Abstract
Endogenous serum testosterone, estradiol, and triiodothyronine concentrations were measured in the northern shoveler (Anas clypeata, n = 20) during different stages of migratory condition at their wintering grounds to look at how these hormones may be linked to the development of migratory condition. These hormones were assayed by enzyme-linked immunosorbent assay (ELISA). The changes in hormones concentrations were parallel to each other and correspond to the increasing day lengths of spring. Furthermore, the northern shovelers showed a distinct seasonal cycle in serum concentrations of triiodothyronine, estradiol, and testosterone. It is suggested that these seasonal cycles in the northern shovelers are endogenously programmed and their entrainment to the environmental photoperiod ensures the occurrence of different physiological functions at temporally fixed time of the year. The results of this study offer a captive model of the seasonal changes in physiology and behavior that occur in the months preceding and including vernal migration of the northern shovelers. J. Exp. Zool. 317:216226, 2012. (C) 2012 Wiley Periodicals, Inc.