Double-crested cormorants (Phalacrocorax auritus) are large waterbirds found across North America. They are distinguished by the two tufts of black feathers on their heads during breeding season. Cormorants are colonial nesters, forming dense breeding colonies near waterways. Their populations can number in the tens of thousands in a single colony.
Cormorants form monogamous pair bonds and share parental duties like nest building, incubating eggs, and raising young. But an interesting question arises – do cormorants mate for life, remaining with the same partner year after year? Or do they form new pair bonds each breeding season?
Courtship and Pair Bonding
Cormorants arrive at breeding colonies in spring after migrating from wintering areas. Courtship rituals soon commence, with males displaying to females by stretching their necks up, flapping their wings, and emitting guttural sounds. The crests on a male’s head become more prominent during breeding.
Females ultimately select a male to pair with for the breeding season. The male will gather nesting material like sticks and seaweed, which the female then uses to construct a nest. Nests are built on the ground, in low shrubs, or in trees.
Cormorants appear to form strong pair bonds once they have paired up for breeding. The male and female cooperate in incubating the 3-5 eggs and later provisioning the chicks with regurgitated fish. Both parents feed and protect the young.
This parental cooperation and coordination requires maintained pair bonding throughout the breeding season. But does the bond last for future years?
Site Fidelity and Mate Fidelity
Cormorants show high levels of site fidelity, returning to breed at the same colony site year after year. However, studies show they may not necessarily breed with the same mate from one year to the next.
Researchers have color-banded cormorants to follow individuals across breeding seasons. They found that a significant portion of pairs reunite at colony sites the next year and breed together again. However, mate-switching also commonly occurs.
Reasons for Mate Switching
There are several explanations for why cormorants may switch mates between breeding seasons:
- If one member of a pair dies, the surviving bird must find a new mate.
- Divorce may occur if the pair experiences low reproductive success.
- Young or newly matured birds may replace older birds in a pair bond.
- Mates from a previous season may not reconnect at colony sites.
Environmental factors also likely influence mate fidelity. In years with low food availability, more birds may skip breeding or nest elsewhere, disrupting prior pair bonds.
Genetic Studies
Analyses of genetic relationships provide further evidence that cormorants frequently mate with different partners across seasons.
Researchers extracted DNA from cormorant chicks in colonies and compared their genetic profiles. Chicks from the same nest tended to be full siblings, indicating monogamy within a breeding season. However, half-siblings were also commonly found, reflecting different parentage across years.
These genetic findings align with field observations of mate switching in banded birds. Together, the data suggest cormorants display serial monogamy across breeding seasons rather than lifelong pair bonds.
Do Cormorants Reunite Outside Breeding Season?
Cormorants seen together during the breeding season go their separate ways after nesting is complete. They migrate and overwinter independently rather than as pairs.
So mate reunion likely only occurs in the spring if both birds survive migration and return to the same colony site. Any synchronized behaviors seen during breeding do not carry over into the non-breeding period.
Conclusion
In summary, the evidence indicates that double-crested cormorants are seasonally monogamous but not lifelong mates. While they form strong social pair bonds during breeding, these bonds do not usually persist across years.
Cormorants exhibit site fidelity to nesting colonies but frequently switch mates between seasons. Genetic data confirms reproduction with multiple partners across years. So although cormorants display coordinated courtship, nesting, and parenting behaviors, they are serially monogamous rather than mating for life.
Comparison with Other Bird Species
How do cormorant mating behaviors compare to other similar waterbirds? Several patterns emerge:
Examples of Lifelong Monogamy
Species | Mating System |
---|---|
Albatrosses | Lifelong monogamy, mate every year unless a partner dies |
Laysan albatross | Over 99% mate fidelity in each breeding season |
Black vultures | Nearly 100% long-term monogamy in studied populations |
Barnacle goose | Long-term pair bonds lasting up to 10 years |
These species demonstrate nearly absolute mate fidelity across years. Pairs reunite year after year at colony sites with extremely rare divorce or mate switching.
Examples of Serial Monogamy
Species | Mating System |
---|---|
Ring-billed gulls | Serial monogamy with high mate switching between years |
Snowy egrets | Frequent mate switching between breeding seasons |
Red phalaropes | Polyandry – females form bonds with multiple males per season |
Double-crested cormorants | Seasonal monogamy but not lifelong pairs |
These species demonstrate habits of forming monogamous seasonal pair bonds but choosing new mates in subsequent years. This serial monogamy differs from lifelong pair bonding.
Why Do Some Birds Mate for Life While Others Don’t?
Evolutionary theory holds clues that may explain variable mating systems in birds.
Lifelong monogamy likely evolves when birds must coordinate and cooperate in order to successfully reproduce. Feeding and protecting altricial chicks is labor intensive, favoring coordinated biparental care.
Conversely, serial monogamy occurs when birds derive no evolutionary benefits from sticking with the same mate year after year. Cormorants can successfully reproduce without a consistent partner.
Habitat variability may also influence mating systems. For species concentrated in just a few breeding colonies, reuniting with previous mates is simpler. But for widespread species like cormorants, mates from prior years may nest elsewhere.
Researchers continue working to uncover the ecological factors driving reproductive behaviors in diverse bird species. Monogamous mating clearly occurs along a continuum in birds, ranging from lifelong pair bonds to flexible serial monogamy.
Future Research
Further studies on cormorant mating systems could focus on these questions:
- How do factors like age, experience, or reproductive success influence mate switching?
- Can we identify ecological predictors of mate fidelity versus divorce?
- Do pairs that reunite have higher breeding success than new pairs?
- How do population demographics like adult sex ratios affect pair bonding?
Tracking banded birds and their reproductive outcomes remains an important approach. Comparing related versus unrelated nestlings also gives insights into mate fidelity. And genetic techniques allow assessing relationships between offspring and parents.
Combining long-term field observations with emerging genomic analyses will continue clarifying the selective forces and evolutionary causes underlying diverse avian mating systems.
References
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