Birds display a wide variety of mating systems, ranging from monogamy to polygamy. Monogamy refers to a mating system where a male and female pair bond and mate exclusively with each other. Polygamy refers to mating systems where an individual of one sex mates with multiple individuals of the opposite sex. Polygamy can take the form of polygyny (one male with multiple females) or polyandry (one female with multiple males). The mating system adopted by a bird species depends on various ecological, biological and behavioral factors. In this article, we will examine the evidence for monogamy and polygamy in birds, look at examples of bird species exhibiting each mating system, and discuss the evolutionary factors that shape mating systems in birds.
Evidence for Monogamy in Birds
There is good evidence that monogamy is a common mating system among birds:
– Social monogamy, where a male and female form a pair bond and cooperate in tasks like nest building, defending territory and raising young, is estimated to occur in around 90% of bird species. Genetic monogamy, where the male and female only reproduce with each other, occurs in an estimated 75-85% of socially monogamous species.
– Long-term studies tracking the relationships between individual birds show repeated associations between the same males and females across multiple breeding seasons. This indicates prolonged pair bonding.
– Molecular studies using genetic markers like DNA fingerprinting have confirmed genetic monogamy in many socially monogamous species by showing low rates of extra-pair paternity.
– Cooperative breeding systems, where non-breeding helpers assist a monogamous breeding pair, provide additional evidence of multi-year pair bonds.
– Sexual monomorphism, where males and females look alike, and coordinated courtship displays are common features of monogamous bird species.
Examples of Largely Monogamous Bird Species
Some examples of bird species considered predominantly monogamous include:
– Swans and geese: Most species mate for life, with low divorce and re-mating rates. Extra-pair paternity is rare. Long term pair bonds are facilitated by coordinated courtship displays.
– Many seabirds like albatrosses and penguins: They show mate fidelity, bi-parental care of the young and coordination of nesting activities by the pair. Divorce and re-mating is uncommon.
– Many songbirds like corvids, tits, finches, thrushes, woodpeckers etc.: They demonstrate social and genetic monogamy via life-long pair bonds and low extra-pair paternity rates. However, some songbird species like sparrows tend to have higher rates of cheating and divorce.
– Birds of prey like eagles, hawks and owls: They display prolonged pair bonds, coordinated territorial and breeding behaviors, and low rates of extra-pair mating.
– Turacos: DNA fingerprinting studies confirm genetic monogamy and life-long pair bonds in these African birds.
Evidence for Polygamy in Birds
While monogamy may be the predominant mating system, there is also evidence that polygamy occurs in many bird species:
– Polygyny, where one male mates with multiple females, has been documented through behavioral observations and genetic studies of mating systems in birds like Red-winged Blackbirds, Great Rheas, Red Jungle Fowl, Sage Grouse and Wild Turkeys.
– Polyandry, with one female mating with multiple males, is rarer but does occur in species like Jaçanas, African Jacanas, Dunnocks, Galapagos Hawks and Acorn Woodpeckers.
– In socially monogamous species, most individuals mate outside the pair bond at least occasionally, known as extra-pair copulations. Extra-pair paternity rates average around 14% across socially monogamous species.
– Hybridization between closely related species provides genetic evidence that individuals of some species may mate with multiple heterospecific partners.
– Large sexual size differences within species, elaborate male visual or auditory displays, and male-biased adult sex ratios also provide clues to polygamous mating systems in some birds.
Examples of Polygamous and Polyandrous Bird Species
Examples of species where polygamous mating has been well-documented:
| Species | Mating System |
|---|---|
| Red-winged Blackbird | Polygynous. Males defend territories and mate with several females. |
| Dunnock | Polyandrous. Females form pair bonds but also mate with extra males who provide care. |
| Galapagos Hawk | Polyandrous. Females mate with up to eight males who help provision young. |
| Great Rhea | Polygynous. Males maintain harems of 2-12 females. |
| Acorn Woodpecker | Cooperatively polyandrous. Groups contain up to four breeding females and seven males. |
Evolution of Mating Systems in Birds
The mating system adopted by a bird species depends on both ecological and social factors including:
– Parental care requirements: Monogamy is favored when bi-parental care is crucial for offspring survival and reproductive success. Polygamy occurs when males provide little or no care.
– Environmental constraints: Under harsh conditions, monogamy may be necessary to raise young. Under more favorable conditions, polygamy can emerge.
– Temporal and spatial distribution of resources: Defendable, clustered resources promote polygyny while dispersed resources favor monogamy.
– Adult sex ratios: Skewed sex ratios promote polygamy while equal ratios favor monogamy.
– Life history: Long-lived species with low annual mortality favor prolonged pair bonds. Short lived species with high mortality show higher turnover.
– Extra-pair mating benefits: Extra-pair reproduction may provide genetic benefits that shape willingness to mate outside the pair bond.
– Mate-guarding tradeoffs: Males face tradeoffs between closely guarding the mate versus seeking extra-pair copulations.
Conclusion
In summary, while genetic and social monogamy are very common in birds, polygamy also occurs in many species. The balance between monogamy and polygamy in any given species depends on a variety of ecological factors including parental care needs, resource distribution, adult sex ratios, life history traits, and extra-pair mating benefits. While a simple dichotomy between “monogamous” and “polygamous” species may be tempting, most bird mating systems encompass a spectrum rather than absolutes. The diversity of avian mating systems provides insights into the evolutionary forces shaping social and genetic mating patterns. Understanding why and how mating systems evolve remains an active area of research in behavioral ecology.
