Variations in plasma sex steroids concomitantly to nest building and early breeding phases in an opportunistic breeder, the zebra finch (Taeniopygia guttata).

In seasonally breeding birds, circulating sex steroids typically rise during early breeding and are thought to influence reproductive behaviors through central action. Much less is known about changes in reproductive steroids in relation to breeding activities in opportunistic breeders such as the zebra finch (ZF). In this study we have determined circulating Testosterone (T) and 17β-Estradiol (E2) profiles at different points during early breeding in males and females, respectively, to provide the first detailed report of circulating steroid dynamics concomitantly to early breeding, and in relation to nest building (NB), in this species.

Plasma samples were collected for T (RIA) and E2 (EIA) measurement from two groups of adults (n=16 mixed-sex pairs/group) during single-sex pair housing, after 48 h of male-female pair-housing, on day 2 of NB and 48 hours post-building. Although males’ circulating T was high across both courtship and NB, its level sharply dropped 48h after NB had ceased, contrarily to E2 in females which slowly increased and then plateaued during the period investigated.

Our data suggests that T is transiently increased and specifically associated with courtship and nest building in male ZFs (the building sex in this species), which is in line with previous reports of sex steroid manipulations (Harding et al., 1983, Walters & Harding, 1988). One question our data raises is whether inter-individual differences in T levels relate to inter-individual differences in courtship and NB, and how specific sex steroid sensitive and insensitive regions (Hall et al., 2014) interact together to support NB.