Eastern bluebirds (Sialia sialis) are a popular and easily recognized songbird found in open and semi-open habitats throughout much of eastern North America. Male Eastern bluebirds are known for their brilliant royal blue upperparts and rusty reddish throat and breast, while females are mostly grayish blue above and pale below with some blue on the wings and tail.
Eastern bluebirds are socially monogamous, meaning a male and female form a pair bond and work together to raise young during the breeding season. But are they sexually monogamous as well? Do they mate exclusively with their social partner throughout the breeding season and from year to year? Or do they ever seek “extramarital” copulations outside of their pair bond? These are important questions when considering the evolution of mating systems and parental care strategies in songbirds like the Eastern bluebird.
What is Social and Sexual Monogamy?
Social monogamy refers to a social living arrangement in which a male and female pair up and work together to defend a territory, build a nest, and raise young. It does not necessarily imply sexual fidelity or exclusive mating between the pair. Sexual monogamy refers to exclusive mating between a male and female pair throughout the breeding season or over multiple years.
Some key points about social and sexual monogamy:
- Many bird species are socially monogamous but not necessarily sexually monogamous.
- Sexual monogamy in birds is quite rare—only around 10-15% of all bird species are considered sexually monogamous based on genetic paternity studies.
- Even in species considered “monogamous,” infrequent cases of extra-pair mating may occur.
- Males are often more likely to pursue extra-pair copulations than females in socially monogamous species.
- The degree of sexual monogamy can vary considerably among populations of the same species based on ecological factors.
So social monogamy does not guarantee sexual exclusivity. Male and female partners in a socially monogamous pair may still seek extra-pair matings given the right circumstances. This highlights the need for genetic paternity studies to confirm whether a socially monogamous species like the Eastern bluebird is truly sexually monogamous as well.
Evidence for Social and Genetic Monogamy in Eastern Bluebirds
Several lines of evidence indicate that Eastern bluebirds are primarily socially and genetically monogamous, with a relatively low frequency of extra-pair matings:
High Mate Fidelity
Studies tracking banded Eastern bluebirds have found very high mate fidelity from year to year. In one 20-year study in Michigan, researchers found that mate retention across breeding seasons averaged 96% for females and 64% for males. Both males and females will often return to the same breeding territory and partner in subsequent years.
Low Rates of Extra-Pair Young
Analyses using genetic markers have shown low levels of extra-pair paternity in Eastern bluebird broods:
- One study found only 2.7% of nestlings resulted from extra-pair mating.
- Another broader analysis detected even lower levels—only 1.5% extra-pair young across all studied broods.
This indicates the vast majority of Eastern bluebird chicks are sired by the female’s social mate, suggesting a high degree of sexual fidelity.
Aggressive Mate Guarding
Eastern bluebird males closely guard their mates during the female’s fertile period, following them closely and chasing away potential intruders. One study found they spend 70% of their time near the female 5 days before and during egg laying. Such mate guarding behavior is expected for a predominantly monogamous species needing to ensure paternity.
Lack of Sexual Dimorphism
Eastern bluebirds lack strong physical differences between the sexes. This again points to a mating system based on social and genetic monogamy rather than extra-pair mating. In contrast, sexually promiscuous bird species often show stronger sexual dimorphism and “sexier” traits used to attract multiple mates.
Evidence | Description |
---|---|
High mate fidelity | 64-96% mate retention between breeding seasons |
Low extra-pair matings | Only 1.5-2.7% of young genetically unrelated to social father |
Aggressive mate guarding | Males closely follow females before egg laying |
Lack of strong sexual dimorphism | Males and females are physically monomorphic |
Why Such Strong Monogamy in Eastern Bluebirds?
What evolutionary factors may have led to such a high degree of social and sexual monogamy in Eastern bluebirds? Here are some of the leading hypotheses:
Need for Biparental Care
Eastern bluebirds exhibit high levels of biparental care. Both parents participate in choosing nest sites, building nests, incubating eggs, and feeding nestlings. Having a reliable, investing partner is critical to successfully raising young. Extra-pair mating could jeopardize this parental collaboration.
Low Annual Reproductive Output
Eastern bluebirds in northern parts of their range normally only raise one or two broods per breeding season. Producing a few broods with a reliable partner may lead to greater reproductive success than seeking frequent extra-pair mating but less male parental care.
Dense Nest Site Limitation
Competition for tree cavities and nest boxes that bluebirds require for nesting is intense. A male who loses his partner risks losing access to suitable nesting sites, which are in short supply. This may select for maintaining pair bonds.
Female Mate Choice for Parental Care
Females may favor male partners who demonstrate willingness to provide care over males pursuing more frequent mating. By mating with her social partner, a female ensures continued male investment in the young.
Hypothesis | Description |
---|---|
Need for biparental care | Both parents required to successfully raise offspring |
Low annual reproductive output | Only 1-2 broods per season in north |
Dense nest site limitation | Intense competition for tree cavities and nest boxes |
Female preference for parental care | Females favor males that provide care |
Do Extra-Pair Mating Patterns Vary?
While Eastern bluebirds are largely monogamous across their range, a few studies suggest there may be regional and individual variation in levels of extra-pair mating:
- Higher latitude populations show lower extra-pair mating rates than southern populations.
- Age may influence infidelity—older males were found to pursue more extra-pair matings than yearling males.
- “Divorces” in which birds switch mates between breeding seasons are more common in females than males.
- Individual variation occurs—a small number of males gain a disproportionate share of extra-pair matings.
So while exceptions occur, social and genetic monogamy remains the dominant mating strategy across the species as a whole. Small degrees of flexibility may help maximize individual reproductive success when opportunities arise.
Conclusion
In summary, Eastern bluebirds demonstrate very high levels of social monogamy, with most males and females breeding exclusively with one partner during a breeding season. Genetic analyses confirm a correspondingly high rate of sexual monogamy as well, with around 95-98% of young sired by their social father.
This degree of monogamy appears related to strong needs for biparental care, limited annual reproductive output, and competition for nest sites in Eastern bluebirds. However, some variation exists, with Souther populations and older males showing slightly higher rates of extra-pair mating. But overall, Eastern bluebirds provide a prime example of the monogamous mating systems predominating among temperate zone passerine birds. Their reliance on monogamy continues to fascinate biologists while delighting backyard bird enthusiasts across North America.