Feathering the Nest: The Nesting Nuances of Colorado Birds

According to the 2016 Colorado Breeding Bird Atlas, there are 285 bird species that breed in Colorado. As you might imagine with that many birds, you can find an incredible variety of nests and nesting behaviors throughout the Centennial state.

For instance, some birds, like the Cliff Swallow, are amazing engineers. While this widespread, colonial bird used to nest primarily on cliff sides (hence the name), they are now more likely to be found on or near human structures, especially bridges. Hundreds or even thousands of birds may make up a single colony.

These colonies are full of gourd-shaped nests, which both the male and female swallow construct from hundreds of tiny mud “bricks” that they gather from a nearby puddle or shoreline and attach to an overhang. Once the outer walls are nearly complete, they line the inside of the mud gourd with grass to cushion the eggs and offer softness to future hatchlings. This nest building can be labor-intensive, so Cliff Swallows will often first attempt to reuse a nest before building one from scratch.

Another remarkable architect? The tiny, but mighty Bushtit! In a Bushtit breeding pair, both members will construct their nest over the course of one to two months. They weave the outer covering out of lichens, moss, leaves, and grass, which are held together using insect cocoons and spider silk. Then, they line the nest with plant down, fur, and feathers. These characteristics lead to an extremely well-camouflaged nest that is sock-like and very stretchy. The elasticity is important because some Bushtit pairs have helpers (usually unpaired adult males) that assist with incubation and feeding duties, and all these adults and chicks sleep in the nest at night until fledging.

It can not be said that all birds are great builders though. Take the Peregrine Falcon, for example, who barely makes a nest at all. Traditionally, the peregrine digs out a shallow depression on a rocky outcropping or cliff ledge- otherwise known as a scrape- in which to lay its eggs. However, in the last few decades they have also adapted to nesting on city high rises where they can prey on the abundance of Rock Pigeons.

Mourning Doves, too, are notoriously “simple” in their nest-building. They typically construct their flimsy nests in less than a day and place it almost anywhere, including, as I have personally witnessed, on top of a parked tractor’s tire. The male gathers materials such as twigs, grass, and pine needles, and the female arranges them into a loose, unlined platform. Due to a number of factors, not least the quality of the nest, Mourning Doves often hedge their bets against nest failure by having multiple broods in a season.

On the other end of the spectrum, some birds, like the Great Horned Owl, do not build a nest at all. In fact, most owls don’t build nests, as their short, hooked beaks are not well-designed for carrying and manipulating nest materials. Instead, the Great Horned Owl chooses a tree cavity or co-opts an unused nest from another species like the Red-tailed Hawk, Common Raven, or Great Blue Heron. Because they nest early in the year, they often have their choice of nesting spots before other birds even begin to think about starting their own broods.  

Finally, there are those birds, like the Brown-headed Cowbird, that do not even nest, nor raise their own hatchlings. Known as a brood parasite, this member of the blackbird family used to follow bison herds across the Great Plains. Because of this nomadic nature, they evolved to lay eggs in other birds’ nests, deflecting all parental duties onto the new foster parents. Cleverly, the cowbird is able to lay eggs in multiple colors and patterns to help them blend in with the host’s eggs. In fact, the camouflage is so spectacular that the number of species parasitized by the Brown-headed Cowbird numbers over 220.

But the evolutionary craftiness does not stop there! Because the cowbird chick tends to hatch sooner and grow quicker than the host species’ own chicks, they are usually the only one to survive. While cowbird eggs mostly go unnoticed, some species do recognize the parasitism and respond accordingly. For example, the Yellow Warbler, whose bill is too small to remove the cowbird egg, will simply build a new nest on top of the parasitized nest. 

Nesting strategies within Colorado birds are as diverse as the birds themselves. Now that spring has sprung, it’s the perfect time to get outside and see what your local feathered friends are building and braiding, and who is becoming the next generation of beautiful birdies.

This spring at the Denver Audubon Kingery Nature Center, you can see not only the wonderful migrants, but an array of nests from the tiny masterpieces that are hummingbird cup nests, to the gigantic Osprey nests that rest atop telephone towers. Be sure to join us this season for Bird walks, talks, and events, our calendar is full of opportunities to get to know the wild life we love so much.

‍