Nesting Behaviors of Eastern Bluebirds in the USA

Found in North America, there are three known species of bluebird; the Eastern 

Bluebird, the Western Bluebird and the Mountain Bluebird. 

Living up to their name, bluebirds are, well, blue. 

However, it is not just their striking colouration that makes the bluebird stand out. It is also their interesting approach to nesting. 

Being secondary cavity-nesting birds, they rely on other species to create spaces for them to nest. Once in the vacant cavity, each sex has a defined nesting behavior. Female Eastern Bluebirds build and contrast the nest, whilst the male guards his mate.

This article will aim to explore the nesting behaviors of the Eastern Bluebirds.

What Are Eastern Bluebirds?

Before we delve into the world of Eastern Bluebird nesting, first we need to understand what exactly an Eastern Bluebird is. 

The Eastern bluebird is a small passerine bird species belonging to the thrush family, not too dissimilar from the European robin.

Although the Eastern Bluebird has a strikingly vibrant blue head and wings, their breast is a mix of orange and white plumage.

These colors become even more pronounced in males during the breeding season, which occurs throughout the Spring and Summer months – typically between April to July. 

Fractionally larger than their Western cousins, the aptly named Western Bluebird, Eastern Bluebirds can reach lengths of 21 cm and weigh up to 32g. They are omnivorous foragers, feeding on a range of insects and seeds and berries. 

Eastern Bluebirds are a highly social bird species, often observed flocking together in groups of over 100. 

Geographical Distribution of Eastern Bluebirds

Eastern bluebirds have a relatively large range and, as the name suggests, can be found in much of Eastern North America. They typically occur East of the Rocky Mountain range.

There are 8 recognised subspecies of the eastern bluebird. Some can be found as far North as South Canada. Other populations can be found as far South as Nicaragua. 

Eastern Bluebirds living in warmer regions, such as Southwest USA and Central America, will remain year-round. 

Other populations, especially those living in Canada and North USA, are partial migrants. 

This means, during certain unfavorable months, individuals migrate from their breeding sites to new areas with plentiful food and a stable climate. 

Eastern Bluebirds are heavily dependent on weather and the climate. If conditions are too cold, many individuals will perish. 

To avoid high chick fatality rates, adult Bluebirds nest in enclosed cavities within trees. Not only does this protect developing eggs and chicks from potential predators, it also provides the young some respite from the elements. 

Where Do Eastern Bluebirds Nest?

Bluebirds are secondary cavity-nesting birds. 

Despite its long-winded, and somewhat complicated, name, secondary cavity-nesting birds really do what they say. Let’s break it down. 

The “cavity-nesting” aspect is derived from where they nest. For Eastern Bluebirds, which are susceptible to cold weather and high predation risks, holes within trees, or cavities, offer increased protection.

However, Eastern Bluebirds lack the necessary adaptations that would allow them to create these cavities themselves – their beaks and feet simply aren’t strong enough to pick away at tree bark. 

Instead, they must rely on other species to construct these initial cavities. Such species, such as woodpeckers, are referred to as primary cavity-nesting birds. These are the species that make the original holes. Fortunately for the Bluebirds, woodpeckers typically use their cavities for just one season, before moving on to create a new hole. 

Vacant holes are prime real estate, and they are free for the taking. So, can you see where the “secondary” aspect comes from?

However, the best sites come at a cost. Before the breeding season, males will compete with one another to gain the best nesting cavity. 

Although Bluebirds can be found across much of North America, they are found in particularly high densities throughout human-modified environments, such as pastures and agricultural land, orchards, parks and gardens.

Typically, when humans alter wild landscapes, it is to the detriment of the species living there. 

For the eastern Bluebird, however, human modification has allowed populations to flourish. Rotting fence posts provide an excellent place to bore. Fruit orchards contain stable food sources. Suburban parks and backyards may have artificial nesting boxes. 

All these factors have allowed the eastern Bluebird to nest successfully in an ever-changing world. But more on that later. 

Copulation In Eastern Bluebirds 

Copulation, otherwise known as sexual intercourse, is the process of transferring sperm from the male, to the eggs of the female. 

This is an incredibly energetically costly investment for the female, so she needs to make an informed decision before mating with just any old mate. 

And the male knows this. 

So, in order for the male to secure mating rights with the female, he must impress her. 

Not only does he do this with his brightly coloured plumage (the brighter the feathers, the healthier the male), but also by his erratic displays. 

After selecting a nesting cavity – be it an old woodpecker hole, a rotting fence post, or an artificial nest box – the male Eastern Bluebird sings in front of a potential mate.

If the female is interested, the male will begin bringing nest material to the cavity. Entering in and out of the hole, the male will partially spread his wing and tail feathers. 

If he is successful, the female will initiate copulation. Both facing the same direction, the male Eastern Bluebird will mount the female’s back, attempting to make cloacal contact. This is typically done from a perch, close to the nest cavity. 

The cloaca, present in both males and females, mostly in birds, reptiles and amphibians, is a dual-functioning opening for the digestive, reproductive and urinary tracts. Talk about efficiency. 

In mere seconds, copulation is complete. In most bird species, courtship behaviors often last significantly longer than the main event. 

Now, preparations for nesting can begin. 

Does Sex Affect Nesting Behavior?

Sex plays a significant role in the nesting behaviors of Eastern Bluebirds. 

Most Eastern Bluebirds are monogamous or, in other words, they have just one mate. 

However, this monogamous behavior seems to vary. 

Some eastern Bluebirds show signs of lifelong monogamy, where mates form strong bonds with one another for the duration of their lives. 

Other individuals display seasonal monogamy, whereby partners will mate for the duration of the breeding season, then proceed to find other mates. 

Regardless of the type of monogamy displayed, one would expect an equal relationship when it comes to nest building. 

However, in the case of the Eastern Bluebird, it is the females that do most of the work. 

Over the duration of around 10 days, the female lines the cavity with a variety of materials – most of which consists of grass – that is loosely woven together. Some individuals take nest building one step further by adding good insulating materials, such as horse hair or feathers. 

Although he has no input in the construction of the nest, the male remains close by. Ever vigilant, the male guards his mate, warding off any predators or potential male competition. 

Once the female has constructed her nest, she will lay up to 7 teal-colored eggs. She will incubate the eggs for around 2 weeks, and then an additional week after her chicks have hatched. During this nesting period. The female will rely on her male partner to bring her food. 

Like many bird species, baby Eastern bluebirds are altricial. They are born blind and featherless, and must rely on their parents until they become nutritionally independent.

Older Sibling Nesting Behaviors 

The world can be a scary place, especially for an Eastern Bluebird. 

For some individuals, it pays to stay close to the nest. This is especially true of those who hatch towards the end of the hatching season. 

Once fledged, these juveniles may stay in the territory of their parents. 

Occasionally, these young birds may help their parents raise the brood of the following season. 

But why? 

Well, it is speculated that the young birds stick around for selfish behaviors. They feed their siblings to gain knowledge on nesting behaviors that may inadvertently increase their own nesting successes when they breed. 

This behavior is defined as alloparental care, whereby care is given to non-descendent young. 

Although there are some costs for the caregivers, such as delayed personal reproduction, the benefits outweigh these costs. Chicks have a higher likelihood of survival with the additional support, and the caregivers receive knowledge that will increase their chances of a successful brood. 

Another theory suggests that, by helping to raise sibling chicks, it ensures at least some of the genes of the helper are passed on. 

How Humans Can Help

By the turn of the 20th century, Eastern Bluebird populations had plummeted by nearly 90%.

Owing to habitat loss, competition with introduced species, pesticide use, and climate change, just 18,500 bluebirds were recorded. 

However, in a remarkable display of human cooperativity, artificial nest boxes were erected nationwide. 

With the additional space, Eastern Bluebirds were able to recognize areas once deemed unsuitable due to habitat loss. 

These new nesting boxes were designed with the size of the eastern Bluebird in mind. The entrance hole was big enough for the Bluebird, but too small for the introduced and invasive starling, eliminating interspecific competition for nesting sites. 

Today, the Eastern Bluebird is one of the most numerous bird species in North America, with populations estimated to be close to 23 million individuals. A truce conservation success story. 

Final Thoughts

In today’s modern world, conservation success stories are few and far between. 

With a range of threats facing the natural world, it is hard for wildlife to thrive. 

However, with the help of humans, Eastern Bluebirds are thriving. 

Owing to the installation of nest boxes, these secondary-cavity nesters were able to successfully raise chicks without the risk of competition, habitat loss and extreme weather.