Author Institution: University of Illinois, Champaign, and Metropolitan Park District, Cleveland, OhioThe thyroid glands of the house sparrow and other small birds in the Cleveland region have greater secretory activity during late autumn and winter than during late spring and summer. Evidence for this is the presence, during the winter, of high epithelial cells surrounding the follicles, of smaller follicles, and of lesser volumes and weights of the whole thyroid. Large-sized species have larger thyroids, both absolute and relative to body weight, than do small species. With certain precautions, changes in the size of the thyroids serve as a useful index of inverse variations in secretory activity intra-specifically, but not inter-specifically
