Same-Sex Pairing in Birds: What the Nests With Too Many Eggs Revealed

In the early 1970s, on Santa Barbara Island off the coast of southern California, George and Molly Hunt had a problem with the arithmetic. Western Gulls (Larus occidentalis) normally lay a clutch of two or three eggs. Some of the nests they were finding held five or six, and birds were recorded incubating as many as seven. These oversized clutches, which ornithologists call supernormal, were not new to science, and the usual assumption was that a second female had dumped her eggs into a nest that already belonged to someone else.

The Hunts checked, and the assumption did not hold. Two details gave it away. The eggs in the supernormal clutches were laid at shorter intervals than eggs in normal clutches, which is impossible for one female and exactly what you would expect from two laying at the same time. And when they looked at who was attending those nests, they found territories with no resident male at all. The pairs holding them were two females, and they were staying together from one year into the next.

What the gull study actually found

The Hunts published the result in Science in 1977, in a short report that has been cited and re-cited ever since. Female-female pairs were a substantial minority of the pairs on the island, and figures between 10% and 14% circulate in later writing about it. We would treat any precise percentage carefully, because the numbers quoted today generally come from downstream literature rather than from the original two-page report.

Two further findings matter for understanding the phenomenon. Most of the eggs in these clutches were infertile, which tells you the females were not routinely obtaining sperm, though some did, since a proportion of eggs hatched. And in three of the pairs, one female performed behaviours that observers had previously recorded only in males. The pair bond was not a partial or degraded version of the normal arrangement. It was the full behavioural package, running with two females in it.

What happened to the finding

The paper arrived in 1977, in the middle of a fierce public argument in the United States about homosexuality, in which the claim that it did not occur in nature was doing a great deal of rhetorical work. A study documenting stable female-female pairs in a wild animal was therefore never going to stay in the ornithological literature.

It did not. The finding was picked up by the national press, argued over by people with no interest in gulls, and George Hunt received a large volume of mail from both directions, gratitude from some correspondents and fury from others. In 1979 the musician Tom Wilson Weinberg wrote a song called "Lesbian Seagull," which reached a far wider audience seventeen years later when it appeared in the film Beavis and Butt-Head Do America. The gulls have had a second life in podcasts and popular science writing ever since.

We report this because it is part of the story and because it explains why the science is less widely understood than the controversy. It is also a reminder worth carrying: the birds were doing this long before anyone published on it, and they were entirely indifferent to what it was taken to mean.

The albatrosses, and the arithmetic underneath

The gull study is the famous one. The best-quantified case is elsewhere.

At Kaena Point on Oahu, researchers found that 31% of Laysan Albatross (Phoebastria immutabilis) pairs were female-female, in a population whose overall sex ratio ran to 59% female as a result of female-biased immigration. That is the arithmetic of the whole phenomenon in a sentence. There were not enough males to go round.

Follow-up work put numbers on what the arrangement costs. Females in same-sex pairs produced 80% fewer chicks than females in male-female pairs, survived less well, and skipped breeding more often. This is not a cost-free alternative, and the researchers are explicit that it carries lower fitness than male-female pairing. It is still better than the alternative, because a Laysan Albatross cannot raise a chick alone. The egg needs continuous incubation across months while the other bird travels enormous distances to feed, so a female with no partner at all fledges nothing.

One detail undercuts the assumption that one bird in such a pair is merely a helper. Genetic work showed that in most female-female pairs which raised a chick in more than one year, at least one chick was related to each female. Both birds were reproducing, in different years, out of the same partnership.

The same research turned up something less comfortable. Females in same-sex pairs that successfully raised a chick sometimes acquired a male mate the following year, while females in failed same-sex pairs never did. The authors read this as males assessing female quality and effectively relegating lower-quality females into same-sex pairs, which makes this a story about male choice as well as about a shortage of males.

The terns, and what it costs a colony

Roseate Terns (Sterna dougallii) at Bird Island, Massachusetts show the same pattern with the mechanism laid bare. Supernormal clutches have made up somewhere between 1% and 7% of clutches at that colony since at least 1970, attended by female-female pairs and other multi-female groupings, some of them lasting up to five years. The breeding population there runs at about 56% female, and the bias appears early, with roughly 55% of chicks hatching female.

This case carries a consequence the cheerful coverage tends to skip. Roseate Terns are endangered in the northeast, and the female-biased sex ratio reduces average productivity across the colony, which researchers have suggested is one reason the population has been slow to recover its former range. Same-sex pairing here is not a curiosity. It is a visible symptom of a demographic problem, and the birds are making the best available response to it.

Why it happens

Three things, working together, account for nearly all the documented cases in the wild.

The sex ratio is skewed. This is the common thread across gulls, albatrosses and terns. When a breeding population contains substantially more females than males, some females will not find a male partner no matter how the pairing shakes out. The surplus is real, and it has to go somewhere.

Raising young is a two-bird job. In these species a single parent cannot rear a chick, so the value of having any competent partner is high. A partner who cannot fertilize the eggs is still a partner who can incubate, defend the territory, and share the feeding. The arrangement is not a substitute for reproduction. For the birds involved it is the only route to reproducing at all.

The pair bond machinery is already there. Social monogamy in birds runs on courtship, mutual display, territory defence and coordinated care, none of which depends on the sexes of the two individuals performing it. Where the demographic conditions create surplus females, the existing behaviour does not need modifying to accommodate them. It simply runs.

Where fertile eggs do appear in these nests, they generally come from a female copulating with a male who is socially paired to someone else. That distinction between who a bird pairs with and who fathers the chicks runs through this entire subject, and we cover it fully in our guide to how birds mate.

What this does and does not tell us

It is worth being precise here, because this is a subject on which imprecision gets weaponized in both directions.

What the research documents is behaviour: which birds pair, how long they stay together, who incubates, and how many chicks fledge. It does not document anything about the inner life of a gull, and the human categories people reach for when they discuss it do not map onto birds in any straightforward way. Biologists use terms like same-sex pairing rather than borrowed human vocabulary for exactly that reason, and it is a description of what the birds do rather than a claim about what they are.

Equally, the frequent framing of these pairs as failures or aberrations does not survive the data either. A behaviour that lets a bird fledge a chick instead of none, sustained across years, with both partners reproducing over time, is functioning. The honest position is the mechanistic one, and it is more interesting than either of the positions people try to recruit it into.

Reading pairs in the field

One practical note. Western Gulls, Laysan Albatrosses and Roseate Terns are all species in which males and females look essentially alike, which is precisely why this went unnoticed for so long. Nobody was overlooking an obvious visual cue. Sexing these birds reliably requires measurements in the hand or genetic work, and observers watching a colony are reading behaviour and inferring the rest. That is worth remembering the next time you feel confident about which bird in a pair is which.

If you want to watch pair behaviour properly, our field guide to how to watch bird courtship displays covers the etiquette and the optics. For what happens when established pairs come apart, see divorce in birds, and for the question of who does the work once a pair forms, see why some male birds are deadbeat dads.