Author: Bethany Beekly
Editors: Will Dana, Claire Shudde, Christina Del Greco, and Jennifer Baker
Illustrator: Katie Bonefas
Picture this: It’s Cinco de Mayo, but you don’t know it as a day of tacos, tequila, and mariachi. From your perch, about 40 feet off the ground in the branches of a cottonwood tree, you observe a different kind of revelry.
All around you, birds are engaging in their rituals of spring. Here in Ann Arbor, you might see a red-bellied woodpecker excavate a new cavity above the one he created last year while a bluebird constructs a nest in that vacant space downstairs. The woodpecker has a brief altercation with a starling who’s hoping to take a shortcut in the nest-building department, but today he triumphs over his assailant. In a neighboring tree, blue jays come and go periodically with mouthfuls of sticks and rootlets. Their progress is slow and almost careless; they’ve been working for two weeks, and they’re only just now nearing the final stages of construction. The trees go briefly silent and still as a red-tailed hawk passes overhead, but she isn’t hunting right now. She is engaged in an elaborate courtship dance with her partner of many years, soaring and gliding in circles before suddenly taking a nosedive straight down into the treetops.
Meanwhile, you, a cowbird, are simply watching, surveying all the activity. You have a very different reproductive strategy: wait until other birds around you are done with their nests, then stealthily lay one of your own eggs inside—as many as 25 per year. With that many eggs to find homes for, you’ve got your work cut out for you. Birds make many types of nests, and cowbirds aren’t very picky, so you have lots of opportunities for success! Most people picture a bird’s nest as a bundle of sticks nestled in a tree, and some birds certainly do this. However, bird nesting habits are far more diverse than you might think!
Some birds construct cup- or basket-like nests primarily made of grasses and sticks—the quintessential bird’s nest out of a picture book. Many will line the nest with mud to create a smoother surface. Almost all will make the innermost layer out of soft, warm materials like feathers, spiderwebs, and fur. These may be in trees or shrubs, or even thick, tall grasses like cattails, where I have observed red-winged blackbird nests. Less commonly, birds such as the Baltimore and orchard orioles will build hanging nests of grass woven into baskets that dangle off the ends of tree limbs. Cavity nesters lay their eggs in holes in snags (dead trees) or even, in my experience, dead branches of living trees. They may make their own each year (like woodpeckers), find preexisting holes (like bluebirds), or take over another bird’s partially-complete nest (like starlings). A red-bellied woodpecker might spend two weeks excavating a cavity only to lose it to a starling and have to start over! Some birds, like geese and killdeer, are ground nesters. This tactic may sound dangerous, but despite being on the ground, these birds and their eggs are extremely well-camouflaged. I have found that it can be nearly impossible to spot the same killdeer nest two days in a row (and one must take care not to step on them!) Finally, cliff-dwelling birds like cave and barn swallows will build mud-cup nests on the sides of cliffs or, in more urban environments, in the rafters of picnic shelters and other man-made structures.
A small handful of species—such as the common cowbird—will take a parasitic approach and simply lay their eggs in others’ nests. While no one is completely sure why they do this, cowbirds earned their name because of their habit of trailing behind herds of bison and domestic cattle, then gorging on the invertebrates stirred up by their hooves. Cowbirds in urban settings today generally don’t have to look far to find a meal, but the nest parasitism may have been retained from a bygone era in which they lived a more nomadic life. It would have been impossible to stay in one place long enough to build a nest, incubate eggs, and raise young without losing track of the animals they depended upon for food. So, the cowbirds pass those responsibilities on to someone else, and hope their young catch up eventually.
As absentee parents, cowbirds are highly irregular in the bird world. However, like humans, birds take a variety of approaches to the division of labor. In some cases, homemaking is mainly undertaken by one parent. Alternatively, some birds bond by playing house together. A male Carolina wren will select a few potential nesting locations and take his mate on a tour to visit them. She picks her favorite, and then the two construct a nest together at that site. And mourning doves have a fascinating creative process: once a location is selected, the female stays put while the male flies back and forth at breakneck pace, delivering her sticks and grasses by landing on her back, reaching over her shoulder, and passing them into her beak. She weaves the materials in while he makes another trip, essentially building the nest around herself.
Once the nest is built and eggs are laid, some male birds hit the road (and may even seek out additional females to mate with). However, between 80-90% of birds engage in some level of biparental care. There is a loose but interesting relationship between the sharing of parental responsibilities and the degree of monogamy observed in a given species. For instance, red-winged blackbirds are highly promiscuous, with males having up to 15 mates in a season. Male red-winged blackbirds also have virtually no interaction with their chicks. Mourning doves, on the other hand, form powerful pair bonds with their mates and are also some of the most egalitarian parents of the bird world. The males and females take turns incubating the eggs and, after they hatch, brooding the young. Both male and female doves of numerous produce a substance called “crop milk” in their esophagi with which they feed the chicks in the first days of their lives—an ability that is unique to just a handful of species. The diversity of reproductive strategies is reflective of the incredible diversity and adaptability of birds across the world.
It’s important to note that the behavioral patterns discussed here aren’t rigid. Flexibility is often essential, especially for wildlife dealing with urbanization and climate change. When it comes to nest location, some birds get creative depending on what’s available. While wrens are accustomed to nesting in tree cavities, birders have anecdotal evidence of them incubating eggs in coils of Christmas lights, old boots, light fixtures, and plant pots. This type of adaptability makes them resilient to the ever-increasing strain that urban sprawl places on wildlife habitats. Most of the birds (and, indeed, other types of animals) that you see in your day-to-day life have at least partially adapted to urbanization. Unfortunately, populations of less- flexible species across the animal kingdom are in sharp decline. Birds that are pickier about their environment can get pushed out by savvier adapters, leading to pockets of extinction or extreme reductions in their numbers. Biodiversity is critical to the preservation of healthy ecosystems, which is why it’s important to preserve wild spaces as much as we can within urban areas.
Even before human interference, avian parenting strategies were flexible, particularly for males. Their decisions can vary based on environmental conditions, such as food availability, to maximize their own reproductive success. When food availability is high, males may engage in more promiscuous behavior because there are enough resources to support multiple broods. On the other hand, if food is scarce, they may stick with one female and participate more actively in chick-rearing to ensure the success of a single brood. When animals conduct this type of cost-benefit analysis, it is known as a behavioral “trade-off.” However, climate change could adversely impact their ability to make these decisions. Birds use information like temperature and precipitation to determine the best reproductive and parenting strategies. With the intense and erratic weather patterns that are becoming the norm, food availability is unpredictable. As a result, birds could suffer either opportunity loss or offspring death due to predation and starvation.
It’s clear that birds need our help. In addition to the effects of climate change, deforestation, skyscrapers and windmills, artificial light at night, and noise pollution have detrimentally impacted avian biodiversity. However, in some ways birds are more resilient than we typically give them credit for. For instance, while artificial light at night seems to cause cell death in the brains of zebra finches in the short-term, they largely recover normal neuronal density after 6 weeks of the nighttime light exposure. And common urban birds like the cowbird and red-bellied woodpecker show us that many birds are happy to learn how to live amongst humans (especially when it means free handouts). Moving forward, it will be essential for us to adapt as well. We must prioritize research on how anthropogenic changes affect behaviors like nesting and parenting, which starts with a deep understanding of birds’ natural habits. This is especially critical for those species of birds that are shy or less flexible in their habits and thus suffer greater losses when human civilization encroaches upon their lives. Armed with a greater knowledge of how our choices harm birds, we can make both personal and policy decisions with their well-being in mind. Your Cinco de Mayo picnic just wouldn’t be the same without them.
Special thanks to Leonard Weber, a longtime volunteer with the Detroit Audubon Society and guide to the flora and fauna at Eliza Howell Park in Detroit! Thank you, Leonard, for sharing the decades of invaluable wisdom you have accumulated through your loving and meticulous observations. You can learn more at Leonard’s blog or by signing up for a field course or nature walk through Detroit Audubon.
Thank you also to Nicholas Hinnant, who took many of the magnificent photographs used as references for this piece’s illustrations during Leonard’s six-week Birds Nesting field course this spring.
Bethany Beekly is a Neuroscience student in the Elias Lab (UM Department of Molecular and Integrative Physiology). She studies the interplay between sleep and the neuroendocrinology of reproduction. She is also heavily involved with the Graduate Employee Organization (GEO), serving as Steward for the Neuroscience Program and the Chair of the Climate Caucus. When she isn’t doing science or fighting for worker’s rights, she can often be found outside hiking, camping, reading, doing yoga, or just sitting with a cup of coffee listening to the birds.