Birds form same-sex pairings, a phenomenon documented in over 130 avian species worldwide. This behavior includes same-sex sexual interactions, complex courtship rituals, and long-term social pair bonding. These pairings are virtually indistinguishable from male-female pairings, involving consistent behavioral displays, mutual affection, and cooperative living arrangements between two individuals of the same biological sex.
Observing Same-Sex Pairings in Avian Species
Same-sex pair bonding mirrors the full suite of behaviors observed in opposite-sex pairs, extending beyond transient sexual encounters. These pairings involve mutual preening, synchronized vocalizations, and joint defense of a territory or nesting site. For instance, male Black Swans (Cygnus atratus) form long-lasting bonds, with estimates suggesting up to one-quarter of all pairings in some populations are male-male.
Male swan pairs perform courtship displays and mating behaviors similar to heterosexual couples, demonstrating a deep social connection. Female-female pairings are prominent in species like the Laysan Albatross (Phoebastria immutabilis). In one colony on Oʻahu, around 31% of all pairs were found to be female-female, with their bonds lasting for many years.
Other examples include various penguin species, such as the Adelie penguin, where same-sex courtship and pairing were documented as early as 1912. Same-sex bonds in species like the zebra finch are as intense and stable as male-female relationships, with pairs sharing nests and exhibiting strong affiliative behaviors. These pairings demonstrate that birds actively choose a social partner for reasons beyond immediate reproduction.
Hypotheses for Non-Reproductive Pair Bonding
The persistence of same-sex pair bonding, which does not result in direct genetic reproduction, has led scientists to develop several hypotheses focused on social and ecological advantages. One explanation, the practice hypothesis, suggests that same-sex interactions allow young, inexperienced birds to practice complex courtship and pairing behaviors. This practice may increase their competence for successfully establishing a heterosexual pair bond later in life.
Another framework centers on social status or dominance signaling. Engaging in same-sex mounting or displaying aggressive pair behavior can help establish a bird’s social ranking within the flock or colony. Such dominance can secure better access to shared resources, like food or preferred nesting sites, providing an indirect survival benefit.
A drive for social bonding provides a compelling explanation, particularly in socially monogamous species where a cooperative partner is necessary for survival. For birds like the zebra finch, the advantage of finding a long-term social partner for cooperative defense and resource sharing may outweigh the immediate need for an opposite-sex mate. In environments with a skewed sex ratio, finding a same-sex partner may be the best available option for forming a cooperative unit.
Cooperative Parenting and Nesting Roles
Same-sex pairings often translate into functional, cooperative units capable of successfully raising offspring. Male Black Swan pairs, for example, exhibit a higher success rate in fledging cygnets (about 80% compared to 30% for male-female pairs) when they acquire eggs. These pairs typically obtain eggs by briefly forming a trio with a female before driving her away, or by stealing eggs from an opposite-sex pair.
The two males can more effectively defend a larger territory and secure better nesting locations, sharing the duties of incubation and chick-rearing more evenly. For female Laysan Albatrosses, same-sex pairs often form due to a shortage of available males caused by female-biased immigration into the colony. In this arrangement, one or both females mate with an already-paired male from another nest, resulting in an egg incubated and raised by the two females.
Although these female-female pairs have a lower overall reproductive success rate than male-female pairs, the arrangement is a better alternative than not breeding at all. The cooperative effort allows both females to share the demanding parental care required by the long-lived seabirds. This cooperative parenting demonstrates that same-sex pair bonds can be an adaptive strategy, enabling individuals to reproduce and secure resources even under challenging demographic conditions.