Why Do Small Birds Chase Large Birds?

Two crows harassing a perched bald eagle for article on why small birds chase large birds
Crows mobbing a bald eagle. Photo by OutRIAAge02 at the English Wikipedia, Creative Commons Attribution-Share Alike 3.0 Unported

I recently put out a call for questions and article topics to folks on my monthly email list. I got some great suggestions and I’ll be addressing them in future article. (By the way, if you want to suggest a topic or ask a question for me to explore in an upcoming article, feel free to contact me!) Anyway, one of the topics jumped out at me as being very seasonally on-topic here in the temperate Northern hemisphere: why do small birds chase large birds?

It’s a question plenty of people have; after all, the large bird being chased after is often a dangerous predatory species like a hawk, raven, or eagle. And the small birds are usually much tinier than their target, and could easily be a snack if the large bird decided to go on the attack in turn. What could be so important that these little critters are willing to put themselves in such danger?

Providing Some Context

First, let’s look at the timing. You’re most likely to see this behavior happening in spring and early summer. This is the height of nesting season, when birds of all species are busy finding (or reconnecting with) mates, locating a suitable nest site, laying eggs, and raising young. It’s a noisy time, with plenty of songs from birds of both sexes, though females’ songs may be a little quieter or less frequent.

Red-winged blackbird in hot pursuit of a turkey vulture for article on why small birds chase large birds
Red-winged blackbird chasing a turkey vulture. Photo by Matt MacGillivray, Creative Commons Attribution 2.0 Generic.

These songs aren’t just pretty tunes to pass the time away. Rather, different songs serve different purposes. Some are intended to warn rivals away from a claimed territory. Others are meant to attract prospective mates, and in some species there are courtship songs as well. In addition to this musical repertoire, many birds also have alarm calls to warn of approaching predators (some even mimic the sounds of the predator itself!)

All this fuss is for a good reason: birds sitting on nests, young birds, and birds distracted by defending territory (and bringing attention to themselves by singing) are all more vulnerable to predation. The parents have invested a lot of energy and time into raising their young, and they want to protect that investment as fiercely as they can. This means they may be willing to take bigger risks than they would the rest of the year.

The Reason Small Birds Chase Large Birds

By now you’ve probably got a pretty good idea of the answer to our question. When you see one, two, or even a group of smaller birds going after one much larger than they are, they’re likely defending a nest site. Even if the larger bird was just passing through, the parents aren’t going to take any chances, and will dive-bomb and peck at the interloper. They’ve been known to target any perceived threat, from other birds to snakes to mammals (including humans!) This is to prevent potentially problematic animals from even thinking about going after vulnerable eggs or young, whether in the nest or out of it.

But why doesn’t the bigger bird just turn around and defend itself? First of all, maneuvering in mid-air can be more dangerous than it looks, even for an experienced flier; catching a moving target is easier said than done. Any collision could cause a serious injury that violently grounds the bird. And injury is possible when the larger bird is perched in a tree, too. Those little birds may only weigh a few ounces, but a well-aimed beak or claw can take out an animal’s eye or badly scratch a mammal’s nose. With enough determination a small bird can cut through the scaly hide of a snake or other reptile. And if a larger bird gets its feathers damaged, whether due to attack or in the process of escaping, this could seriously affect its ability to fly, potentially leading to starvation. Any injury that breaks skin can lead to a bad infection and possibly death.

A white gull chases after a fleeing brown owl for article on why small birds chase large birds
A lesser black-backed gull (Larus fuscus) chases a short-eared owl (Asio flammeus). Photo by Stephan Sprinz, Creative Commons Attribution-Share Alike 4.0 International 

Since the smaller birds are usually faster and more agile than their target, and often attack in groups of two or more (a practice known as mobbing), the bigger animal usually finds it most prudent to beat a hasty retreat in order to avoid being injured. During the noisy pursuit, pretty much everyone in the neighborhood knows that the hawk, falcon, or other predator is in the area, which means in order to stealthily sneak up on a meal it’s going to need to leave the area entirely–which makes the young in this place safer for the time being.

So is the world made up of the chasers and the chased? Not necessarily. Crows, for example, will angrily mob any hawk, raven, or other threat they find. But they may also find themselves on the receiving end of a similar roust by smaller songbirds. Clearly chasing and mobbing predators are tactics that have worked well enough to be used by many species of bird–even those who know what it feels like to be on the receiving end.

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