The eastern kingbird (Tyrannus tyrannus) is a medium-sized songbird found across North America. These birds are territorial and monogamous, often pairing up for breeding and migration. But do eastern kingbirds actually mate for life? Let’s take a closer look at the mating habits of this species.
Courtship and Pair Bonding
Eastern kingbirds form pair bonds during the breeding season, which lasts from late April to August. Courtship displays like aerial pursuits and gift-giving help establish the bond between a mating pair. Once paired, they will defend a nesting territory together and share parenting duties.
Research shows that kingbird pairs do reunite year after year. A five-year study found that if both birds survive the winter, they are likely to breed together again the following spring. The female returns to the same breeding territory and her distinctive voice helps the male identify and locate her. So there is definitely site fidelity and mate fidelity from season to season.
Cooperative Breeding
Interestingly, eastern kingbirds may accept a new mate while the previous partner is still alive. This cooperative breeding strategy is thought to maximize the number of offspring. One mate may not be enough to successfully raise multiple broods in a single breeding season.
A study in New York found that 20% of kingbird pairs involved males or females retaining their original partner, but breeding with a new additional mate. Almost a third of nests contained young fathered by different males. So while kingbirds are monogamous and reunite each spring, their bond may only last a single breeding season.
Divorce and Re-mating
There are a few scenarios that can lead to “divorce” in eastern kingbirds:
- One mate does not return from migration, likely due to mortality
- One mate returns to the breeding site later, causing the early returnee to find a new partner
- Incompatibility between the mates leads one bird to seek out a new partner
Researchers found that when forced to pick a new mate, female kingbirds almost always select a familiar male from a previous season over an unknown male. Even if he was not her partner, familiarity seems to be a key factor.
Males are less selective and may form a new pair bond with any female that occupies their former nesting site. This may explain why females return to the same territory, while males base their choice on the female herself.
Nesting Site Fidelity
While eastern kingbirds may or may not reunite with their previous mate, they do show fidelity to their nesting site. Females in particular exhibit strong nest site fidelity, returning to the exact same territory year after year.
A study in Michigan found that females returned to within an average of 60 feet from the previous year’s nest. It seems that site familiarity perhaps outweighs mate fidelity in some cases. Immigration and emigration rates are low for breeding females, but higher for males.
Migration Habits
The fact that eastern kingbirds migrate over 3,000 miles to Central and South America each winter may impact their ability to mate for life. Kingbirds migrate at night in solitary fashion, not in pairs or flocks. This makes it more challenging to keep track of a previous mate.
Research shows that females depart earlier than males in the fall. And the larger, older males tend to arrive back on the breeding grounds first in the spring. So timing differences in migration may cause previous pairs to miss each other. Finding a new mate may be necessary in these cases.
Lifespan
Eastern kingbirds are relatively short-lived songbirds, with an average lifespan of just 2-3 years in the wild. The record longevity is 12 years and 9 months for a banded kingbird. Their short lifespans, coupled with high mortality during migration, limits how many breeding seasons a pair can stay together.
Few kingbirds survive more than 4-5 years. So while some pairs may reunite for a few breeding seasons, most are simply not living long enough to mate for life in the true sense. In species with longer lifespans, long-term monogamy is more common.
Conclusion
Eastern kingbirds do exhibit site fidelity and mate fidelity between breeding seasons. Many pairs reunite year after year, especially if both birds survive the winter and return to the same location. However, they may also “divorce” and find new mates for various reasons.
Cooperative breeding is common as well, with birds taking on new mates while the previous partner is still alive. Given their relatively short lifespans and challenging migrations, most eastern kingbirds only manage to stay together for a breeding season or two. The evidence suggests they are serially monogamous across years, but not truly monogamous for life.
Year | Original Mate | New Mate |
---|---|---|
2021 | Male A | – |
2022 | Male A | Male C |
2023 | – | Male D |
This table shows an example of the serial monogamy and mate switching that can occur across breeding seasons. The female breeds with Male A in 2021, reunites with Male A but also breeds with Male C in 2022, and finally breeds with a new Male D in 2023 after Male A dies or fails to return.
Key Takeaways
- Eastern kingbirds form seasonal pair bonds and many reunite each spring
- Cooperative breeding with new mates is common
- Short lifespans and migration challenges prevent lifelong monogamy
- Females exhibit strong nest site fidelity across years
- Serial monogamy across seasons is more accurate than lifelong monogamy
In summary, the evidence shows that while eastern kingbirds do mate faithfully during a given breeding season, they should not be considered truly monogamous for life. Serial monogamy and new pair bonds each spring are more typical due to various environmental and biological factors. Their mating system maximizes reproductive success despite constraints like migration and mortality. Lifelong monogamy is rare in eastern kingbirds, but they do reunite with previous mates at moderately high rates.