Females need a bond: what can we learn about calling behavior and opportunistic breeding?

Opportunistic breeders typically express reproductive behaviors only when specific environmental conditions co-occur. In this respect, food and water availability have been described as major cues for many species. But, can social bonds influence an individual to respond to these cues by breeding? Is this reflected in their hormonal response? We tackled these questions by investigating soft call usage and sex steroid hormones in male (n=15) and female (n=18) captive zebra finches (Taeniopgygia guttata). We formed same-sex trios and equipped the birds with miniature wireless microphones that continuously recorded individual vocalisations during the eight days of experimentation. During the first four days, we simulated a “drought-like” environment and then triggered breeding onset by adding a variety of food, excess water, green plants, nesting material and nest boxes for an additional four days. Hormonal levels were measured at the beginning and at the end of the experiment and daily focal observations were made. The strongest bond within each trio was identified and the bird’s social status was classified as “bonded” or “single”. Males increased the amount of breeding calls after the breeding onset, but no physiological changes were detected independent of social status. Intriguingly, all females showed a physiological response towards the breeding onset, but only females with a bond changed their vocal behavior consequently to the breeding onset. Overall, our findings reveal sex-specific strategies and sensitivity towards breeding triggers, and a decoupling between vocal and physiological adjustment in response to the breeding onset.