Wild birds occasionally get into fights, skirmishes, and tussles with each other. This is common if birds are in competition for the same food source, nesting site, or mate. However, certain birds (sparrows, wrens, blue jays, etc.) can get aggressive even to the point of killing other birds. I have even been dive-bombed by “peace-loving” bluebirds a handful of times. If you are trying to attract bluebirds to your back yard, you have probably wondered if bluebirds are aggressive. So, here’s the truth.
Bluebirds are not very aggressive birds. They spend most of the day hunting for insects, leaving other birds, animals, and people alone. Bluebirds do occasionally dive-bomb or claw at other birds or animals when trying to defend their life, their nest, their eggs, or their mate. They are not a threat to people.
In this article, we will cover some common bluebird behaviors that people often interpret as aggressive, which they shouldn’t. Even so, we don’t want to view bluebirds with rose-colored glasses, thinking that they always avoid conflict at all costs. That’s not true either. There are times when bluebirds have to get scrappy to defend themselves. That’s just nature – but they’re definitely not aggressive.
Let’s cover some common scenarios that you might see as you are monitoring bluebirds during the nesting season.
Are Bluebirds Aggressive toward People?
Bluebirds often swoop down and “dive-bomb” people if they approach too closely to their nest. This tends to happen during peak nesting season once the female has already laid eggs or the eggs have hatched. This behavior shouldn’t be seen as aggressive; the male bluebird is simply trying to defend his nest and keep you (a potential predator) away from the nest.
During your weekly nest box monitoring, if you approach a nest box with an active nest inside of it to open it up, don’t be surprised if a male bluebird swoops down from a tree to try to scare you away from the nest box. He might swoop down a foot away from your head, or even graze your hair if he is particularly brave.
If bluebirds are nesting in a natural cavity in a tree in your yard, this probably won’t happen to you. They only tend to dive-bomb people who are opening up a nest box, like the side-opening nest box I recommend here.
If having a bluebird swooping down near your head bothers you, try to keep your nest box monitoring relatively quick. Just peek inside the nest box once a week to make sure that everything is going well. Get in and get out. Even though bluebirds may swoop down at you, it’s still important to maintain a weekly nest box monitoring routine to keep bluebirds safe from predators and invasive bird species like House Sparrows. Speaking of sparrows, let’s talk about them next.
Are Bluebirds Aggressive toward Other Birds?
House Sparrows
Bluebirds will fiercely defend their nesting site from invasive species like House Sparrows and European Starlings. If bluebirds have already claimed a nest box and started building a nest, they will defend the site as much as they can, even if that means fighting.
Fortunately, starlings are larger birds, so they are easy to exclude from bluebird houses with the proper size of entrance hole. House Sparrows, on the other hand, can fit into any size of entrance hole that works for bluebirds. Therefore, they are a constant threat to bluebirds.
If you see a bluebird fighting with a House Sparrow, it is likely out of self-defense or defense of a nest box. House Sparrows are an invasive (non-native) species in North America and their aggression could be partially responsible for their proliferation throughout the continent. When bluebirds fight with sparrows, it ends in a stalemate many times, but if one of the two birds is going to be injured or killed, it is more likely to be the bluebird.
Recommended Reading: How to Keep House Sparrows Out of Bluebird Houses
House Wrens or Other Native Birds
Bluebirds are not usually too aggressive toward House Wrens or other native cavity-nesting birds, but injuries do happen. House Wrens are usually quicker and sneakier than bluebirds, so they can escape if there is a territory dispute. But bluebirds have injured and killed Tree Swallows when fighting for the same nest box.
Between bluebirds and House Wrens, it’s the wrens that are typically the aggressors when competing with bluebirds for nest boxes (piercing eggs and killings birds). Bluebirds usually end up injured and wrens leave unscathed. But with Tree Swallows it’s different.
Recommended Reading: How to Keep House Wrens Out of Bluebird Houses
Tree Swallows and bluebirds often compete for the same nesting locations, and since Tree Swallows are smaller and weaker than bluebirds, they usually end up losing when there is a physical fight. Bluebirds have even been shown in studies to kill Tree Swallows while they’re on the nest (source). That’s pretty aggressive, right? It all comes down to what the situation is, and whom they are paired up against.
With a weaker nesting site competitor, and a limited number of nesting sites, it appears bluebirds do show aggression from time to time. Luckily, there is a way to pair bluebird nest boxes in order to support the nesting of both bluebirds and Tree Swallows in the same area. Check out this article here to learn more about that strategy.
Are Bluebirds Aggressive toward Other Bluebirds?
Generally, you won’t see too much aggression from one bluebird to another. Bluebirds spend time in larger groups during the winter, getting along fine, and then they pair off in the spring and keep to themselves and their family. However, you might see sex-specific bluebird aggression during the early nesting season.
One 1988 study found that female Eastern Bluebirds exhibited more aggression toward other females of the same species early in the nesting cycle. Researchers thought that this behavior might prevent other female bluebirds from laying eggs in a nest that wasn’t theirs (source).
While bluebird-bluebird aggression can result in injury or death, it isn’t too common, and it’s just a part of life. If you observe bluebirds fighting on your nest box trail, just consider letting nature take its course. I, myself, have never seen bluebirds fighting for more than a minute or two near a nest box before one of them packed up and moved on.
Are Bluebirds Aggressive toward Predators?
Just as bluebirds might swoop down at a person who walks too close to their nest box, bluebirds also dive-bomb squirrels and other crawling predators who approach their nest boxes during day.
Squirrels can cause devastation if they discover how to get inside a bluebird house, so bluebirds have a good reason to chase them away! Here is a fairly funny video of a squirrel being “attacked” by two bluebirds who are just trying to keep the animal away from their nest:
Bluebirds may also swoop at house cats who approach their house during the day, but nocturnal predators like raccoons usually sneak in while bluebirds are asleep in the trees or in the nest box.
As the above video shows, climbing predators have easy access to bluebird houses when they are mounted to a tree. That’s why I recommend in this article to mount your nest boxes to a metal pole instead of to a tree. Read that article for more in-depth information on why it’s so important.