Male and female Eastern Bluebirds. Photo: Patricia Pierce/Flickr/CC by 2.0.
Most Bluebirds (95%) mate for life and mated pairs can stay together for as long as they survive. In the event of the death or disappearance of the male or female, the remaining bird replaces it with a new mate. In spite of long-term relationships between mated pairs, studies have shown that both males and females routinely engage in extra-pair copulations. Genetic analysis of nestling bluebirds demonstrated that a male bluebird sired 20-30% of multiple broods analyzed.
Bluebird divorces may occur when a mated pair has repeated nest failures or when females attempt multiple broods with more than one male. After a divorce, the pair splits and each bird tries with a new partner.
Only on rare occasions, one male pair up with two females or two males pair up with one female.
Do bluebirds mate for life?
Studies on Western Bluebirds found that the vast majority of mated pairs stay together for life.
In long-term studies of Western Bluebirds’ natural history, scientists found that only about 6% of 117 pairs studied changed partners when both members of the pair were still alive.
A separate study conducted by Dickinson et al. (1996) found that only 3% of the 106 mated pairs studied switched partners. The switching of partners took place as females attempted multiple broods in a single breeding season and changed males between attempts.
Not such a study exists for Eastern Bluebirds, but field observations suggest that most birds pairs mate for life while both male and female are alive.
There is a lot we do not know about the mated life of the bluebird.
Bluebird pair formation?
Field observations indicate that many birds arrive on the breeding grounds as pairs. It appears that they begin to form pairs either while still in the wintering grounds or during the migration north when they congregate in flocks.
Field observations suggest that migratory bluebirds that are not paired upon arrival in the breeding grounds establish a breeding territory and find a mate within about a week.
Non-migratory Bluebirds generally pair up with the last year’s partner. Even though mated pairs appear not to interact very much after the breeding season, year-round resident or non-migratory Bluebirds are likely to re-mate with the same partner.
The timing of pair formation varies with latitude.
Pairs start forming first during mid-January in the warm southern states. Birds in the northern regions begin to form pairs approximately during February and March.
When is the pair bond official?
An indication that bluebirds are mated is when both the male and female interact and are seen together. But perhaps the most telling sign of pair formation is when both the male and female enter together a nesting cavity or nest box.
But whether the initial association is to remain as a breeding pair depends on additional pair interactions.
The male performs a nest demonstration consisting of bringing nesting material to the cavity. Once she enters the cavity for a first inspection of the interior, the deal appears to be sealed.
The female then accepts the demonstration and approves of the cavity, and starts bringing nesting material herself.
The male can change his mind about a female and reject her by removing the nesting material she started to bring to the nesting cavity.
If the male accepts the female as a mate, he will start offering her food as a sign of reaffirming acceptance of the breeding mate. Then they both enter the nesting cavity confirming the pair formation for the breeding season.
After pair formation and territory establishment, bluebirds start copulating. Mated pairs copulate for approximately eight days before the first egg is laid and continue six days after the clutch’s last egg has been laid.
The female appears to initiate a copulatory event. While perched, she crouches, keeping her back horizontal while dropping and shaking her wings and slightly cocking her tail. This action is called a female’s solicitation posture.
Eastern Bluebird copulations are seldom observed in the field.
The male mounts the female’s back and tries to make cloacal contact. Copulations generally last about 3 to 5 seconds.
Most copulations take place on perches. Although sometimes males try to mount females within the nesting cavity, these attempts are unsuccessful as females do not show interest while in the nest.
Mating for life does not mean copulating with only one partner.
Field ornithologists seldom report copulations outside the mated pair. However, genetic analysis of broods in several Eastern Bluebird populations studied showed that 20-30% of nestlings are sired by more than one male (Gowaty 1996 and Gowaty and Bridges 1991b).
This relatively high percentage of chicks sired by other males suggests that both males and females evade each other’s attention to sneak extra-pair copulations with neighboring birds.
Interestingly, this is the period when the construction of the nest and copulation are occurring, and the male keeps a close watch of the female to precisely keep her from straying and mating with another male to ensure that the brood he is going to raise is his. However, the males do the same and go around mating with other females.
This behavior is not rare among birds. Many studies have found similar behavior among several species of birds.
In areas densely populated by bluebirds where breeding territories are adjacent to each other, the percentage of nestling sired by more than one male is higher than in areas less densely populated by bluebirds.
If breeding territories are of low quality, birds have to travel outside their territories to find food.
Females in low-quality territories that traveled outside during their fertile period had more extra-pair copulations. The genetic analysis of their offspring determined this.
Mating with other birds outside the mated pair may be opportunistic rather than something sought after by the bird engaging in extra-pair copulations.
These studies took place in North Carolina, U. S., and Ontario, Canada. The wide separation between study sites suggests that the practice of extra-pair copulation is widespread across the bluebirds’ range.
The male Bluebird tries to make sure the brood is his.
Males guard their females closely during the period of high fertility, which is when the mated-pair is copulating before and during egg-laying.
Interestingly, males that guard their females more closely have more young sired by other males. Perhaps these males are familiar with their mate’s tendencies to stray onto other territories. It may also suggest that males do not guard the females all the time.
Another interesting fact regarding bluebird extra-pair copulations is that first-time breeders have a higher percentage of broods sired by more than one male than pairs that have been together and bred for an extended period of time.
There are limits to staying together
There is conflicting evidence on whether pairs that fail to succeed on a nesting attempt split to try again with another partner.
Ornithologists have found that most pairs that had successful nesting attempts stayed and re-nested with the same partner.
The same study also found that only 30-50% of pairs that failed on a nesting attempt stayed together and re-nested with the same partner.
Said it differently, 70 to 50% of the pairs that failed on a nesting attempt will switch partners hoping for a successful nesting attempt with another partner.
Eastern Bluebirds re-mate with another partner if one dies. Surprisingly, re-mating with another partner can happen within hours after one member of the pair’s death.
Rarely, 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 vast majority of breeding Bluebirds are monogamous. Gowaty (1980) observed 177 nesting attempts by color-banded birds. The majority of pairs (95.5%) was composed of males and females.
On rare occasions, and perhaps due to the lack of nesting cavities, two females and a male form a breeding family. The two females build the nest and lay eggs in the same nest. Both partake in the incubation of the eggs taking turns or often sitting on the eggs simultaneously.
Once the eggs hatch, both females and the males help raise the brood.
In the case of one male and two females breeding arrangement, it is not clear if the second female or both females mate exclusively with the one male or if one female comes from elsewhere with already fertilized eggs.
Even more rare is when two males breed with a single female. As with the other breeding arrangements, all members of the family help with all breeding activities.
On rare occasions, Eastern Bluebirds can form communal breeding families. Cooperative or family breeding arrangement occurs when 1 or 2 young birds from an earlier brood of the same year stay in the parents’ territory and help with the following brood or their younger siblings.
- 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.