There are a lot of birds that mate for life, but what about bluebirds? Do any of the bluebirds in North America mate for their entire lifetime?
The answer is yes…sort of.
Bluebird mating habits are actually quite fascinating. While a male and female bluebird will typically pair up and mate for life, there is fascinating research into the genetics of bluebirds that shows that females aren’t monogamous–and the young in a bluebird nest may or may not belong to the male defending that nest.
Let’s learn more about the interesting mating habits of bluebirds (including all three types of bluebirds in North America: Eastern, Western, and Mountain Bluebirds).
How Do Bluebirds Choose a Mate?
Bluebirds are generally monogamous, but how do they end up in their chosen pair?
Although some bluebirds are migratory and others are year-round residents, males still want to get to their breeding grounds as early as possible. This is because they want to outcompete other birds for the ideal nesting spot.
Bluebirds are cavity-dwellers, which means they live in holes in trees or in nesting boxes. They aren’t capable of excavating their own cavities, as they have very small bills. Instead, they are dependent upon claiming a cavity before any other birds have the chance.
They often find themselves competing with Northern Flickers, Tree Swallows, and invasive European Starlings for territory.
The single most important thing for a male to do if he wants to secure a mate is to find a desirable nesting cavity.
He will show off potential nesting sites by singing, drawing attention to himself, and displaying nesting materials to the female.
The female bluebird is very selective–she wants a safe, secure place to build the nest, incubate her eggs, and raise her hatchlings.
How Do Bluebirds Mate?
Copulation occurs after the female has chosen her mate and the two birds have determined their territory and nesting cavity.
Observation of bluebird copulation behaviors shows that the female typically initiates the act by crouching low, dropping her wings, and raising her tail. She will do this while perched on a branch, as opposed to in the nest. Although males will sometimes attempt to copulate in the nest, females are disinterested and uncooperative in this location.
Like other birds, males and females both have a cloaca, which is the opening below the tail that serves multiple purposes.
The cloaca receives or expresses sperm, depending on the sex of the bird. It is also where birds release urine and feces, and where the female passes her eggs.
Copulation between bluebirds lasts between 3-5 seconds. The male will mount the female, and the birds’ cloacas will make brief contact to transmit sperm from the male to the female.
Bluebird Pairs Stay Together for Life–Mostly
Nest Hollow has provided a great overview of the studies related to bluebird monogamy. Here is the chart they produced:
| 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 |
What can we take away from these studies?
It becomes clear that even though bluebirds tend to mate for life, they are not strictly monogamous.
25% of nests that were studied by Gowaty & Karlin in 1984 found that the young were fathered by different males. This means that the female had also copulated with males other than her chosen mate.
Later, in 1991, Gowaty & Bridges confirmed these findings, documenting that 35% of hatchlings born in any nest were from a pairing outside of the original mated pair.
That increased by 2003 when Dickinson confirmed that 42% of hatched bluebirds were not the offspring of the mated pair. However, a 2010 study by Steward, Westneat & Richardson found that only 11% of hatchlings had different parentage than their nesting parents.
The numbers vary, but there is plenty of clear evidence: female bluebirds are not monogamous.
Reasons for Females to Mate With Additional Males
There are several reasons why a female might choose to mate with more than one male. Even if she is bonded to one specific male partner, she may breed with another male for the following reasons:
- Improving genetic diversity
- Improving breeding synchrony rates (ensuring that babies hatch around the same time as others in the area)
- Access to additional, unbonded males
- Her bonded male is protecting their territory, but not his mate, from additional males
What About Males Mating With Other Females?
Males will also breed outside of their bonded pair.
For example, if a female is not fertile in synchrony with the appropriate breeding window, he may choose to breed with a female who is fertile, thus creating more opportunities for genetic reproduction success.
Nest Failure & Bonded Pairs
Sometimes, a nest will be predated by a common bluebird predator (hawks, snakes, and cats are common culprits), and none of the hatchlings survive.
Disease and natural disasters could also cause a catastrophic nesting failure for a pair of bluebirds.
Sometimes when this happens, bluebirds will “break up” and find a new mate.
FAQs About Bluebird Mating Habits
Here are some other questions we often get asked about bluebirds and their mating habits.
Do Male Bluebirds Care For Other Males’ Offspring?
Female bluebirds do most of the rearing of hatchlings. However, males protect the nest and the territory while the female is raising the young. They also bring food to the female bird while she is incubating the eggs.
To a male bluebird, the only thing that matters about the babies is that they are in his nest. He will protect them, regardless of their paternity.
The only way to determine the genetic father of a nestling is to do genetic testing, so it is not as if males can differentiate between their own hatchlings and another male’s offspring.
Do Bluebirds Return to the Same Nest Every Year?
Bluebirds may return to the same nest every year, although it’s not a guarantee that they will do so. Some estimates state that they return to the same nest about 33% of the time.
The earlier you put out a nest box, the more likely you are to have a nesting pair in your yard.
What Kind of Nesting Box Is Best for Attracting Bluebirds?
Bluebirds are drawn to nesting boxes because there is so much competition for natural cavities–especially in areas where European starlings have an established population.
The ideal nesting box is made from cedar or recycled plastic, rather than cheap wood. It will have predator protection and a mesh floor.
Install your nesting box so that it faces the southwest, protecting the birds against wind and precipitation. Ideally, you will mount it on a 4×4 pole or post–rather than on a fence post or on a tree. It should be 5-6 feet above the ground.
Do Bluebirds Visit Birdfeeders?
Bluebirds will visit birdfeeders that are out in the open with plenty of mealworms. They might visit for other food sources, but mealworms are definitely their preferred food at a birdfeeder.
What Other Birds Mate for Life?
Some of the other kinds of birds that are generally monogamous include:
- Some owls
- Sandhill cranes
- Bald eagles
- California condors
- Golden eagles
- Sandhill cranes
- Canada geese
- Black vultures
- Atlantic puffins
- Trumpeter swans
- Mute swans
- Lovebirds
- Scarlet macaws
- Carolina wrens
- Penguins
- Ospreys
- Whooping cranes