American kestrels (Falco sparverius) are small, colorful falcons found throughout most of North and South America. Known for their acrobatic courtship flights and ability to hover while hunting, kestrels are a delight to observe in the wild. Their lifespan is around 12 years in the wild, though they may live longer in captivity. Given their relatively long lives, an interesting question arises – do American kestrels mate for life?
TL;DR
While American kestrels may remain paired for multiple breeding seasons, they do not mate for life. Pairs form each spring, produce a clutch of eggs, and rear the young together. However, the bond dissolves after the breeding season and adults may take new mates the following year.
Courtship and Pair Formation
American kestrels reach sexual maturity and begin breeding around one year of age. Courtship begins in late winter or early spring, when unmated males begin displaying to attract females. Courtship displays involve the male kestrel calling loudly and performing dramatic aerial maneuvers, including steep dives and rapid climbs. If a female is interested, she may join the male in flight and the two birds will exchange food mid-air as part of courtship feeding. Once bonded, the male and female remain close together. They will perch together, preen each other, and the male will fiercely defend the territory around the nest site from intruders.
Duration of Pairs
While American kestrels form strong seasonal pair bonds, the majority do not remain together from year to year. One long-term study that uniquely color-banded kestrels found that most mated with new partners each breeding season, rather than re-pairing with previous mates.
However, there is some evidence that kestrels may remain paired across multiple seasons under certain conditions. A five-year study of American kestrels in Pennsylvania found that about one third re-paired with their prior mate from the previous breeding season. Re-mating was more likely to occur when the previous season’s breeding had been highly successful, such as when the pair fledged a large number of chicks. If the prior breeding attempt had failed or chicks died, the adults were more likely to seek new mates.
Do They Remain Monogamous Within a Season?
American kestrels are generally considered to be socially monogamous, meaning a mated pair remains faithful and does not mate with other individuals during a given breeding season. However, there is some evidence that occasional “cheating” may occur.
One study conducted DNA analysis of kestrel chicks and found that around 15% were unrelated to their social father, suggesting some instances of females mating outside the pair bond. However, extra-pair mating is difficult to conclusively demonstrate in wild populations and rates may be overestimated. Overall infidelity appears to be rare, as kestrel pairs are highly territorial and guard access to the female.
Reasons American Kestrels Don’t Mate for Life
There are several evolutionary theories that may explain why most American kestrels form new pair bonds each spring rather than mating for life:
- Avoiding inbreeding – By mating with new individuals, kestrels maintain genetic diversity in the population. Mating for life risks inbreeding if close relatives are unable to disperse and breed elsewhere.
- Bet-hedging reproductive strategy – Bonds may dissolve after failure to increase chances of breeding successfully with a new partner next season.
- Seasonal optimizing of fitness – Annual courtship rituals allow individuals to assess potential mates each year and select the highest quality option available.
Comparison to Other Falcon Species
While American kestrels do not mate for life, some other falcon species do form long-term monogamous bonds. Examples include:
- Peregrine falcons – Often mate for life, with pairs staying together across multiple breeding seasons until one mate dies. May reuse the same nesting site for many years.
- Prairie falcons – Thought to maintain long-term pair bonds spanning multiple seasons. Site fidelity to breeding areas may encourage remating.
- Gyrfalcons – DNA analysis suggests most maintain the same mate year after year. One wild female gyrfalcon was observed with the same partner for 10 consecutive years.
The reasons why certain falcon species, but not American kestrels, form permanent pair bonds are not fully understood. Factors like longevity, site fidelity to breeding grounds, and geographic isolation may promote lifelong monogamy in some species.
Breeding Behavior
Although they do not mate for life, American kestrel pairs cooperate extensively in breeding activities within each season:
- The male locates a suitable nest cavity in a tree or building and both birds defend the territory around it.
- The female lays a clutch of 3-5 eggs which are incubated for about 30 days.
- Both parents share brooding and feeding responsibilities for the altricial chicks, which fledge in 4-5 weeks.
- The male provisions food to the female during egg-laying and much of incubation.
- Lifelong monogamy may not be essential since both sexes invest heavily in offspring care.
Parental Investment Table
Breeding Duty | Male Investment | Female Investment |
---|---|---|
Territory Defense | High | High |
Nest Site Selection | Primary | Some |
Egg Production | None | High |
Incubation | Minimal | Primary |
Brooding | Shared | Shared |
Feeding Chicks | Shared | Shared |
Impact of Mate Switching
The seasonal dissolution of kestrel pairs and re-mating with new partners each spring may have some broader impacts on the population:
- Increased gene flow and genetic diversity from mate switching each year.
- Promotion of stronger pair bonds within each season, since effort is not divided across years.
- Reduced risk of population decline if a mate dies, as the surviving bird can easily find a new partner next season.
- Potential spread of parasites and diseases through greater contact between adults each breeding season.
Is Divorce Adaptive?
Studies show that American kestrels who “divorce” and switch mates between seasons actually have higher fitness on average than those who remain paired. This suggests the strategy of seasonal serial monogamy provides overall benefits that may outweigh the costs of finding and courting new mates annually.
Conclusion
In summary, while American kestrels form close seasonal pair bonds and cooperate in rearing young, they do not mate for life across years. Most individuals breed with new partners each spring through an elaborate courtship process. Serial monogamy allows kestrels to optimize mate choice and breeding success annually. Though they don’t maintain multi-year bonds, both sexes invest heavily in their offspring during each breeding season before the temporary pair dissolves until the next spring.