Monogamy, meaning mating for life with a single partner, is rare in the animal kingdom. Only about 3% to 5% of mammal species are known to form lifelong pair bonds. However, monogamous mating systems are more common among birds. Over 90% of bird species are monogamous, at least socially if not sexually. This means they form pair bonds and cooperate in raising young, even if they occasionally seek extra-pair copulations. There are many theories for why monogamy evolved in birds, and the answer may be different for different species. But in general, monogamy is thought to have evolved because of the demands of raising altricial young that require extensive biparental care.
What is monogamy?
Monogamy refers to a mating system where one male and one female form an exclusive mating pair and cooperate to raise offspring. Monogamy can take different forms:
Social monogamy
Two individuals display pair-bonding behaviors including close association, cooperation in territorial defense and raising young, but may still seek extra-pair copulations.
Sexual monogamy
A pair mates exclusively with each other. Extra-pair copulations do not occur.
Genetic monogamy
A pair mates exclusively, resulting in all offspring being sired by the social male partner. Rarely confirmed outside of laboratory settings.
Most monogamous bird pairs are socially monogamous, but extra-pair paternity does occur in many species. So sexual and genetic monogamy are less common.
Why did monogamy evolve in birds?
There are several leading hypotheses for why monogamy evolved as the predominant mating system in birds:
Biparental care hypothesis
Most bird species have altricial young that require intensive care and feeding by both parents. Monogamy evolved so the male would stick around to share the burden of raising offspring.
Territoriality hypothesis
Many birds defend territories for nesting and foraging. Having a bonded pair facilitates cooperative territory defense against intruders.
Mate-guarding hypothesis
Males can better guard and maintain exclusive sexual access to females when pairs are monogamous.
Mate choice hypothesis
In species where females invest heavily in producing eggs, they are choosy. Monogamy allows males to demonstrate their quality over time before being selected as a mate.
The strongest support is for the biparental care hypothesis, but multiple factors were likely at play.
What bird families contain mostly monogamous species?
Most bird families contain predominantly monogamous species. Some examples:
Bird Family | Example Species |
---|---|
Sparrows and allies (Emberizidae) | Chipping sparrow |
Finches (Fringillidae) | Zebra finch |
Hummingbirds (Trochilidae) | Ruby-throated hummingbird |
Pigeons and doves (Columbidae) | Mourning dove |
Crows and jays (Corvidae) | Blue jay |
Waxwings (Bombycillidae) | Cedar waxwing |
Nearly all songbirds (Oscines) are monogamous. Non-songbird groups like pigeons, doves, crows, and allies also mostly contain species that form monogamous pair bonds.
What are some specific examples of monogamous bird species?
Some well-studied examples of monogamous birds include:
Albatrosses
Albatrosses have very long-term pair bonds that can last for decades. Pairs engage in mating rituals like billing, vocalizations, and synchronized dances to reinforce the pair bond each breeding season. They forage widely at sea but return to breed at the same nest with the same partner year after year.
Bald eagles
Bald eagles form permanent pair bonds reinforced by cartwheeling courtship displays. They defend large nesting territories together for years. If one mate dies, the surviving eagle may find a new partner after several years alone.
Flamingos
Flamingos have monogamous pair bonds that last multiple breeding seasons, though they do not always remain faithful sexually. Both parents build the nest mound together, incubate eggs, and care for the chicks.
Puffins
Atlantic puffins form long-term pair bonds and exhibit high mate fidelity, returning to the same burrow and partner year after year. Both parents take turns incubating the single egg and feeding the chick.
Wolves
While not birds, wolves demonstrate one of the rare monogamous mating systems in mammals. Wolf pairs jointly hunt and raise pups together and may remain paired for life unless a mate dies.
How common are extra-pair copulations in monogamous birds?
Social monogamy with occasional extra-pair copulations appears to be common in many supposedly monogamous bird species:
Species | % of offspring from extra-pair matings |
---|---|
Blue tit | 29% |
Reed bunting | 15-42% |
Savannah sparrow | 17-20% |
Tree swallow | 44-46% |
Western bluebird | 13-15% |
Rates vary greatly by species and population, but on average, 10-20% of offspring in monogamous birds may be sired by an extra-pair male. Females likely play an active role in soliciting extra-pair copulations in many species.
How do extra-pair copulations impact monogamy?
The occurrence of moderate levels of extra-pair paternity in ostensibly monogamous birds has led to debate about the degree of “true” sexual monogamy. There are several possible interpretations:
Socially monogamous does not always mean sexually monogamous
Most birds form lasting pair bonds but do seek some opportunities for extra-pair mating. Social monogamy provides benefits like biparental care even when sexual monogamy is not absolute.
Extra-pair copulations may be adaptive
In species where females invest greatly in producing eggs, extra-pair mating allows females to obtain superior genes from extra-pair males while retaining male parental care. Thus, moderate promiscuity may be favored.
Extra-pair copulations may be non-adaptive outcomes of sexual conflict
While females may benefit from extra-pair fertilizations, they can also impose fitness costs on males. Extra-pair copulations may persist as a byproduct of sexual conflict even if males attempt to control female mating.
Most evidence indicates social monogamy in birds is a widespread and evolutionarily stable strategy, even if perfect sexual monogamy is rare.
Do all bird species mate monogamously?
No, while the majority do, some exceptions occur:
Lek mating
In lek species like grouse, males congregate to display and compete for mates while females visit only to copulate. Leks include no male-female pair bonds.
Polygyny
Some raptors like hawks and eagles are polygynous, with one male controlling access to multiple females. Females raise young alone.
Cooperative breeding
In some species like the Australasian babbler, breeding groups contain multiple males and females that all contribute to raising young. Monogamy is absent.
Promiscuity
The ostrich shows promiscuous mating with no stable bonds. Males maintain no territories and may mate with any available females.
So while monogamy predominates in birds, it is not the only mating system found. Complex social and ecological factors determine each species’ strategy.
Conclusion
Monogamy is the norm rather than the exception among bird species. Most form social pair bonds for breeding and biparental care, though sexual monogamy is often not perfect. A variety of evidence supports evolved benefits from pair bonds in birds with altricial young, including joint territoriality, mate-guarding, and mutual mate choice. Though extra-pair copulations occur in many socially monogamous species, the prevalence of enduring pair bonds across diverse bird families attests to the general evolutionary success of avian monogamy.