Anna’s hummingbirds (Calypte anna) are small, brightly colored birds found along the west coast of North America. Known for their iridescent feathers and remarkable flying abilities, these hummingbirds have complex mating behaviors that include both polygamy and serial monogamy.
What is monogamy?
Monogamy refers to a mating system where an individual forms an exclusive mating bond with a single partner. In a monogamous pairing, both the male and female partner will only mate with each other during a breeding season or for life. This is in contrast to polygamy, where an individual may have multiple mates.
There are two main types of monogamy:
- Social monogamy refers to a social living arrangement where a male and female pair bond. However, there may still be cases of extra-pair copulations outside the primary partnership.
- Sexual monogamy refers to a mating system where both partners exhibit complete fidelity – there are no extra-pair copulations and all offspring are sired by the resident male partner.
In birds, most socially monogamous species will participate in some level of extra-pair mating, making sexual monogamy less common. However, the level of extra-pair paternity varies greatly across species.
The breeding season
Anna’s hummingbirds breed between November and June depending on their geographical location, with peak breeding in January and February. During this time, the males establish breeding territories centered around food resources that will attract females.
The males perform elaborate courtship displays, flying in U-shaped or J-shaped patterns to attract females to their territory. If a female enters his territory, the male will continue these courtship flights and also vocally defend his territory from intrusions by other males.
Territory and nesting
Once paired with a female, the male Anna’s hummingbird will continue to defend his breeding territory from other males. The female builds a small cup-shaped nest out of plant down, spider webs, and lichens, typically on a downward sloping tree branch.
She incubates the eggs alone for 14-19 days, and cares for the chicks once they’ve hatched. The female may lay 2-3 broods in a season, with 2 eggs per clutch. The young fledge from the nest in 18-26 days.
Male infidelity
Although Anna’s hummingbirds form monogamous pair bonds during breeding, the males are not always sexually faithful to their mate. DNA paternity studies have found that between 10-20% of chicks are sired by an extra-pair male rather than the resident mate.
Male Anna’s hummingbirds may opportunistically mate with females intruding into their territory while continuing to provide care for their original mate’s brood. Due to the high energy demands of their flight, male hummingbirds require frequent foraging. This necessitates leaving their mate alone for periods of time, which creates the opportunity for sneaky extra-pair matings in the absence of the male partner.
Female fidelity
In contrast to the moderate level of infidelity shown by males, female Anna’s hummingbirds appear to be entirely faithful within a breeding season. DNA analysis reveals near zero incidence of extra-pair young in broods. Females likely exercise complete fidelity and mate solely with their resident male partner.
Several factors may contribute to the female’s sexual fidelity:
- Need for male parental care – Hummingbird chicks require extensive care and feeding. Extra-pair mating would jeopardize male care.
- Limited opportunities – Females have fewer chances to copulate outside of their territory due to their full-time incubation and care of young.
- Risk of injury – Attempting to mate with extra-pair males leads to aggressive interactions and potential harm.
While female hummingbirds are physiologically capable of reproducing via extra-pair mating, they appear to avoid seeking extra-pair copulations in order to maintain their current mate’s parental investment in their offspring.
Is Anna’s hummingbird monogamous?
So in summary, while Anna’s hummingbirds form monogamous breeding pairs, they are generally considered serially monogamous rather than sexually monogamous due to moderate levels of male extra-pair mating:
- Social monogamy – Individual males and females form an exclusive pairing bond during breeding season and work cooperatively to raise young.
- Moderate male infidelity – Males will mate outside the pair bond if given the opportunity. About 10-20% of offspring are sired by extra-pair males.
- Complete female fidelity – Females appear to resist extra-pair mating, largely to retain male parental care.
- Seasonal monogamy – These partnerships only last for a single breeding season. New pairs form each year.
The seasonal social monogamy and moderate rate of extra-pair paternity places Anna’s hummingbird on the spectrum between polygamy and sexual monogamy. Their mating system can be classified as serial monogamy with limited polygamous tendencies in males.
Monogamy spectrum across hummingbirds
Anna’s hummingbirds display an intermediate level of extra-pair paternity compared to other hummingbird species:
| Species | Extra-pair paternity |
|---|---|
| Ruby-throated hummingbird | 40-55% |
| Black-chinned hummingbird | 18-25% |
| Anna’s hummingbird | 10-20% |
| Costa’s hummingbird | 5% |
| Buff-bellied hummingbird | 0% |
As this comparison shows, Anna’s hummingbirds tend towards more monogamy than some related species like the ruby-throated hummingbird. However, other hummingbirds like the buff-bellied display complete sexual monogamy within a season.
Why be monogamous?
What evolutionary benefits does monogamous pairing provide for Anna’s hummingbirds? There are several potential advantages:
- Maximize mating opportunities – By maintaining a territory and displaying courtship behaviors, paired males are able to maximize their reproductive success with a female.
- Ensure paternity – Males can be more certain of parentage if they prevent rivals from mating with their partner.
- Shared parental care – Both parents cooperatively raise the offspring, providing food and protection.
- Find the best mate – Longer-term partnerships allow both sexes to assess mate quality over time.
However, these must be weighed against the lost opportunities for additional matings. The moderate infidelity of Anna’s hummingbird males likely represents an equilibrium balancing the risks and rewards of extra-pair mating versus pair bonding.
Serial monogamy across years
The pair bonds between breeding Anna’s hummingbirds dissolve at the end of each season. Both males and females will seek new mates in subsequent years rather than maintaining long-term partnerships with a single bird.
There are no documented cases of Anna’s hummingbirds maintaining multi-year monogamous bonds. Banding studies have shown that individual birds may display site fidelity to the same breeding locations year after year, but will still pair with new mates each season.
Frequently switching partners allows both sexes to optimize reproductive success. For males, it provides increased access to novel females. For females, it allows them to assess the quality of potential new mates and select the best partner each year.
Conclusion
Anna’s hummingbirds display a complex mix of monogamous and polygamous behaviors. While they form temporary seasonal pair bonds and cooperate to raise young, moderate rates of male extra-pair mating and serial mate-switching across years places them somewhere between sexually monogamous and fully polygamous species.
Their mating system can best be classified as serial social monogamy with limited sexual polygamy in males. This represents a balance between the reproductive benefits of pair bonding versus extra-pair matings. Understanding the complex mating behaviors of hummingbirds like the Anna’s continues to provide insights into how different evolutionary strategies shape social systems across species.
