Tree swallows are small, migratory songbirds that breed across North America. They get their name from their habit of making nests in holes in dead trees or nest boxes put up by people. Tree swallows form socially monogamous pair bonds – a male and female will mate and raise offspring together. However, recent research suggests that females may sometimes mate with males besides their social mate, leading to “extra-pair copulations”.
Background on Tree Swallows
Tree swallows (Tachycineta bicolor) are members of the swallow and martin family Hirundinidae. They have iridescent blue-green upperparts and clean white underparts. Their wings are long and pointed, adapted for fast and agile flight to catch insects while airborne. Tree swallows breed in open areas across northern North America from Alaska to Nova Scotia, and south to California and the Carolinas. They migrate to the southern United States, Mexico, and the Caribbean in winter.
Tree swallows nest in cavities, including natural tree holes and nest boxes put up for them. They readily take to nest boxes, allowing researchers to easily monitor their breeding biology. Females build a nest out of grasses and feathers and lay 4-7 white eggs. Both parents incubate the eggs for around 14 days until hatching. Nestlings fledge around 18-24 days after hatching. Tree swallows are prone to competition for nest sites from other cavity nesters like house sparrows.
Social Monogamy and Extra-pair Copulations
Tree swallows form pairs during the breeding season, with a male and female defending a nest site together and sharing parental duties. This is called social monogamy, when two individuals function as a monogamous social unit. However, with advances in genetic analyses in recent decades, researchers have found that social monogamy does not always reflect sexual or genetic monogamy in birds.
In many socially monogamous bird species, DNA fingerprinting has revealed cases of “extra-pair copulations” – where the female will mate with and have offspring fathered by a male other than her social mate. This phenomenon has now been documented in over 200 species of socially monogamous birds, including around 75-95% of surveyed passerine (songbird) species. Rates of extra-pair paternity vary across species but can affect up to three-quarters of broods in some populations.
Extra-pair Copulations in Tree Swallows
Several studies have now shown that extra-pair copulations occur in tree swallows, meaning that females sometimes mate with males besides the one she is paired with. However, estimates of rates of extra-pair paternity have varied across different tree swallow populations.
One early study on a population in Saskatchewan, Canada in the late 1980s found that 27% of nestlings were sired by an extra-pair male. The female tree swallows in this population averaged 1.27 mates per nest. More recent work has revealed even higher levels of extra-pair mating by female tree swallows in some areas:
- A 2000 study in Ontario, Canada showed around 46% of nestlings resulted from extra-pair mating, with an average of 1.46 mates per female.
- In New York state, 39% of broods showed extra-pair young, with an average of 1.39 mates per female.
- However, other populations have much lower rates, such as one in Alaska with only 5% extra-pair nestlings.
Overall, most estimates indicate 15-45% of nestlings are sired by an extra-pair male in tree swallows. So while social monogamy is the norm, many females are gaining “side” matings.
Why Extra-pair Mating Occurs in Tree Swallows
Scientists have proposed several evolutionary explanations for why extra-pair matings are so common in birds supposedly paired for life. These include gaining “good genes” or more compatible genes for offspring, having higher-quality or larger broods, and hedging against mate infertility. The prevalence can depend on factors like breeding density, nesting synchrony, and male ornamentation.
For tree swallows, several drivers of extra-pair mating have been suggested:
- Good genes hypothesis – Females may mate with extra males who have “good genes” to increase the genetic quality and viability of some of their offspring.
- Fertility insurance – Extra matings could serve as “insurance” against infertility in a female’s social mate.
- Trade-off between parental care and genetic quality – Females may trade off some paternal care from their social mate for increased genetic benefits from extra-pair mating.
The good genes idea is supported by findings that extra-pair males tend to be older with more tail streamers, indicative of viability. Fertility insurance could explain why females are more likely to mate outside the pair early in breeding season. But females also seem to balance extra-pair matings with maintaining enough care from their mates.
Do Male or Female Tree Swallows Have More Partners?
The evidence so far indicates female tree swallows are more likely to mate with multiple partners than males. This matches theory, since a male’s reproductive success tends to be limited by access to fertile females rather than vice versa. Moreover, the sex that provides more parental investment (females incubating eggs) is expected to be more selective in mate choice.
Studies quantifying within-pair and extra-pair young show female tree swallows averaging about 1.2-1.5 mates per clutch. In contrast, male tree swallows tend to get over 85% of their young from their social mate. Some males get 100% within-pair paternity. But a small minority of highly successful males can sire many extra-pair young in the population.
The greater variance in mating success for male vs female tree swallows is illustrated in the table below comparing estimates from an Ontario population:
Measure | Male | Female |
---|---|---|
Mean number of mates | 1.85 | 1.46 |
Max number of mates | 6 | 4 |
A small number of “super dad” males mate with many more females than average. But most males largely remain socially paired. So female tree swallows definitely engage in more extra-pair mating than males.
Do Tree Swallows Divorce or Reunite with Previous Mates?
Tree swallows exhibit site fidelity, returning to the same breeding areas year after year. However, they do not always reunite with the same social mate from one year to the next. One long-term study in Saskatchewan found:
- About 61% of pairs remained together from one breeding season to the next
- 31% divorced and paired with a new mate
- 8% reunited with a past mate from 2-3 years prior
Younger females were more likely to “divorce” and take new mates between years. Beyond age, factors like nesting success, relative mate quality, and availability of the previous partner likely contribute to mate switching versus reuniting in tree swallows.
Divorcing a mate doesn’t necessarily mean the pair failed to reproduce. In many cases, a female “traded up” to a higher quality male the next year. This further supports the idea females seek extra-pair matings and switch mates to improve their offspring’s genes. Lifetime monogamy is rare, but social re-pairing with previous mates does occur in a moderate proportion of tree swallows.
How Common is Multiple Mating in Other Swallow Species?
Extra-pair paternity has been confirmed through genetic testing across the swallow family Hirundinidae. Besides tree swallows, high rates of multiple mating have been documented in:
- Barn swallows – Up to 65% of nests contain extra-pair young, with 35-40% on average
- Bank swallows – Around 20% of broods have extra-pair paternity
- Cliff swallows – Up to 27% of offspring come from extra-pair mating
In barn swallows, large tail streamers in males are linked to extra-pair success. Cliff swallow colonies exhibit synchronous breeding, which facilitates extra-pair mating. So despite monogamous social structure, multiple mating is the norm rather than the exception across swallows.
Conclusion
In summary, while tree swallows form socially monogamous pair bonds, many females also mate with males outside of the pair to fertilize some offspring. Multiple mating is common across swallow species despite their outwardly monogamous appearance. Extra-pair paternity in tree swallows likely results from females seeking benefits like better or more compatible genes for their young. Males remain largely paired to a single female, but a minority of highly successful males can sire many extra-pair offspring. Multiple mating strategies reflect the complex evolutionary balances between parental investment, mate quality, and offspring fitness in birds like tree swallows.