For generations, anthropologists have argued whether humans are evolved for monogamy or some other mating system, such as polygyny, polyandry, or promiscuity. But any exploration of monogamy must begin with a bifurcation of the concept into two completely different phenomena: social monogamy and sexual monogamy.
Sexual monogamy is just what it sounds like: The restriction of sexual intercourse to within a bonded pair. Social monogamy, also known as economic monogamy, describes the bonding itself, a strong, neurohormone-driven attachment between two adults that facilitates food and territory sharing, to the exclusion of others, for at least one breeding season, and generally purposed towards raising offspring.
Because these two aspects of monogamy are so often enjoined among humans, they are considered two sides of the same coin. But, as it turns out, they are entirely separable among animals. In fact, social monogamy is extremely common in birds and somewhat common in mammals, while sexual monogamy is vanishingly rare among any species. Because of the unique way their embryos develop — externally but with constant warmth required — birds are the real stars of monogamy and have thus borne the brunt of its misconceptions.
The marriage (if you’ll pardon the pun) of two very different behaviors into one concept is — and always was — unsupported by evidence from the natural world. Monogamy, as it is commonly understood, was the invention of anthropomorphic bias. Naturalists in the 19th and 20th centuries documented how pairs of various bird species dutifully toiled together building a nest, protecting the eggs, mutually feeding each other and their offspring, before eventually flying off into the sunset together. These prim and proper Victorians didn’t have to squint very hard to see a perfect model in nature of what they valued most in human society — lifelong and sexually exclusive marriage.
Had they looked closer, they would have seen a lot more. While about 90 percent of migratory bird species form stable dyads, about half of those last only for a breeding season. The remaining half are a mixed bag and dissolutions can occur even after many years together. Further still, even in the most durably pair-bonded birds, sexual exclusivity is almost never a component of their partnership.
Social monogamy is extremely common in birds and somewhat common in mammals, while sexual monogamy is vanishingly rare among any species.
The idea that bird pairs always raise their own genetic offspring went unquestioned for centuries, but came crashing down in the late 1980s with the advent of DNA-based paternity testing. Beginning with indigo buntings and red-winged blackbirds, ornithologists began scrutinizing the paternity of bird clutches and, one-by-one, the results came back like an episode of the “Maury” show: “You are NOT the father!”
These DNA tests finally forced biologists to reexamine what mating and monogamy are really about, since sexual exclusivity was definitely not the central plank that everyone thought it was. The clunky term extra-pair copulation, which had appeared in the ornithology literature only a handful of times prior, suddenly exploded in use.
But these results also revealed the prevalence of another phenomenon: brood parasitism, or the sneaking of eggs into other birds’ nests in an attempt to steal parental care.
Extra-pair copulations, or EPCs, are often described as moments of reproductive conflict. When it comes to incubating, protecting, hatching, feeding, and caring for their own offspring, the genetic interests of mated male and female birds are well aligned. But EPCs introduce conflict. Among other reasons, females likely pursue EPCs in order to diversify their clutch with genes from different males, while males’ goals appear to be mostly numeric.
A male can pursue paternity with any female he finds at essentially no cost and no risk (although in some species, males often return to feed their past EPC partners). But for females, there is a substantial minimum cost in producing and incubating an egg, and feeding and protecting the hatchling, even if she has help from her mate. For a female, there is no free lunch because maternal investment is always high in birds.
Except when it isn’t. Female birds actually can manage a free lunch through brood parasitism by forcing other birds to raise their hatchlings. When successful, this is a superb strategy because not only is the parasite able to pilfer care for their offspring, but it will come at the expense of their competitors’ own children.
This gives the sneaky crooks a huge competitive advantage, so it is no surprise that brood parasitism has been observed in hundreds of nest-building socially monogamous species, from migrating songbirds to the colorful birds of the tropics, and is especially common in waterfowl, having been observed in more than one-third of Anseriformes species.
Most species practice intraspecific brood parasitism, which is the laying of eggs in the nests of other birds of the same species. This results in not just extra-pair paternity, but extra-pair maternity as well. The eggs and chicks are the progeny of neither of the parents that actually raise them. Ornithologists seem pretty convinced that this is all deceptive.
Remarkably, some birds pass their eggs into the nests of birds that are an entirely different species. As absurd as this sounds, it works often enough that several bird species have evolved to do this exclusively, something we call obligate brood parasitism.
The most famous of the obligate brood parasites is the common cuckoo (Cuculus canorus) and these birds parasitize several species. Interestingly, adult cuckoos can resemble an entirely different species — the sparrowhawk — very closely. The sparrowhawk is an aggressive bird of prey, adept at hunting and fighting, while the cuckoo’s nest targets are smaller birds and mostly insectivores. By mimicking vicious sparrowhawks, the cuckoo is able to scare their targets away from their nests so that they can lay their eggs in them in peace.
There are five species of cowbird (genus Molothrus) and all of them are obligate brood parasites. Their whole reproductive strategy involves finding, rather than building, a nest to lay their eggs. While most brood parasites have evolved to parasitize a specific host, right down to mimicking the size and coloring of the hosts’ own eggs, three of the cowbird species will attempt to parasitize any nest they can find and have done so successfully with over two hundred target species that we know of.
It seems puzzling that brood parasites get away with their treachery. Birds are highly intelligent and have excellent vision. Are these victims really just hapless fools with no idea they’re being duped into caring for alien eggs and hatchlings? If not, why do they tolerate this extreme form of mooching?
In most cases, we’re not sure, but scientists have recently uncovered some of the ways that hosts are coerced into performing this unpaid labor. For example, the brown-headed cowbird watches her eggs carefully after laying them in a host nest. If the host pushes them out, the cowbird will then repeatedly attack and destroy the nest itself along with any eggs left in it. In other words, they extract retribution.
In 2007, researchers in Southern Illinois quantified just how serious the threat of cowbird violence can be on a specific host, the prothonotary warbler (Protonotaria citrea), through a four-year study examining more than a hundred nests. The scientists found that, once cowbird eggs were laid in a warbler nest, if they were accepted, that nest was successful more than 95 percent of the time, meaning that warbler eggs were hatched. But, if cowbird eggs were lain and then removed by the warblers, the nests were successful only about 60 percent of the time. The researchers noted that more than half of the unsuccessful nests were completely destroyed. (The overall nest success rates in this study are inflated because the warbler nests had been artificially protected as part of a conservation experiment.)
Brood parasitism has been observed in hundreds of nest-building socially monogamous species, from migrating songbirds to colorful tropical birds to waterfowl.
The retaliatory destruction of host nests, first documented in great spotted cuckoos (Clamator glandarius), appears in some cowbirds as well, but each species uses the tactic differently, in accordance with the various nuances of their preferred hosts. For example, screaming cowbirds (M. rufoaxillaris) in Argentina conduct extensive surveillance on their potential targets, visiting repeatedly for days before placing their eggs. If they were merely scanning for the best target, they wouldn’t need to make all those repeated visits. Instead, the birds may be timing their egg laying precisely. Too early and it will be obvious that the eggs are parasitic; too late, and they might not be incubated long enough.
Another possibility is that the cowbirds want their potential victims to know they are being targeted and watched. Indeed, it has been shown that the more frequently brown-headed cowbirds (M. ater) visit a parasitized nest, both before and after laying eggs, the less likely the hosts are to eject the eggs. Intimidation seems to work!
A reed warbler feeding a common cuckoo chick in a nest.
Visual: Per Harald Olsen/Wikimedia
Shiny cowbirds (M. bonariensis), on the other hand, also heavily surveil their targets, but only before laying eggs, not after. This is because whenever they lay their eggs, they also pierce any host eggs that are already there. I guess it’s not enough to add their eggs to the clutch, they attempt to hijack all host parental effort. And, because even they themselves can be fooled by how closely their eggs mimic the targets’, they never return, lest they accidentally pierce their own eggs.
Researchers use the term mafia behavior to describe all the surveillance, intimidation, and retaliatory destruction of nests by brood parasites. (I can picture cowbirds making the rounds and greeting roosting warblers with the avian equivalent of, “That’s a nice nest you got there. It’d be a shame if something happened to it.”)
Interestingly, resisters of brood parasitism can also fight back using organized violence. Warblers have been known to attack cuckoos on sight. Unlike cowbirds, who use muscle and might, cuckoos tend to act stealthily, and, as mentioned above, have even evolved mimicry. In 2022, researchers in China reported that a group of Oriental reed warblers (Acrocephalus orientalis) mounted an organized attack on an approaching cuckoo, pecking and scratching it to death, the first such report of a coordinated mortal attack by warblers on their main brood parasite.
Brood parasitism also affects a bird species’ approach to social monogamy, because it is usually (but not always) linked to the cooperative rearing of offspring. When the lifestyle of a bird species requires the incubating of one’s own eggs and subsequent care for the hatchlings, there is an evolutionary reward for couples sticking together and taking turns with those tasks. Hence, most bird species are socially monogamous.
But when the lifestyle does not require it — as with the brood parasites — there isn’t as much incentive for mating couples to pair-bond and stick together. Accordingly, three of the five species of cowbirds do not form socially monogamous pair-bonds. The trend is even stronger among the cuckoos, a family of around 150 species. Most of the cuckoos that are obligate brood parasites do not pair-bond, since they don’t build nests, incubate eggs, or feed chicks. But among those that do nest and provide their own parental care, such as the three species of anis, most form monogamous pair-bonds. The exceptions help prove the rule.
Nathan H. Lents is professor of biology at John Jay College, of the City University of New York. His research lab studies human genome evolution and forensic genetics. He is also the author of “Not So Different: Finding Human Nature in Animals” and “Human Errors: A Panorama of Glitches, from Pointless Bones to Broken Genes.”