This study examined flexibility and choice in same-sex pair-bonding behavior in adult zebra finches (Taeniopygia guttata). Zebra finches form life-long monogamous relationships and extra pair behavior is very low, making them an ideal species in which to study same-sex pairing. We examined same-sex behaviors using both semi-naturalistic choice paradigms and skewed sex ratios. In the first experiment, we allowed zebra finches to pair in aviaries with equal sex ratios as part of multiple experiments. On average, 6.4% (N = 78) of unmanipulated pairs were same-sex: all but one was female-female. In a second experiment, we identified pairs from same-sex cages and selected 20 total same-sex pairs (10 of each sex). We then gave pairs a chance to court and pair with members of the opposite sex and observed their behavior for three days. Females did not retain their partner, but most paired with males. In contrast, some males did retain their partner. Similarly, females were more likely to engage in pairing behaviors with males than with their partners or other females whereas males were equally likely to engage in same-sex and opposite-sex pairing behaviors. These findings suggest that same-sex partnerships in zebra finches can be facultative, based on the sex ratio of the group in which they live, but can also be a choice, when opportunities to pair with opposite-sex individuals are possible. Furthermore, it is possible that females are more flexible in this choice of same-sex partnerships than are males.

Opioids are implicated in social attachments, but their role in avian pair bonds is not well understood. The present study tested the effects of naloxone, an opioid antagonist, on pairing using both a forced-choice and a mixed-sex aviary paradigm. First, three doses of naloxone were systemically administered in males using a repeated measures forced-choice design, partner preference formation was tested on the second day. Males treated with 20 mg/kg sang less undirected song. Males treated with 10 mg/kg of naloxone sang less to the familiar partner than when treated with saline and were less likely to form a partner preference than were other treatments. In females, 10 mg/kg of naloxone in a forced-choice paradigm increased preference for the unfamiliar over the familiar male. Finally, males and females were administered either naloxone (10 mg/kg) or saline in a mixed-sex aviary. In females, naloxone increased pairing behaviours, but had no other effects in either sex. Our findings suggest that the effects of naloxone on pairing-related behaviours are context-dependent; male–male competition may decrease the effects of naloxone on male song and a choice of mates may increase affiliation in females in a semi-naturalistic paradigm, and increase preferences for an unfamiliar partner in a forced-choice paradigm. Our findings highlight the importance of using multiple paradigms to test mechanisms of behaviour. These findings contribute to our understanding of the mechanisms of monogamous relationships and suggest that opioids play a role in male courtship, female affiliation and partner preferences in both sexes of zebra finches, but that context is important.