasing hormone [GnRH] and -inhibitory hormone [GnIH]) and gonadal (baseline plasma testosterone [T] and ‘GnRH challenge’-induced) levels, as well as the morphology dependent on these processes (paired testes mass, seminiferous tubule diameter, and cloacal protuberance width).