An Eye-Opening Exploration of the Unexpectedly Complex Sex Lives of Birds

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Investigating the complexities of relationships among migratory birds reveals the nuances of monogamy and parental care.

Nathan H. Lents, Undark

April 11, 2025 8:00 a.m.

In the vibrant skies of southern Arizona, brown-headed cowbirds embody a surprising facet of avian life. This species is known as a brood parasite, meaning they lay their eggs in the nests of other birds to exploit the parental efforts of those unsuspecting hosts.

The debate around human mating systems—whether they favor monogamy, polygyny, polyandry, or promiscuity—has long captivated anthropologists. When examining monogamy, it is essential to differentiate between two distinct concepts: social monogamy and sexual monogamy.

Social monogamy, sometimes referred to as economic monogamy, entails a strong partnership prompting shared responsibilities such as food and territory, crucial during breeding seasons and typically aimed at rearing offspring. On the other hand, sexual monogamy restricts sexual interactions strictly to a bonded pair.

While many people often conflate these two forms of monogamy, they are not necessarily linked in the animal kingdom. In fact, while social monogamy prevails in many bird species, true sexual exclusivity is notably rare. Birds, experiencing unique reproductive pressures due to the external development of their embryos, serve as key subjects in discussions about monogamy, challenging prevailing misconceptions.

Historical documentation by naturalists in the 19th and 20th centuries painted a romantic picture of bird pairings, showcasing their cooperative nesting and caretaking. This perspective sensationalized a model that mirrored the Victorian ideals of lifetime commitment and exclusivity, valuing human-like relationships in nature. However, a closer examination uncovers a more complex reality.

The Sexual Evolution

In this segment, author Nathan H. Lents shares insights excluded from his book The Sexual Evolution: How 500 Million Years of Sex, Gender, and Mating Shape Modern Relationships (Mariner Books).

Observations indicate that while roughly 90 percent of migratory bird species form stable pairs, only about half of these alliances last beyond a single breeding season. Moreover, those pairs that endure for years often lack any sexual exclusivity.

The long-held assumption that bird pairs exclusively raise their genetic offspring was critically challenged in the late 1980s through the advent of DNA paternity testing. Pioneering studies on species such as indigo buntings and red-winged blackbirds revealed surprising truths: many birds were not raising their biological young, leading to a widespread reassessment of monogamous behavior. The term extra-pair copulation quickly became commonplace among ornithologists, emphasizing that sexual exclusivity was not a universal norm.

This influx of knowledge also highlighted another intricate behavior known as brood parasitism, wherein birds deposit eggs into other birds’ nests to benefit from their care. This strategy introduces an intriguing layer of competition, as it allows the parasitic bird to divert parental resources away from the host’s own offspring.

Extra-pair copulations often create a dynamic tug-of-war in the reproductive strategies of birds. While mated pairs may generally align in their interests regarding the care of their young, females engage in EPCs to introduce genetic variety into their clutches, while males seem motivated primarily by quantity.

Interestingly, the cost of reproduction for females is significant; laying an egg and nurturing a hatchling is resource-intensive, contrasting with the relatively low-cost reproductive pursuits available to males. However, some females can bypass this resource investment through brood parasitism, allowing them to capitalize on the nurturing efforts of other species.

This tactic is prevalent among many socially monogamous species, including waterfowl, where over a third of Anseriformes exhibit this behavior. Generally speaking, birds engage in intraspecific brood parasitism, wherein they lay eggs in the nests of the same species, markedly leading to the phenomenon of extra-pair paternity and maternity.

Remarkably, some birds employ parasitism at interspecific levels, depositing eggs in the nests of entirely different species—a behavior termed obligate brood parasitism. This strategy culminates in evolutionary adaptations, where species like the common cuckoo (Cuculus canorus) have developed mimicry that helps them avoid detection by potential hosts. By resembling predatory species such as sparrowhawks, the cuckoo can deter host birds while laying their eggs in their nests.

Beyond the cuckoos, members of the cowbird genus (Molothrus) represent other examples of obligate brood parasites, preferring to locate rather than construct their nests. Although many brood parasites have specialized host preferences, some cowbird species have successfully parasitized over 200 different target species.

It may seem extraordinary that these brood parasites succeed without detection. Birds are equipped with acute senses and intelligence, prompting questions about why host birds accept foreign eggs and chicks. Emerging research has started to unravel this mystery, illustrating that the dynamics of coercion play a significant role. For example, brown-headed cowbirds maintain a watchful eye on their laid eggs; if their eggs are ejected by host birds, they may retaliate by destroying the entire nest.

In a notable study from 2007, researchers examined the impact of cowbird behavior on prothonotary warblers (Protonotaria citrea). They discovered a stark difference in nest success: nests accepting cowbird eggs had a success rate of over 95 percent, while those that rejected them experienced only a 60 percent success rate, a difference attributed to the potential for violent reprisals against the nest.

Cowbirds and cuckoos demonstrate varying strategies to ensure their reproductive success. The term “mafia behavior” has emerged to encapsulate these intimidating tactics. In contrast, warblers have exhibited group defensive behaviors, as seen in reports of coordinated attacks on cuckoos. This highlights the ongoing evolutionary arms race between host species and brood parasites, reflecting intricate interactions within ecosystems.

The influence of brood parasitism extends to the social structures of bird species. Species reliant on nurturing their own eggs typically show a clear evolutionary benefit for forming lasting pair bonds, fostering cooperation in offspring rearing. However, for brood parasites, the lack of such requirements leads to a decrease in these social bonds. Notably, of the five cowbird species, three do not observe socially monogamous partnerships, mirroring trends observed in many cuckoo species.

This article was originally published on Undark. Read the original article.

Nathan H. Lents is a biologist at John Jay College, City University of New York. His research includes human genome evolution and forensic genetics, and he is the author of Not So Different: Finding Human Nature in Animals and Human Errors: A Panorama of Glitches, from Pointless Bones to Broken Genes.

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