House sparrows and European starlings are often misclassified as predators of bluebirds and other cavity nesting birds. However, they are not bluebird predators by definition. Instead, they are invasive competitive species.
Predators are animals that hunt, kill and eat other animals for food. These are very consistent elements of the definition.
While house sparrows and European starlings do kill bluebirds, tree swallows, chickadees, purple martins and other cavity nesting birds, they are not hunting and killing them in order to eat them.
Sure, we wouldn’t put it past a starling or house sparrow to pick at the carcass of a dead bird – but their purpose for killing native cavity birds is not for food or food potential.
The primary reason house sparrows and starlings will kill a bluebird is for its nesting location. Another reason would be for territory defense.
This is what makes them a competitive species rather than being predators of bluebirds.
Defining predators
Just to be very clear about what a predator is, let’s review a few definitions of predators and look at how predators are defined in a more scholarly fashion.
Basic dictionary definitions of predators:
- Merriam-Webster: “an organism that primarily obtains food by the killing and consuming of other organisms : an organism that lives by predation.”
- Cambridge Dictionary: “an animal that hunts, kills and eats other animals.”
- Encyclopedia Britannica: “an animal that lives by killing and eating other animals : an animal that preys on other animals”
The trouble is that the definition of what a predator is can get really complicated, especially in different contexts.
And not all predators will eat their prey. Consider a cat playing with a mouse. They may not necessarily eat it, but they will spend time hunting and chasing it. Ultimately, their hunting and battering of the mouse can lead to its eventual death while the cat goes off to eat its kibble instead.
When talking about predators and prey, scientists are describing a specific relationship as well that is separate from competition.
A really great comparison comes form Professor William Gaud at Northern Arizona University, “In competition, individuals seek to obtain the same environmental resource. In predation, one population is the resource of the other. One can conceptualize competition as occurring horizontally on the same resource level, while predation takes place vertically between different resource levels (1999).”
Therefore, with the cat and mouse example, even if the cat chooses to eat something else, the mouse is still on a different resource level from the cat, and it could still be a resource for the cat.
A more scholarly definition of a predator is an animal who expends a considerable amount of energy to locate, seize and kill its target animal.
With this as our definition, there are a few things to point out.
First, context is very important. You wouldn’t really consider a songbird to be a predatory animal in general. However, songbirds do prey on insects and other arthropods. Therefore, in some situations a bluebird is a predator, and in others it is a prey animal.
Second, if we apply any of these definitions provided, we can say that starlings and house sparrows are in competition with other songbirds but they are not predators of bluebirds or other cavity nesting birds because their target is not another animal. And further, bluebirds, house sparrows, starlings, tree swallows and other cavity birds exist on the same level, fighting over the same resource.
Each cavity bird is in search of a good nest box. In the case of house sparrows and starlings, if another animal happens to be in the nest box, they will kill for it, but these birds are not hunting or seeking out songbirds to kill.
What are some predators of bluebirds and other cavity nesting birds?
Click here to learn more about predators of bluebirds and how to protect your bluebirds.
Based on ecological definitions of predators, true predators of bluebirds include raccoons, squirrels, chipmunks, possums, cats, snakes and birds of prey.
Crows and other corvids can also be considered songbird predators since they are known to prey on fledgling birds.
But predators of bluebirds may involve a wider scope than what we commonly understand. Based on how predators are defined, anything that will expend effort to hunt and kill a bluebird for a food item or to be used as a resource could be considered a bluebird predator.
We have a lot of great resources about predators of bluebirds and how to protect bluebirds in our bluebird resource hub.
Understanding interspecific competition and intraspecific competition
Just because house sparrows and starlings are not true predators of bluebirds, chickadees, tree swallows and other cavity nesting birds doesn’t make them any less dangerous to the ecosystem and populations of native North American cavity nesting birds.
House sparrows and European starlings take a huge toll by killing native North American cavity nesting birds.
And while we may like to mark them as predators in order to bring more awareness and attention to the danger they pose, it would be fairly biologically incorrect to do so.
Instead, they are an invasive species that outcompetes native cavity birds by killing them. And we can more appropriately call them an invasive competition/competitive species.
Graduate researcher Jennifer Lang and Assistant Professor Eric Benbow define competition as “the interaction of individuals that vie for a common resource that is in limited supply, but more generally can be defined as the direct or indirect interaction of organisms that leads to a change in fitness when the organisms share the same resource (2013).”
When talking about house sparrows and starlings, the typical limited resource is a cavity nesting location, whether a nest box or tree hole.
But, within the cavity nesting space, there is a lot of competition. In truth, all cavity nesting birds are in competition with each other when ideal nesting locations are limited (sometimes even when they aren’t limited).
House wrens are a very common example of a native cavity nesting bird having direct impact on bluebirds, tree swallows and chickadees by disposing eggs or nestlings in order to take over a nesting site.
House wrens invading a bluebird nest would be an example of interspecific competition. This is where an animal of one species competes with an animal of another species.
House sparrows taking over a bluebird nest and attacking a bluebird for it would also be an example of interspecific competition.
But all birds, whether house sparrows, starlings, wrens, bluebirds or others will also compete with members of their own species. This is what ecologists call intraspecific competition.
An example of this, which has been reported by trail monitors would be if a house wren has an established nest, only for another house wren to invade that nest and take it over.
Territory defense tactics are also examples of intraspecific competition behavior.
When it comes to native birds and ecology, intraspecific competition, interspecific competition and predation are very important mechanisms that promote the overall welfare of an ecosystem.
However, when it comes to invasive species like house sparrows and European starlings this kind of competition is not only disruptive, it is ecologically devastating.
How house sparrows and starlings harm bluebirds
House sparrows and starlings have been an ecological problem in North American for more than a century.
In fact, a really great case study was published in 1923 hypothesizing that the introduction of house sparrows to the United States was the cause of a famous Purple Martin colony disappearing for more than a decade (Ziegler, 1923).
Nowadays Purple Martins are a great example of colonial cavity nesting birds that are almost entirely dependent on human provided and human monitored nesting sites.
Habitat loss is definitely a contributing factor for this, but as we’ve discussed, the combination of invasive competition with habitat loss and natural and predation creates desperate ecological situations such as this.
House sparrows are strong birds with a powerful beak. When they infiltrate a nest box with an adult bird within, they have no trouble pinning down the native bird, trapping it inside and then killing it with their beak.
House sparrows are also known to build their new nest overtop the bird carcass rather than trying to dispose of it.
Outside of a nest box, tree swallows and bluebirds can usually get away from a house sparrow. And in some cases, they can chase them off. But once inside the nest box, it is very difficult for them to escape with their lives.
Nestlings and eggs are even easier for house sparrows to kill and discard or build their nest overtop.
They are also known to invade nests, kill the bird and then not take over the nest at all.
Since their introduction in 1850, house sparrows have dramatically proliferated making it easy for them to outcompete the native cavity birds.
Likewise, European starlings attack birds in the same way. They’ll invade tree swallow and bluebird nests, but will also go after bigger cavity nesting birds like various woodpecker species.
Approaches to reducing house sparrow and starling populations
While house sparrows and starlings are not considered a classic predator of bluebirds, tree swallows, chickadees, Purple Martins and other cavity nesting birds, they do outcompete these birds in a dangerous and ecologically taxing way.
Reducing the overall numbers of these invasive birds in North America is crucial for the survival of native cavity nesting birds as well as for the balance of local ecosystems.
Approaches to population management include discouragement, trapping and euthanizing, or passive management techniques.
Raising awareness and being more responsible hosts to backyard birds will also not only reduce populations but help increase the number of native birds.
House sparrow and starling discouragement: Discouraging house sparrows and starlings involves methods such as putting out seed that is less attractive to house sparrows, refraining from using suet which is very tempting to starlings, and installing sparrow spookers on the property or on nest boxes. Putting out predator decoys is another option, but is less selective and may discourage native birds as well.
Trapping and euthanizing: Trapping and euthanizing house sparrows and starlings are important tactics in conservation. Because these birds are nonnative and invasive, they are not federally protected. Most states do not require permission or permitting to trap and euthanize these birds.
Euthanasia tactics include shooting with a pellet gun, cervical dislocation, CO2 gas, and the blow method.
Passive management techniques: Passive management techniques can be attractive to people starting out wanting to help lower the invasive population without directly euthanizing invasive birds. Passive management techniques include coordinating with a falconer (ideal) or wing clipping making clipped birds more vulnerable to predators.
We do not recommend wing clipping because it will attract more predators to a given area, which is ecologically disruptive and can bring harm to all birds and native animals within a local ecosystem.
Responsibility and awareness: While raising awareness and becoming a more responsible bird landlord are indirect methods of population control, they can dramatically help the situation overall. Not very many people know that a problem exists in North America with European starlings, house sparrows and other old world sparrows. Bringing attention to this in compassionate ways can change the story and inspire people to become more responsible when feeding birds and creating a backyard sanctuary.
Ultimately, house sparrows and starlings are not technically considered bluebird predators, or predators of other songbirds, but they are no less a threat. In fact, many experts consider house sparrows and starlings to be a more significant threat than certain classical bluebird predators.
By researching more, you will learn how to mitigate these threats and allow your native cavity birds to thrive.
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References
Britannica Dictionary. (n.d.). Encyclopædia Britannica. Retrieved April 1, 2023, from https://www.britannica.com/dictionary/predator
Cambridge Dictionary. (n.d.). Predator. PREDATOR | definition in the Cambridge English Dictionary. Retrieved April 1, 2023, from https://dictionary.cambridge.org/us/dictionary/english/predator
Chase, J. M., Abrams, P. A., Grover, J. P., Diehl, S., Chesson, P., Holt, R. D., … & Case, T. J. (2002). The interaction between predation and competition: a review and synthesis. Ecology letters, 5(2), 302-315.
Lang, J. M. & Benbow, M. E. (2013) Species Interactions and Competition. Nature Education Knowledge 4(4):8
Merriam-Webster. (n.d.). Predator definition & meaning. Merriam-Webster. Retrieved April 1, 2023, from https://www.merriam-webster.com/dictionary/predator
Ziegler Jr, G. F. (1923). Notes on a Purple Martin colony. The Auk, 431-436.
