Eastern Bluebirds are renowned for their striking plumage and melodious songs, but their romantic lives hold fascinating intricacies. While many assume these birds engage in lifelong monogamy, the reality is more complex. This article delves into the mating behaviors of Eastern Bluebirds, examining the strength of their pair bonds, instances of infidelity, and the dynamics of their social structures.
The majority of Eastern Bluebirds, approximately 95%, form lifelong partnerships. These bonds can endure for the entirety of their lives, and if one partner perishes, the surviving bird will seek a new mate. However, this apparent monogamy doesn’t preclude extramarital affairs. Studies reveal that both male and female bluebirds frequently engage in extra-pair copulations, with genetic analyses showing that a single male can sire up to 20-30% of nestlings within multiple broods. Bird behavior examples often highlight such complex social dynamics.
Instances of “divorce” can occur in bluebird pairs, typically following repeated nesting failures or when a female seeks multiple partners within a single breeding season. When a split happens, each bird endeavors to find a new mate. Though rare, situations where one male pairs with two females, or vice versa, have also been observed.
Bluebird Pair Formation
Field observations suggest that many bluebirds arrive at their breeding grounds already paired. Pair formation seems to begin either in their wintering grounds or during their northward migration when they gather in flocks. For migratory bluebirds that are not paired upon reaching breeding territories, establishing a mate typically occurs within a week.
Non-migratory Bluebirds often reunite with their previous year’s partner. Despite seeming to have limited interaction after the breeding season, these resident pairs are likely to reform their bond. The timing of pair formation is influenced by latitude, with pairs in warmer southern states beginning to form in mid-January, while those in northern regions initiate this process around February and March.
When is the Pair Bond Official?
An observable sign of a mated pair is when both the male and female are frequently seen together. However, the most definitive indicator of pair formation is their joint entry into a nesting cavity or nest box. The solidification of the pair bond depends on further interactions.
The male initiates a “nest demonstration” by bringing nesting material to the cavity. If the female enters the cavity for an initial inspection and approves, the bond is largely sealed. She then begins contributing nesting material herself. The male can withdraw his acceptance by removing the material she brings, signaling a change of heart. Upon mutual acceptance, the male reaffirms the bond by offering her food, a gesture of acceptance as a breeding mate. Together, they enter the nesting cavity, confirming their partnership for the breeding season.
Copulatory Behavior
Following pair formation and territory establishment, bluebirds engage in copulation. Mated pairs copulate for about eight days before the first egg is laid and continue this behavior for six days after the final egg is laid. The female typically initiates copulation by adopting a “solicitation posture,” involving crouching with her back horizontal, dropping and shaking her wings, and slightly cocking her tail.
Eastern Bluebird copulations are seldom witnessed in the wild. The male mounts the female’s back, attempting cloacal contact, with copulations usually lasting between 3 to 5 seconds. Most copulations occur on perches, though attempts within the nesting cavity by males are generally unsuccessful due to the female’s lack of interest while inside the nest.
Lifelong Mating Doesn’t Mean Exclusive Mating
While field ornithologists rarely report copulations outside established pairs, genetic analyses of Eastern Bluebird broods reveal that 20-30% of nestlings are sired by multiple males. This suggests that both sexes discreetly engage in extra-pair copulations with neighboring birds during the nest construction and early copulation phases. The male’s close observation of the female during this period aims to ensure paternity, yet males also engage in similar behaviors with other females.
This practice is not uncommon among bird species. In areas with high bluebird density and adjacent breeding territories, the rate of nestlings sired by multiple males increases. Females in territories of lower quality, where they must travel further for food, exhibit higher rates of extra-pair copulations during their fertile periods, as confirmed by genetic analysis of their offspring. These extra-pair mating behaviors might be opportunistic rather than actively sought. Studies conducted in North Carolina and Ontario indicate that this practice is widespread across the bluebird’s range.
The Male Bluebird’s Paternity Assurance Efforts
Males intensify their guarding of females during the peak fertility period, which coincides with copulation before and during egg-laying. Interestingly, males who guard their females more closely sometimes have a higher percentage of young sired by other males. This could indicate an awareness of the mate’s propensity to stray or that male guarding is not constant. First-time breeders also show a higher incidence of broods sired by multiple males compared to pairs with established, long-term relationships.
Limits to Pair Bonds
Evidence is mixed regarding whether pairs that experience nesting failures separate to find new partners. Pairs with successful nesting attempts predominantly remain together for subsequent nesting. However, between 50% to 70% of pairs that fail in a nesting attempt will seek new partners, hoping for greater success. Eastern Bluebirds will also re-mate rapidly if one partner dies, sometimes within hours.
Bluebird multiple partnersRarely, two males form a breeding arrangement with a single female. All participants help with all breeding activities.
More Than Two Birds in a Mated Pair?
The overwhelming majority of breeding bluebirds are monogamous, with studies showing approximately 95.5% of pairs consisting of one male and one female. In unusual circumstances, potentially due to a scarcity of nesting cavities, two females and a male may form a breeding family. Both females contribute to nest building and egg-laying in the same nest, sharing incubation duties. Following hatching, both females and males participate in raising the brood. The dynamics of mating in these arrangements, whether exclusive or involving external fertilization, remain unclear. Even rarer are instances of two males breeding with a single female, where all family members contribute to breeding activities.
Occasionally, Eastern Bluebirds form communal breeding families. This cooperative arrangement occurs when one or two young birds from an earlier brood within the same year remain in the parents’ territory and assist with the subsequent brood or their younger siblings. Fledging bluebirds often benefit from such family support systems.
Conclusion
Eastern Bluebirds exhibit a complex mating system characterized by strong pair bonds that often last a lifetime, yet are not exclusive. While monogamy is the norm, extra-pair copulations are common, highlighting the birds’ strategic approach to reproduction and genetic diversity. Understanding these intricate behaviors provides deeper insight into the social ecology of these beloved North American birds. For more detailed information on bluebird life cycles and behaviors, explore our other articles on Eastern Bluebirds.
References:
- Gill, Frank (1995). Ornithology. New York: W.H. Freeman.
- Gowaty PA, Plissner GH. (1998). Eastern Bluebird (Sialia sialis). The Birds of the World Online. Cornell Lab of Ornithology, Ithaca, New York.
- Eastern Bluebird, Life History. All About Birds. The Cornell Lab of Ornithology.
