If you’ve ever hosted bluebirds, whether it’s eastern, western or mountain bluebirds, then you know too well about the amazing devotion mates have toward each other, and how vigilant they are caring for their young. It might make you wonder, do bluebirds mate for life?
Bluebirds often socially mate for life. However bluebirds will also engage sexually with others who they are not socially bonded to. Molecular studies have confirmed some bluebird nestlings are from another male, though percentages of how often this happens vary widely between studies.
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Article Contents:
- Introduction to bluebird social and sexual mating
- Why female bluebirds engage in extra-pair mating
- What exactly does “mate” mean for bluebirds?
- Do father bluebirds give less care for young that isn’t their own?
- How scientists determine bluebird paternity
- References
Introduction to bluebird social and sexual mating
So let’s unpack this information a little better. Based on what we’ve said, bluebirds sort of engage in life-long bonds, but this doesn’t mean they are sexually faithful.
In a clutch (set of eggs in a single egg-laying period) of four nestlings, it’s possible that some of the eggs are from the bonded male, while some are from another male.
It may seem sad to find out that these devoted birds aren’t as faithful to each other as they appear. However, one happy thought is the fact that band studies have found that during the breeding season, socially bonded pairs often return to each other, if both are still alive, and begin the nesting season with each other all over again.
There have been some situations where a socially bonded pair of bluebirds breakup if they had an unsuccessful nesting event.
An unsuccessful nesting event would be when all or most of the nestlings do not survive to fledge the nest. In this situation, bluebirds will potentially seek new social mates or new nesting sites in an effort to establish better nesting success.
As for extra-pair relationships, it can be hard to estimate or predict the extent of “extra-pair copulations” among bluebirds. There simply is not an average or agreement.
Gowaty & Karlin (1984), for example, cited 5% of males and 15% of female bluebirds were taking care of nestlings that weren’t their own. And in 25% of the cases, they observed multiple parenting events.
In a later study by Gowaty & Bridges (1991), infidelity or multiple parenting events happened in 35% of observed cases.
Another 2010 study showed only 11% of nestlings arose from encounters with male bluebirds who were not the mate of the female. And of that, 26% of the broods had at least one nestling that was fertilized from another male (Stewart, Westneat, & Ritchison, 2010).
Table 1. Abridged list of studies and statistics of bluebird extra-pair mating
| Researcher | Percent of nestlings fathered by another bluebird |
| Gowaty & Karlin (1984) | 25% of observed nests sites had multiple parenting events |
| Gowaty & Bridges (1991) | Extra-pair offspring observed in 35% of studies cases |
| Dickinson (2003) | 42% of observed nestlings were from extra-pair bluebirds |
| Steward, Westneat & Richardson, 2010 | 11% of observed nestlings were from extra-pair bluebirds |
Trail monitors in other locations have reported higher numbers, and instances of lower numbers, which again emphasizes that citing one study is not necessarily enough to confirm a standard.
Now comes the flood of questions: what reasons would a female have for being with another male bluebird? How does mating and egg laying even work with bluebirds? How do researchers know the paternity of each nestling? Don’t the males try to keep other males out of their territory? Do male bluebirds care less for nestlings that aren’t their own?
Why do female bluebirds engage in extra-pair mating?
There are many reasons female bluebirds will breed outside of their pair-bonded male. Reasons include genetic diversity, low breeding synchrony, additional males (migratory) within the population, and nesting on the edge of a territory.
Reproductive synchrony is attributed to being one of the reasons for male bluebird infidelity. What we mean by reproductive synchrony is the alignment of fertility between a population of males and females.
If for instance, the bonded female was not fertile at the moment, this might encourage the male to venture elsewhere (Meek & Robertson, 1994).
Mate guarding, or the lack of mate guarding can also influence mating outside of the pair bond. If a male bluebird is spending more time guarding his territory and not his mate (mate guarding), this can also leave the female more available to copulate with another male.
Mate guarding in birds is where a male bird will keep a close watch on his mate during times of fertility, and follow her around to ensure his paternity over the nest and prevent other males from engaging with the female.
The mated male bluebird may also engage in more copulation events with the female to ensure his paternity.
Interestingly, bluebirds are very territorial. Eastern bluebirds, for example, establish large sized territories. The more dominant the male is, the larger the territory.
Advice to trail monitors is to space nest boxes at least 100 yards apart from one another. However some experienced monitors have found that even this isn’t enough for extremely territorial males.
It’s not just the females who will engage in extra-pair copulations (cheat on their mate). Pair-bonded males will also copulate with other females. However, many studies have focused on female copulation decision and extra-pair paternal events.
There are also different hypotheses for mating outside the bond. First, females are not passive in these events, but active. Second, the events can happen more often when the male is away from his territory during a time when the female bluebird is incubating her eggs. Third, extra-pair copulation events seemed to occur more frequently in newly bonded couples versus pairs who have returned to each other for another breeding season.
So what exactly does “mate” mean for bluebirds?
Bluebirds, like other songbirds, mate socially. And many species are classified as “socially monogamous.” This means a bonded pair will share care of nestlings and fledglings. However, a pair of bonded bluebirds does not make them sexually monogamous.
Defining the term “mate” for bluebirds, therefore, involves two factors: social partnering and copulation partnering.
Based on much of the observation, the social partner will biologically father most of the nestlings within a nest (“within-pair copulation”). However, a female bluebird, as you’re learning, may have engaged with another copulation partner.
In either case, it is the socially partnered male and female who will take the responsibility of caring for the nestlings within their established nest.
Pair bonding begins at the start of the breeding season. Winter flocks of bluebirds will begin to break off and establish breeding territories. More dominant males will establish larger territories by comparison to less dominant males.
And during this period, social paring will begin. Usually, bluebirds will resume partnership with their previous social mate.
The male will show the female different potential nesting sites, and it will be the female’s decision on where nesting will actually take place.
Typically, copulation events begin before the nest is built. However some trail monitors have reported “bluebird honeymoons.” This occurs after the female has built her nest and then the pair seemingly disappears for a period of time. After a week or two, they’ll reappear and the female will begin laying her eggs and incubating.
During the egg incubation period and nest brooding period, the male bluebird will guard the territory, watch the nest and bring food to the female.
Keep in mind, the female does not practice “continuous incubation” where she sits nonstop on the nest. Instead, she will leave the nest for food or to stretch, or to cool the eggs down on a hot day.
Ultimately a songbird, like a bluebird or a chickadee, may be considered “socially monogamous” but not sexually monogamous (Mennill, Ratcliffe, & Boag, 2002).
So when it comes to whether bluebirds mate for life, we’ve established that they usually mate socially for the lifetime of their partner, but they do not sexually mate for life.
There are caveats to the social mating interactions of bluebirds, this is true of each bluebird species as well as other songbirds. In some cases, a second male will help care for the nestlings. In other words, three adult birds are invested in a single nesting site, caring for nestlings and fledglings. A situation like this is extremely rare.
Do father bluebirds give less care to young that isn’t theirs?
It turns out, male bluebirds caring for young will provide the same amount of care despite parentage. Many studies have looked into whether male bluebirds were more attentive to nestlings that were biologically theirs versus nestlings that were fertilized by another bluebird.
Data from research was surprising; however, it makes sense male bluebirds would be equally attentive to all young in the nest, even if some are fathered by another bird because we have seen this example with the cowbird.
A female brown-headed cowbird will lay her eggs in other birds’ nests. If the birds do not detect the difference and the baby cowbird hatches, the adopted parent birds will care for the baby cowbird. Often, required care and demands end up sacrificing original nestlings.
Cowbirds do lay eggs in bluebird nests, and both male and female bluebirds who did not detect the cowbird intrusion or egg difference would provide plenty of care to the baby cowbird.
The research regarding male attentiveness in the 1984 Gowaty and Karlin paper even made researchers further consider overall bluebird habits and whether they might be categorized more as polygamous rather than monogamous.
To further demonstrate reproducibility in these studies, a 1998 study found the same results regarding attentiveness in male bluebirds. In this study, researchers MacDougall-Shackleton & Robertson had controls and experimental nests. In experimental nests, mated males were removed from the area during a 2-day period of female fertility to reduce “paternal confidence.”
Through DNA fingerprinting, researchers were able to determine the exact paternity of the nestlings. And even though the hypothesis still lingered that male bluebirds would reduce care for nestlings they were uncertain of, their findings demonstrated the opposite. Care was equal among nestlings in their study as well.
At this point, we can fully gather that bluebirds and many other songbird species are not monogamous; however, males will make efforts to protect their parentage.
How scientists determine bluebird paternity
To determine bluebird paternity, researchers will use DNA fingerprinting technology, which involves taking a sample of DNA, breaking it up into smaller components and comparing the results against known samples.
Researchers will usually take blood samples of the nestlings, mother bluebird and potential father bluebirds and extract their DNA from the blood samples. Sometimes, the original sample of extracted DNA is not enough for analysis. Therefore, they’ll use the sample DNA to make copies.
The best analogy for comparison would be if you had a form you signed and needed to make copies of. You would scan the form and tell the machine to print you a certain amount of copies.
Likewise, researchers use a molecular technique called “polymerase chain reaction,” or PCR for short, to copy DNA. The original sequence of the double-helix is broken apart and each strand of DNA is used as a template to produce more.
Millions of copies can be produced, yielding enough of a sample for analysis.
The next step is to cut the DNA up into smaller fragments based on cut sites. Now, a bunch of little DNA sequences in each vial will be floating around in a suspension.
Next, researchers will run a test called “gel electrophoresis.”
There are little “wells” in the gel-like structure where samples of DNA are dropped into. The gel is ran on a machine that separates the small DNA fragments based on size.
What shows up on the gel are colorful bands. Researchers can now compare the bands of the nestling to the bands of the mother bird, and potential father birds to determine paternity.
DNA fingerprinting tests are 99.9% accurate. This means the results researchers have found have been significant and eye-opening in terms of overall bluebird fidelity. Male bluebird care of nestlings that might not be theirs
References
Dickinson JL, Koenig WD, Pitelka FA, 1996. Fitness consequences of helping behavior in the western bluebirds. Behav Ecol 7:168-177.
Dickinson, J. L. (2003). Male share of provisioning is not influenced by actual or apparent loss of paternity in western bluebirds. Behavioral Ecology, 14(3), 360-366.
MacDougall-Shackleton, E. A., & Robertson, R. J. (1998). Confidence of paternity and paternal care by eastern bluebirds. Behavioral Ecology, 9(2), 201-205.
Meek, S. B., & Robertson, R. J. (1994). Interspecific competition for nestboxes affects mate guarding in eastern bluebirds, Sialia sialis. Animal Behaviour, 47(2), 295-302.
Mennill, D. J., Ratcliffe, L. M., & Boag, P. T. (2002). Female eavesdropping on male song contests in songbirds. Science, 296(5569), 873-873.
Stewart, S. L., Westneat, D. F., & Ritchison, G. (2010). Extra-pair paternity in eastern bluebirds: effects of manipulated density and natural patterns of breeding synchrony. Behavioral Ecology and Sociobiology, 64(3), 463-473.
