Birds exhibit a wide variety of mating systems and social behaviors. While some species do form monogamous pairs, many others have different arrangements such as polygamy, promiscuity, or solitary living. Let’s take a closer look at the evidence on avian relationships.
Monogamy in Birds
Monogamy refers to a mating system where a male and female form an exclusive pairing during at least part of the breeding season. This allows both parents to cooperate in raising young. Monogamous pairs may stay together for a single season, multiple seasons, or life.
Researchers estimate that about 90% of bird species are at least partially monogamous. Some well-known examples of monogamous birds include:
- Bald eagles
- Mute swans
- Great blue herons
- Peregrine falcons
- Barn owls
- Lovebirds
- Seagulls
In most monogamous species, the male and female work together to build a nest, incubate eggs, and find food for hatchlings. The male often feeds and guards his female while she incubates eggs. Both parents feed and protect the chicks once they’ve hatched.
Scientists have proposed several evolutionary reasons for monogamous pairing in birds:
- Raising offspring is demanding, so having two parents improves success.
- Males can be more certain of paternity if they mate with only one female.
- Females receive better care and resources when bonded to a single male.
- Both sexes reduce risk of sexually transmitted diseases by having one partner.
The degree of faithfulness varies across supposedly monogamous species. While some pairs never stray, extra-pair copulations are common in many socially monogamous birds. Even so, most continue their pair bond while one or both mates sometimes mate outside the pair as well.
Other Mating Systems
While monogamy is common in birds, there are certainly species that employ other mating systems as well. These include:
Polygyny
Polygyny describes a mating system where one male mates with multiple females. The male does not provide direct care to offspring. Polygyny is most common among bird species where males do not contribute substantially to nest-building, incubating, or feeding young. Some polygynous species include:
- Wild turkeys
- Peaco*cks
- Grouse
- Pheasants
- Red-winged blackbirds
In many polygynous species, males perform elaborate courtship displays and fight to establish dominance. Females then choose among available males. The reproductively successful males may mate with multiple females in a season, while less competitive males may not mate at all. Females receive no help from males.
Polyandry
Polyandry occurs when one female mates with multiple males. This is rare in birds since females are more limited by egg production than males are by sperm production. Still, polyandrous systems do exist in some species where males contribute significantly to offspring care. Examples include:
- Phalaropes
- Spotted sandpipers
- Jacanas
In these shorebirds and wading birds, males incubate the eggs and care for young while females often mate with multiple males. The female then moves on while different males raise her various broods. This unusual reversal of sex roles allows females to enhance reproductive success by laying more than one clutch per season.
Promiscuity
Promiscuity refers to mating systems where both males and females mate with multiple partners. Neither sex forms strong pair bonds. Promiscuity occurs in some species where male parental care is limited including:
- Mallards
- Teals
- Pintails
- Widgeon
- Mergansers
The males are not territorial. They provide no care as the female incubates and raises ducklings alone. A female may have a brood fathered by multiple males. Meanwhile, each male mates with many females. This maximizes reproductive success when birds flock together briefly during migration or on wintering grounds.
Solitary Living
Some bird species tend to be solitary rather than social. In these species, adult males and females only interact briefly for mating. They do not form lasting bonds or cooperate in raising young. One or both parents may provide some offspring care, but they forage and nest separately. Examples of predominantly solitary birds include:
- American woodco*ck
- Brown creeper
- Ruby-throated hummingbird
Woodco*cks perform aerial mating displays over a mating ground where females congregate only briefly. Hummingbirds exhibit a promiscuous mating system. Females build tiny individualized nests, lay two eggs, and rear young alone.
Solitary living allows birds to reduce competition for food and nest sites. However, the lack of bonding and biparental care can increase risks to eggs and young.
Evidence of Monogamous Pairs in the Wild
Scientists employ various research techniques to study the relationships between wild birds.
Field Observation
Researchers observe the behaviors of birds as they interact in their natural habitats. Repeated observation of specific individuals can determine if birds are paired monogamously, display mate guarding behaviors, or associate with multiple mates.
For example, observers noted that migrating Magnificent Frigatebirds always nested in the same tree and foraged together after breeding, indicating a monogamous bond.
Banding and Tracking
Unique bands placed on birds allow individual identification over time. Seeing the same banded male and female together repeatedly implies monogamy. Radio transmitters and unique color bands also allow tracking paired birds.
Tracking shows many seabird species reunite yearly with previous mates. However, tracking devices have also revealed cases of “divorce” when birds switch mates between breeding seasons.
DNA Fingerprinting
Collecting and genetically analyzing feathers, eggshells, blood samples, or chicks from a nesting site can identify maternity and paternity. This reveals whether offspring were sired by the male tending the nest or by another male.
In a study of Indigo Buntings, DNA fingerprinting showed 35% of nestlings resulted from extra-pair mating despite strong social bonding of breeding pairs.
Hormone Analysis
By measuring hormone levels, scientists can assess factors like fertility and bond strength. For instance, monogamous prairie voles show distinct patterns of oxytocin and vasopressin release.
Researchers can non-lethally sample blood, saliva, feces, feathers, or albumen of eggs to analyze hormone levels in wild birds.
Bird Species with Looser Pair Bonds
The strength of the pair bond varies across species considered monogamous. Some birds display extremely tight lifelong pair bonds while others are more casual or temporary.
Loose Annual Pairing
Species like wigeon and pintail ducks form new pair bonds during each mating season. These bonds completely dissolve when summer ends. One or both mates may take a new partner the next year.
Serial Monogamy
Many birds remain faithfully bonded for a single breeding season, then split up. The male and female each find a new mate the following year. Examples include razorbills, puffins, gulls, terns, and penguins.
Divorce may serve to correct a poor genetic match or improve reproductive success. For long-lived birds, re-pairing with a younger mate later can enhance lifetime fecundity.
Casual Bonds
In some colonially nesting species, pairs form but exhibit little overt bonding behavior. Both sexes may interact freely with neighbors. Mate switching between nesting attempts is frequent.
This casual serial monogamy occurs in cliff swallows and barn swallows. Females seem to gain reproductive benefits by mating with multiple colony males.
Cheating
Even strong pair bonds do not prevent “cheating” through extra-pair copulations in many species. While helping rear young of their social mate, one or both pair members may also sneak off to mate with neighbors.
Extra-pair mating is common in shorebirds like lapwings and oystercatchers. Females likely create mixed broods to hedge bets against infertile social mates.
Benefits of Avian Pair Bonding
Why do most birds bother with bonding at all? Pairing even temporarily offers a number of advantages.
Increased Chances of Breeding
Finding and defending a suitable nesting site is difficult for a lone female. A bonded male can better establish and protect a territory.
Many unpaired birds never get an opportunity to breed at all. Monogamy provides a reproductive opportunity for a larger number of individuals.
Enhanced Ability to Forage
Mates can more efficiently locate and capture food while one bird watches for danger. They may also cooperate to herd prey or flush it towards each other.
Geese, shrikes, and many raptors showcase synergistic foraging techniques.
Improved Nest Success
With shared duties, monogamous pairs can build higher quality nests in safer, more concealed locations. The male guards the female and nest, while the female incubates eggs.
Biparental care also reduces risk of egg or chick loss. One parent can immediately resume brooding if the other parent is killed or driven off.
Reduced Energy Expenditure
When both males and females contribute, they spend less time overall performing essential breeding behaviors like nest defense and feeding young.
With a partner’s help, each individual can spend more time resting and foraging for self-maintenance.
Negative Aspects of Pair Bonds
Depending on circ*mstances, pairing up may also have downsides leading birds toward looser bonds or alternative strategies.
Decreased Genetic Variation
Lifelong monogamy means an individual bird has offspring with only one mate. This can reduce genetic variation in small isolated populations.
Promiscuity in some populations may counteract potential inbreeding effects.
Inflexibility
If environmental conditions change, a lone bird can easily move to a better breeding site. Paired birds are constrained by obligations to a mate and shared territory.
Divorce allows birds to eventually rectify maladaptive matches.
Enforced Investment
Once they start incubating eggs or provisioning young, pair members must see the breeding effort through. A lone bird encountering limited resources can simply abandon a nest.
This may explain looser summer bonds in waterfowl preparing for challenging migration.
Mate Limitation
If one member of a pair dies during breeding, their mate cannot instantly replace them. This strongly impacts birds like albatrosses with very low annual fecundity.
Temporary monogamy during peak breeding ensures birds have chances with multiple mates.
Conclusion
The majority of bird species demonstrate monogamous pair bonding during breeding to some degree. However, the nature and duration of these bonds varies considerably in different environmental and social contexts. While lifetime monogamy was once considered the avian norm, research reveals more complex mating patterns in many species. The diversity of avian social systems reflects how different reproductive strategies suit particular life histories and ecological situations.
