Monogamy, defined as pairing with a single mate for at least an entire breeding season, is quite common in birds. However, truly lifelong monogamy, where a pair remains together until one mate dies, is less common. The degree of monogamy varies greatly across different bird species.
Why Do Birds Mate for Life?
There are several evolutionary theories as to why some bird species form lifelong pair bonds:
- In birds that raise altricial young (born helpless), having two parents increases the chance of offspring survival. The male can help feed and protect the young.
- Remaining with one partner removes the need to spend time and energy finding and courting new mates each breeding season.
- Close coordination and cooperation between mates improves reproductive success.
- Males can be more certain of paternity if their mate is faithful.
However, monogamy is not the only mating strategy in birds. In some species, males provide no paternal care and seek to mate with as many females as possible. In others, males and females may form temporary pairs that only last for one breeding season. The mating system that evolves depends on a variety of ecological factors.
What Percentage of Bird Species are Monogamous?
Scientists estimate that approximately 90% of bird species demonstrate some level of monogamy, at least for a breeding season. However, the percentage of birds that mate for life with a single partner is lower:
- Around 10% of bird species, including many seabirds, form lifelong pair bonds.
- In 5-10% of species, the bond lasts several years or more but not necessarily for life.
- In 75-80% of species, monogamy lasts only for a single breeding season.
The level of monogamy often correlates with how much paternal care occurs. Species where males invest significant care and provisioning of the young tend to demonstrate stronger pair bonding.
Bird Species That Mate for Life
Here are some examples of birds that demonstrate lifelong monogamy:
Seabirds
Many seabirds form monogamous pair bonds that may last 15-20 years or more, including:
- Albatrosses
- Petrels
- Shearwaters
- Puffins
- Penguins
- Gannets
- Cormorants
- Tropicbirds
- Frigatebirds
- Skuas
Both parents invest significant time and energy rearing a single chick per breeding season.
Waterfowl
Some waterfowl species demonstrate lifelong monogamy, including:
- Mute swans
- Black swans
- Grebes
- Some geese species
- Some duck species
The male guards the female and provides some parental care.
Birds of Prey
Most birds of prey are monogamous, including:
- Eagles
- Hawks
- Falcons
- Owls
- Vultures
The bond may last many breeding seasons or for life. The male assists with hunting and feeding offspring.
Passerines
A minority of passerine songbirds demonstrate lifelong monogamy, such as:
- Crows
- Ravens
- Magpie
- Jays
- Spotted nutcracker
- Pinyon jay
- Florida scrub jay
Both parents care extensively for the young.
Bird Species with Short-Term Pair Bonds
In contrast, other bird groups tend to form shorter-term monogamous bonds:
Galliformes (pheasants, grouse, quail)
Males court and defend females but provide little to no parental care.
Tinamous
Males incubate and briefly care for chicks but pair bonds dissolve after one breeding season.
Parrots
Bond only lasts for a given breeding season.
Pigeons and doves
Mate for a breeding season but will take new mates in future years.
Hummingbirds
Males display site fidelity but females will mate with other males. Bond lasts for breeding season.
Why Are Some Bird Species Monogamous While Others Are Not?
Scientists are still exploring why monogamy as a mating system varies so much among birds. Some key factors appear to be:
- Level of paternal care – Species where males invest significant care in offspring tend toward monogamy. When male care is lacking, pair bonds tend to be weaker.
- Dispersed vs. concentrated resources – When feeding and nesting resources are dispersed, monogamy may lend advantages. With concentrated resources, males can attract multiple mates so polygamy may dominate.
- Annual vs. multi-brooded – Lifelong monogamy more often correlates with having one brood per year. Multi-brooded species may have shorter seasonal bonds.
- Nest type – Cavity and cup nesters more often display monogamy compared to open nesters.
- Breeding colonies – Colonial central-place foragers like seabirds tend toward monogamy whereas solitary nesters or non-colonial species are less monogamous.
Mating Systems in Birds
Here is an overview of some of the mating systems observed in birds:
| Mating System | Description | Example Species |
|---|---|---|
| Lifelong monogamy | Male and female pair bond remains intact until death of one mate | Albatrosses, eagles, geese |
| Seasonal monogamy | Bond lasts for at least one breeding season then dissolves | Warblers, hummingbirds, parrots |
| Serial monogamy | Mate for one season then find a new partner next season | Pigeons, doves, tinamous |
| Polygyny | One male mates with multiple females | Peacocks, grouse, cassowaries |
| Polyandry | One female mates with multiple males | Phalaropes, spotted sandpiper |
| Promiscuity | Members of both sexes mate with multiple partners | Ostriches, chickens |
As shown, avian mating systems encompass a spectrum from lifelong monogamy to promiscuity based on the species’ biology and ecology.
Challenges to Lifelong Monogamy in Birds
Even in purportedly monogamous species, there can be challenges to maintaining a lifelong pair bond:
- One mate dying requires the other to find a new partner.
- Extra-pair copulations – Up to 30% of offspring in some “monogamous” species come from extra-pair matings. However, most species show low rates.
- Divorce or mate switching – In a minority of cases, birds may “divorce” a poor quality mate if given the opportunity to “trade up.”
- Intraspecific brood parasitism – Females may lay eggs in another’s nest, cuckolding the pair.
However, most monogamous species demonstrate high mate fidelity and loyalty through behaviors like mutual courtship displays, coordinated breeding activities, and joint territorial and nest defense.
Examples of Birds That Mate for Life
Let’s take a more in-depth look at a few specific birds known for lifelong monogamous bonds:
Penguins
- Most penguin species are highly monogamous, including the Adelie, emperor, and king penguin.
- Pairs form long-term bonds, meeting yearly to court, breed, and raise their chick.
- They vocalize to identify mates and engage in bonding rituals like bowing and preening.
- If a mate dies, the remaining bird generally tries to find a new lifelong partner.
Albatrosses
- Albatrosses have elaborate mating rituals to form lifelong pair bonds.
- Pairs synchronize behaviors and vocalizations almost instinctively.
- They forage at sea separately but incubate eggs and raise chicks cooperatively.
- Pairs aggressively defend nesting territories.
- One study of a Laysan albatross colony found that divorce was extremely rare, only occurring in 1.4% of pairs observed over 12 years.
Bald eagles
- Bald eagles mate for life unless a mate dies. Then the remaining eagle will choose a new partner.
- Pairs perch close, preen one another, and perform dramatic aerial displays together.
- They defend a large nesting territory and cooperatively incubate eggs and raise eaglets.
- One 15-year study found 90% of bald eagle pairs remained intact.
- If one eagle dies, the surviving mate often tries to find a new partner within 240 days.
Gibbons
- Gibbons are primates, but they demonstrate monogamy like many birds.
- Pairs sing intricate duets together to defend territories and strengthen social bonds.
- Partners stay close and groom one another.
- Mates are fiercely loyal with extra-pair copulations being extremely rare.
- Pairs last for many years until the death of one partner.
These examples illustrate the deep social bonds that lifelong monogamous pairs form in both birds and some mammals like gibbons.
Interesting Facts About Monogamous Bird Species
- The divorce rate in wandering albatross pairs is estimated to be less than 1%.
- Laysan albatross couples have been observed bonding for over 60 years.
- Some parrot species like the Patagonian conure live in large monogamous flocks with multiple breeding pairs.
- Both male and female swans build the nest together and defend their territory as a team.
- Scientists can do DNA testing on chicks to identify cases of extra-pair copulations in otherwise monogamous species.
- The male and female in some monogamous species develop identical plumage color patterns.
- Monogamous bird pairs synchronize their behavior without needing signals or calls from their partner.
- Geese and many seabirds undertake sibling cooperation in raising young, rather than having offspring disperse after fledging.
- Some monogamous bird pairs engage in allopreening where they groom each other’s feathers.
- Both the male and female often develop brood patches for incubating eggs in monogamous species.
Conclusion
In summary, monogamy as a lifelong pairing between mates occurs in about 10% of bird species, while about 75-90% demonstrate shorter-term social monogamy. High levels of parental care by both males and females appears to be a key driving factor in the evolution of lifelong partnerships in birds. Monogamous bird pairs engage in synchronized behaviors to identify each other, defend resources, and cooperate in reproduction. Though monogamy generally leads to greater reproductive success, even monogamous species can occasionally engage in extra-pair matings.
