Same-sex sexual behavior has been documented in more than 130 bird species worldwide, spanning gulls, terns, albatrosses, geese, flamingos, penguins, and many others. These behaviors range from one-off courtship displays to long-term pair bonds in which two males or two females build nests, incubate eggs, and raise chicks together. Whether that qualifies as “gay” in the human sense is a separate question, but the behaviors themselves are real, well-documented, and far more common than most people assume.
How Common Is Same-Sex Behavior in Birds
Researchers have recorded same-sex sexual interactions in birds on every continent where ornithological fieldwork happens. A widely cited figure, drawn from field reports compiled over decades, puts the tally at over 130 species exhibiting some form of same-sex courtship, mounting, pair bonding, or co-parenting.1Behavioral Ecology. Same-sex sexual behavior in birds: expression is related to social mating system and state of development at hatching That number almost certainly underestimates reality. Same-sex behavior in birds has historically been underreported because many researchers treated it as irrelevant noise, a fluke, or even an embarrassment in their data. A 2024 review in the Journal of Avian Biology noted that despite its widespread occurrence, same-sex behavior “remains poorly understood, often dismissed as maladaptive or the result of errors in sex discrimination.”2Journal of Avian Biology. Same‐sex partnerships in birds: a review of the current literature and a call for more data
The behaviors span a wide spectrum. On one end you have brief same-sex mounting, which can look opportunistic or accidental. On the other end, there are stable same-sex partnerships lasting multiple breeding seasons, complete with joint territory defense, nest-building, egg incubation, and chick-rearing. It is the latter category that has attracted the most scientific interest, because long-term pair bonds are costly in time and energy, and birds that maintain them appear to be doing so deliberately rather than making a momentary mistake about their partner’s sex.
The Laysan Albatross and the Most Famous Female-Female Pairs
The most thoroughly studied example of same-sex pairing in birds comes from the Laysan albatross colony at Kaena Point on Oahu, Hawaii. A landmark 2008 study found that roughly 31% of breeding pairs in this colony were female-female.3PubMed Central. Successful same-sex pairing in Laysan albatross That is not a typo. Nearly a third of the pairs raising chicks on that stretch of coastline consisted of two females cooperating to incubate eggs and fledge young.
This was possible in part because the colony’s sex ratio was heavily skewed: about 59% of the birds were female, the result of female-biased immigration into the area. With fewer males to go around, many females faced a choice between pairing with another female or not breeding at all. Laysan albatrosses are obligate biparental species, meaning a single bird cannot successfully incubate an egg and feed a chick alone. A female-female pair was the next best option.
And it worked, to a degree. Female-female pairs did fledge fewer chicks on average than male-female pairs, but they fledged more than zero, which is what unpaired females would have managed. Genetic testing revealed something telling: in most female-female pairs that raised chicks across multiple years, at least one offspring was genetically related to each female in the pair.3PubMed Central. Successful same-sex pairing in Laysan albatross Both females were getting copulations from males outside the pair and then bringing fertilized eggs back to the shared nest. The partnership itself was between two females, but the genetic contribution of males was secured through extra-pair mating. Far from being a reproductive dead end, female-female pairing functioned as a pragmatic reproductive strategy.
Sex Ratios and the Demographics Behind Same-Sex Pairing
The Laysan albatross story points to a pattern that shows up in other species too: skewed sex ratios create the conditions for same-sex pairs. When one sex substantially outnumbers the other in a breeding population, the “surplus” individuals face limited options. In socially monogamous species, where successful reproduction typically requires a partner, those surplus birds can either sit out the breeding season entirely or pair with a member of their own sex.
Roseate terns provide another illustration. Researchers studying a colony at Bird Island, Massachusetts, found that the sex ratio among breeders was roughly 127 females for every 100 males. About 20% of breeding females did not have male mates. Many of these females formed female-female pairs, which were frequently identifiable by their “supernormal clutches” of three or four eggs instead of the typical two. These oversized clutches, comprising 1% to 7% of all clutches in the colony since at least 1970, were usually attended by female-female pairs.4Ibis. Consequences of a female‐biased sex‐ratio in a socially monogamous bird: female‐female pairs in the Roseate Tern Sterna dougallii The researchers concluded that these females had higher fitness by pairing together and raising a few young than by not breeding at all, even though the females involved tended to be of lower “phenotypic quality” based on late laying, smaller clutches, and smaller eggs.
This pattern raises a fair question: if same-sex pairing is just a backup plan when the sex ratio is off, does it really count as anything more than demographic necessity? The honest answer is that demographics clearly drive some of these pairings but not all of them. Same-sex behavior also occurs in populations with balanced sex ratios, and some individuals in skewed populations continue to pair with same-sex partners even when opposite-sex mates become available. Demography is one piece of the puzzle, not the whole thing.
Parental Care and Why Mating Systems Matter
A different line of research approaches the question from the angle of mating systems rather than population demographics. A study published in Animal Behaviour examined same-sex behavior across bird species and found a correlation with how parental duties are divided between the sexes. In species where males provide little parental care and instead invest their energy in acquiring multiple mates, rates of male same-sex behavior were higher. In more monogamous species with roughly equal parental investment, same-sex behavior was less frequent.5Animal Behaviour. Homosexual behaviour in birds: frequency of expression is related to parental care disparity between the sexes
The proposed explanation is that in species where males are essentially “freed up” from parental duties, they engage in more mating behavior across the board, and some of that mating activity ends up directed at other males. In this framing, same-sex behavior is a byproduct of heightened overall sexual activity rather than a targeted preference. Males in these species mount frequently and somewhat indiscriminately because the cost of doing so is low and the chance of missing a genuine reproductive opportunity is high if they are too selective.
This is a plausible explanation for brief same-sex mounting in promiscuous species, but it does a poor job explaining the long-term same-sex pair bonds seen in albatrosses or geese. A goose that maintains a partnership with another male for years, defending territory and co-parenting adopted offspring, is doing something quite different from a male that mounts another male once and moves on. The mating-system explanation covers part of the variation but breaks down for the most committed same-sex partnerships.
What Hormones Do and Do Not Explain
One of the first places people look for an explanation of same-sex behavior is hormones. The intuition is straightforward: maybe birds that pair with same-sex partners have unusual levels of sex steroids like estrogen or testosterone. The evidence, however, is surprisingly thin.
A study on zebra finches tested what happened when researchers chemically blocked both androgens and the enzyme that converts testosterone to estrogen. The expectation was that disrupting sex hormones would change courtship and pairing behavior. It didn’t, much. The treatment had few effects on courtship, pairing success, or the likelihood that a bird would be chosen as a mate by an opposite-sex partner.6Hormones and Behavior. The role of sex steroids in courtship, pairing and pairing behaviors in the socially monogamous zebra finch This suggests that while sex hormones play some role in shaping courtship displays in males, they do not appear to be the master switch controlling who a bird pairs with. Pair-bond formation in zebra finches operates somewhat independently of the hormonal machinery that drives sexual differentiation.
That said, hormones clearly are not irrelevant in all contexts. The picture is complex, and the mechanisms governing avian social and sexual behavior involve interactions between hormones acting during early development and those circulating in adulthood. The current state of the science is that there is no simple “hormone imbalance” story for same-sex behavior in birds. The pairing decisions birds make appear to involve neural and social mechanisms that go beyond what circulating hormone levels alone predict.
Mercury, Endocrine Disruption, and an Unsettling Experiment
While natural hormone variation does not neatly explain same-sex pairing, there is one environmental factor that has been shown to alter mate choice in a striking way: mercury contamination. A study on white ibises exposed captive birds to environmentally relevant levels of methylmercury, the form of mercury that accumulates through food chains and is found in contaminated wetlands. The results were dramatic. Male ibises exposed to mercury showed altered courtship behavior, reduced reproductive success, and increased rates of male-male pairing. These changes were linked to a pattern of endocrine disruption in which dosed males showed a shift in hormone profiles.7PubMed. Endocrine disruption in white ibises (Eudocimus albus) caused by exposure to environmentally relevant levels of methylmercury
This finding matters for two reasons. First, it demonstrates that environmental contaminants at realistic exposure levels can shift mate-choice behavior in birds, which has conservation implications for species living in polluted habitats. Second, and more subtly, it complicates the picture of what “causes” same-sex behavior. In the ibis study, male-male pairing was clearly induced by an outside chemical stressor. But the naturally occurring same-sex pairs in albatrosses and terns show no evidence of contaminant-driven endocrine disruption. Using the mercury study to claim that same-sex behavior in birds is “caused by pollution” would be a serious misreading of the evidence. The mercury finding is one pathway, specific to one set of conditions, that can lead to same-sex pairing. It is not the explanation for the phenomenon across 130-plus species.
Why “Gay” Might Not Be the Right Word
Applying human sexual-orientation categories to birds is tempting but scientifically slippery. When researchers describe a human as gay, they refer to an enduring pattern of attraction to the same sex that persists across contexts, regardless of partner availability. Birds exhibiting same-sex behavior do not always fit that description. Many females in same-sex pairs still mate with males for fertilization. Some birds in same-sex partnerships switch to opposite-sex partners when the demographics shift. Others maintain same-sex bonds even when opposite-sex mates are available. The behavioral range is wide enough that no single label captures it neatly.
Researchers working in this area generally prefer terms like “same-sex sexual behavior” or “same-sex partnerships” precisely because these are descriptions of observable actions rather than inferences about internal states. We can observe that two female albatrosses share a nest for five years. We cannot know whether either bird experiences something like sexual attraction to the other, or whether the arrangement is closer to a practical housing agreement between cooperating adults. The field is careful about this distinction not because same-sex behavior in birds is somehow less real than human homosexuality, but because projecting subjective experience onto animals is not something behavioral science can do rigorously.
That said, the old default of dismissing all same-sex behavior as “mistakes” or “confusion” is losing credibility. The review in the Journal of Avian Biology explicitly challenged the assumption that same-sex partnerships are just errors in sex discrimination. When a pair bond persists for years, through multiple breeding seasons, with both partners investing heavily in shared offspring, calling it a mistake stretches the word past its useful meaning.2Journal of Avian Biology. Same‐sex partnerships in birds: a review of the current literature and a call for more data
The Evolutionary Puzzle
Same-sex behavior presents a genuine puzzle for evolutionary biology because, on the surface, it does not contribute to reproduction. Any behavior that diverts time, energy, or mating effort away from producing offspring should, in theory, be weeded out by natural selection over time. And yet same-sex behavior persists widely across the animal kingdom, not just in birds but in mammals, reptiles, fish, and insects.8PubMed Central. Adaptive value of same-sex pairing in Laysan albatross
Several explanations have been proposed, and they are not mutually exclusive. The demographic argument, already described through the albatross and tern examples, suggests that same-sex pairing can be the best available reproductive strategy when mates of the opposite sex are scarce. Another explanation frames same-sex behavior as a low-cost byproduct of selection for strong mating drive: if being highly motivated to mate increases reproductive success overall, the occasional same-sex interaction is just statistical noise that natural selection tolerates because the net effect is still positive. A third line of thinking focuses on social benefits. In some species, same-sex alliances may confer advantages through shared territory defense, cooperative foraging, or enhanced social status.2Journal of Avian Biology. Same‐sex partnerships in birds: a review of the current literature and a call for more data
The honest state of the field is that no single evolutionary explanation covers all cases. Same-sex behavior in a promiscuous shorebird whose males briefly mount other males is probably a different phenomenon, evolutionarily speaking, from a lifelong female-female albatross pair bond. Lumping all same-sex behavior into one bucket and demanding one explanation is a bit like asking “why do birds sing?” when singing serves different functions in different species. The question itself may be too broad for a single answer.
Same-Sex Pairs in Captive Breeding Programs
Same-sex pairing has practical consequences for conservation, particularly in captive breeding programs for endangered species. When you have a small population of birds in captivity and desperately need every individual to breed, discovering that two of your birds have formed a same-sex pair and are occupying a nest site without producing fertile eggs is a problem. This is especially acute in species where males and females look nearly identical, making visual sex determination unreliable.
The endangered black stilt of New Zealand provides one example. Captive management of this critically endangered wading bird has been complicated by the difficulty of correctly sexing individuals. Same-sex pairs forming in captivity have been explicitly identified as a management challenge, because they tie up breeding-age birds in unproductive partnerships and take up limited aviary space.9Biological Conservation. Captive management and molecular sexing of endangered avian species: An application to the black stilt Himantopus novaezelandiae and hybrids Molecular sexing techniques, which determine sex from DNA in blood or feather samples, have become standard tools in captive breeding precisely because relying on behavior or appearance to assign sex sometimes results in same-sex pairs going undetected for months or even breeding seasons.
Zoos and conservation programs have also learned that simply breaking up same-sex pairs and introducing an opposite-sex partner does not always work smoothly. Birds that have formed strong pair bonds resist disruption, just as opposite-sex pairs do. Some programs have had success by introducing same-sex pairs to eggs or chicks that need foster parents, turning what appears to be a management problem into a parenting resource. This approach has been used with various penguin species in captivity, where same-sex male pairs have successfully incubated donated eggs and raised chicks. The willingness of these pairs to invest in parental care is genuine and functional, even if the pairing itself does not produce genetic offspring.
What Researchers Still Want to Know
For all the attention same-sex behavior in birds has received in popular media, the scientific literature is thinner than you might expect. Most of the detailed case studies involve a handful of well-studied colonial species like albatrosses, gulls, and terns where large nesting colonies make behavioral observation relatively easy. Same-sex behavior in solitary or forest-dwelling species is much harder to detect and has been barely studied. The actual prevalence across all bird species is almost certainly higher than the documented 130-plus figure suggests, but how much higher is genuinely unknown.
Researchers have also called for more consistent data collection. Many older field studies did not systematically record same-sex interactions, and some ornithologists reportedly omitted them from published accounts. The Journal of Avian Biology review explicitly called for more data, noting that the field needs standardized reporting of same-sex partnerships to move beyond anecdotal accounts toward a systematic understanding of how common these behaviors are, what forms they take across different ecological contexts, and whether the same individuals show consistent same-sex preferences over time or shift between same-sex and opposite-sex partners depending on circumstances.2Journal of Avian Biology. Same‐sex partnerships in birds: a review of the current literature and a call for more data Until that data exists, the science of same-sex bird behavior remains a field with more compelling case studies than general principles.