Why do birds make nests?
Ben Mullaly, Interpretive Naturalist
Just about everybody knows that birds build nests, but have you ever wondered why birds build nests? “Of course,” you say. “They build them to protect their eggs and offspring.” Ok, but can you tell me what makes different birds build different types of nests? Of course, this blog post would have to be the size of a college textbook in order to accommodate the minute differences between every single species of bird, so I’ll deal in broader strokes.
To begin, it’s important to know that there are three main types of bird nests: platforms, cavities and cups. Each type looks just like how you’d think they look based on their names. The interesting thing about types of nests, though, is that they’re all used by many kinds of birds. For example, waterfowl like wood ducks are cavity nesters, grebes are platform nesters and Canada geese nest in shallow cups in the ground.
This made me wonder, “why do similar birds nest in such different ways?” The simple answer is competitive exclusion. Competitive exclusion happens when two similar animals change their behavior to avoid competing for space or resources. In this post, I’ll focus on how certain birds have adapted to create nests in surprising and interesting ways.
Let’s start with the sora — a bird that most people have not even heard of, let alone seen in person. These “swamp chickens” are secretive birds that live deep in the tall grasses and cattails of marshy environments. Their shyness cloaks the fact that these are the most common type of rail in America. Even though they’re relatively common, the sora’s wetland habitat is at risk due to human development.
Sora are cup nesters. The females will weave a nest in thick cattails and sedges suspended mere inches above the surface of the water. Once she has completed the bottom, she’ll begin laying eggs and continue working on the nest. Her finished product could be compared to a house, complete with a roof of woven living plants and a ramp leading up to its entrance. Female sora put a large amount of effort into their construction — but why? They could just as easily NOT build a nest suspended over water, but then they would be more susceptible to predators like foxes and raccoons. Constructing a roof provides protection for young birds and mothers alike both physically from the elements and because it camouflages the nest from aerial predators like falcons, eagles and hawks. All of that effort may seem needless, but it would not have come about without the necessity to ensure survival.
Another peculiar nesting bird is the cerulean warbler. Not much is known about the nesting habits of these beautiful blue warblers for one simple reason: their nests are usually built high up in trees. By high up, I mean really high up, as in over 100 feet up into the canopies of trees. The birds nest this high as a countermeasure against egg-eating predators like raccoons, foxes and other birds. As if the height wasn’t a big enough obstacle to anyone searching for these nests, the birds also put their nests at the ends of branches. Understandably, ornithologists have had a difficult time scaling every massive tree in search of tiny nests. What we do know about their nests, however, is that they’re built carefully. The females will weave together materials including pieces of bark, grass and hair and bind it with spiderwebs! Spooky, but logical. As you may know, spider silk is stronger than steel and readily available for collection. The finished nest shape is a shallow, open cup. Cerulean warblers have had it rough over the past 50 years, losing over 70 percent of their population due to habitat loss across their range. Luckily for us here in Summit County, they’re relatively common nesters at Deep Lock Quarry and Hampton Hills Metro Parks. Park staff even found a rare cerulean warbler hybrid several years ago.
Finally, let’s talk about Eastern screech owls. If you’ve ever heard one, you can understand how they got their name. Screech owls are the only owls in Ohio that are obligate cavity nesters, meaning they will only nest in cavities in trees. Other owls, like barred owls, can sometimes be found nesting in cavities, but they can also be found nesting in other ways. Screech owls — even though they’re reliant on cavities in trees to nest — can’t make them by themselves. These birds will occupy preexisting holes made by woodpeckers, rot or squirrels. To explore further this bird’s lack of nest-building effort, screech owls spend no time constructing a nest within the cavity and will lay their eggs on whatever happens to be on the bottom of the space. Some people may equate this lack of effort to poor parenting, but there may be another reason. If a bare-bones nest is good enough, the adult owls can spend more time foraging for food and laying and incubating eggs. Plus, sometimes carrying nesting materials to a nest to build it can attract the attention of predators like larger birds of prey. As it turns out, even doing nothing can be considered an adaptation when done appropriately!
Considering all the odds these birds are up against, you may be wondering: “What can I do to help?” First, plant native plants! Specifically, oak, cherry and willow trees, as well as goldenrod, sunflowers and native strawberries. You may be thinking, “what on Earth does a plant have to do with birds!?” Native plants, particularly the ones mentioned, are common host plants for butterfly and moth caterpillars. Most types of native songbirds exclusively feed their offspring caterpillars while they’re in the nest. That means native plants act like giant natural bird feeders! Native plants also tend to be much easier to keep than ornamentals, even if you don’t have a green thumb, since they grow around us naturally. You can visit https://www.nwf.org/NativePlantFinder/Plants to find which plants are best for your area and which caterpillars they invite.
Another incredibly important thing you can do to protect nesting birds is keep cats indoors. Outdoor cats live significantly shorter and less healthy lives than indoor cats. Plus, they will kill birds — particularly nesting birds — for sport even if they’re fed at home. You can also put out man-made nest structures and provide nesting materials for birds in your yard. Even urban areas have nesting birds that will utilize a bird house or materials if they’re provided. Just make sure that nesting materials are natural and free of chemicals (think dried grass, twigs, moss, plant fluff). You can check the All About Birds website for more specific information. If you know which birds nest in your back yard, you can provide resources just for them by looking at their specific species at nestwatch.org.
