Variation in plasma sex steroids through courtship and nest building 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 behaviours by acting on the brain. Much less is known about how reproductive steroids change through the early breeding phases in opportunistic breeders such as the zebra finch. To start answering this question, in this study we determined circulating Testosterone (T) and 17β-Estradiol (E2) profiles at different timepoints across early breeding in captive males and females, respectively.

Plasma samples were collected 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 nest building and 48 hours post-building. Although males’ circulating T was high across both courtship and nest building, its level sharply dropped 48h after building had ceased, while E2 in females increased and then plateaued during the whole period.

Our data provides the first detailed report of circulating steroid dynamics during early breeding, and in relation to nest building, in this species, and raises the question whether inter-individual differences in T levels relate to inter-individual differences in behaviour(s), and how specific sex steroid sensitive and insensitive regions interact together to support nest building.