Testosterone production in non-breeding Northern Cardinals (Cardinalis cardinalis): is temperature influential?

In seasonally breeding birds annual changes occur in hormone production by the hypothalamic-pituitary-gonadal (HPG) axis as individuals transition between breeding and non-breeding condition. Typically, some external environmental cue induces the observed changes; changes in day length, corresponding to change of seasons, are a strong cue for HPG axis activity in many species. Some species use cues other than day length as their primary initiator of breeding, and a number of light-sensitive species incorporate other cues to initiate breeding. Northern Cardinals (Cardinalis cardinalis) exhibit individual variation in when breeding begins and circulating testosterone is present throughout the year. Chemical tests of HPG axis responsiveness indicate that many individuals have an activated HPG axis as early as December. Here we use GnRH injections to assess whether individuals with active HPG axes prior to the winter solstice and increasing day lengths, may be influenced to initiate breeding by low temperatures experienced at the time of GnRH challenge. We found that while a significant number of individuals had active HPG axes prior to the winter solstice, there was no effect of low temperatures. We suggest that broader environmental patterns and additional cues may be influential to breeding in cardinals as opposed to only immediate temperature changes. Received 9 July 2013. Accepted 24 December 2013.