Despite its unassuming looks, the brown-headed cowbird is one unique bird.
What it lacks in appearance, the brown-headed cowbird makes up for in ingenuity – they effectively take over the nests of other birds, getting them to do the labor-intensive work of raising offspring.
In other words, they’re parasites.
Whilst the parasitic lifestyle isn’t for everyone, the brown-headed cowbird has opted out of the status quo with traditional nest building.
This article will aim to explore what exactly the brown-headed cowbird is, and how it has become one of the most successful brood parasites across North America.
What is a Brown-headed Cowbird?
Dare I say drab? Boring even?
The brown-headed cowbird hasnt gone down the opulent route of evolution.
You’ll see no bright colors or funky patterns on their plumage. Their feathers are short and very average looking.
The brown-headed cowbird has a thick and pointed, yet stubby, finch-like beak. This beak shape allows the cowbird to forage for a range of seeds and insects.
Basic Description
Size & Shape
The brown-headed cowbird is a relatively small yet stocky passerine bird, slightly smaller than a robin.
Typically, they rarely exceed 9 inches in length and have a shorter tail than other species in the same family – such as Brewer’s blackbird and the red-winged blackbird.
Males tend to be slightly larger than females.
Color Pattern
Brown-headed cowbirds are sexually dimorphic.
This means that the males and the females differ in appearances, and the two sexes can be easily distinguished from one another.
Adult males have a plain brown head, neck and, occasionally, upper breast. The rest of their bodies have glossy black plumage, with streaks of almost purple/green iridescence.
Unlike the males, females do not have contrasting plumage. They are a brownish-grey all over, with a very faint gloss – a lot less pronounced than males.
Female cowbirds also have streaking on their underbelly and wings.
Where to Find Brown-headed Cowbirds
The brown-headed cowbird has an extensive range, spanning Southeastern Canada through all of the US, Mexico and into northern Central America.
Their large range, in part, is a consequence of human alteration of the environment.
Unlike many other bird species, the brown-headed cowbird has directly benefited from deforestation caused by humans.
As more of North America was converted from forests to pasture and farmland, the range of the brown-headed cowbird expanded Eastwards.
The brown-headed cowbird favors open or semi-open countries at all seasons.
They can be found in a variety of habitats including prairies, fields, pastures, orchards, and agricultural farms. They avoid dense forests due to the lack of areas that they can forage in.
Many populations found across the US are non-migrants, and remain in their habitats year-round. Occasionally, when conditions are sub-optimal, some flocks travel short distances in search of food.
Some populations, most notably those in Northern regions, are short-distance migrants. This means they stay on the American continent. They have been recorded travelling over 500 miles to reach warmer wintering grounds.
Behavior and Feeding Habits
Brown-headed cowbirds are predominantly ground-feeders, often foraging amongst groups of blackbirds and starlings.
They can be seen in pastures where cattle or horses are present, catching insects flushed out the grass by the large grazers.
This is not a new feeding behavior. Centuries ago, it was a common site to see brown-headed cowbirds following the mighty herds of bison across the Great Plains.
As the bison were hunted to near extinction, the birds sought other feeding opportunities.
Today, their diet consists of seeds, grains and insects.
In the summer months, which coincide with the breeding season, their diet is equal parts insect and plant protein. Insects include grasshoppers, beetles, caterpillars, centipedes, and millipedes.
Female cowbirds can lay as many as three dozen eggs per season. To meet this large calcium requirement, females often supplement their diet with snail and egg shells.
The Impact of Brown-headed Cowbirds on Other Birds
When we think of parasites, what comes to mind?
Fleas? Leeches? Microscopic worms perhaps?
To most, a bird probably wouldn’t come to mind as a parasite.
But that’s exactly what the brown-headed cowbird is: a brood parasite.
Brood parasitism is a breeding strategy in which the parasite, in this case the female brown-headed cowbird, evades all parental care by laying its eggs in the nests of other birds.
This strategy relies on other birds investing their own energy and resources in rearing the chicks of the cowbird.
Brown-headed cowbirds have been known to parasitize the nests of over 220 different bird species, including Red-winged Blackbirds, Ovenbirds, Yellow Warblers and Song Sparrows.
Laying their eggs in the nest of other birds, the brown-headed cowbird will often remove the host bird’s eggs. This has led to a significant population decrease of many North American passerine birds.
As chicks of the cowbird develop quicker than other species, host parents will invest more energy feeding the imposter. Cowbird chicks outcompete other chicks – they raise their heads higher and they beg more loudly.
When they are large enough, cowbird chicks may kill their nestmates; either by kicking them out of the nest, or by smothering them.
Whilst doing research for this article, I noticed many scientific papers, a few as recent as early 2024, starting with the title “First observations of Brown-headed Cowbird parasitism of….” and then the host species will be named.
It seems that the cowbird is an opportunistic nester and, if conditions are right, will take over any nest it can.
Perhaps this is an evolutionary arms race in action?
Are historic hosts, such as red-winged blackbirds or yellow warblers, becoming aware of when their nests have become parasitized, and are they fighting against it?
Yes. Yellow warblers can recognize the eggs of cowbirds. But, due to their smaller size, they cannot remove the eggs. To compensate, yellow warblers build new nests on top of parasitized nests.
FAQ About Brown-headed Cowbirds
Can a cowbird fit in a bluebird house?
Yes, female cowbirds can fit through holes of bird houses. However, due to their parasitic nesting behaviors, it is relatively uncommon for brown-headed cowbirds to parasitize a bluebird house.
Most bluebird houses have an entrance hole of 1.5 inches in diameter. This is small enough to prevent the invasive European starling from gaining entry, but does not keep other birds out.
How can I get rid of cowbirds in my yard?
If you want to remove cowbirds from your yard, always opt for non-intrusive and humane methods. Oftentimes, simplicity is the best approach.
You can change the type of birdseed you use. Choose smaller seeds, such as safflower or nyjer seeds, instead of sunflower seeds or cracked corn.
As cowbirds are ground feeders, ensure you clean up any spilt seeds that fall to the ground. This also prevents pests, such as rats.
You could also introduce specialized feeders, such as tube feeders. This attracts a certain type of bird, such as hummingbirds.
What do cowbirds eat?
Brown-headed cowbirds eat seeds, grains and insects.
In the summer, their diet consists of 50% insects and 50% seeds. However, in the winter, seeds make up nearly 90% of their diet.
Where do cowbirds live?
Brown-headed Cowbirds live in temperate to subtropical grasslands and pastures across North and Central America.
They can also be seen in woodland edges, brushy thickets, prairies, fields, pastures, orchards, and urban areas like suburban residential lands and agricultural farms. They avoid dense forests where feeding opportunities are limited.
Conservation Status
According to the IUCN, the brown-headed cowbird is considered Least Concern.
Although their numbers are thought to be decreasing, their estimated population size is 130 million individuals.
How to Coexist with Brown-headed Cowbirds
Sure, brown-headed cowbirds aren’t the best backyard guest.
But that doesn’t mean you can’t live alongside them.
By taking appropriate actions, such as cleaning spilt seed or changing bird feeders, you can limit the interactions you have with them in your own yard.
Whilst it’s easy to villainize the cowbird, we need to remember that this behavior is completely natural.
Parasitism throughout the natural world is a common phenomenon and can be beneficial in controlling populations.
So, before hating on the cowbirds, just take some simple steps to reduce their presence.
By adding more shrubs and refugia into your yard, you can increase your biodiversity without attracting cowbirds.
Final Thoughts
Due to their extensive range, brown-headed cowbirds are some of the most easily found birds in the United States.
Males are easy to identify with their brown heads and glossy black bodies and conical beaks.
Human behavior, most notably our affinity with deforestation, has contributed to the widespread distribution of the cowbird.
Brown-headed cowbirds do not construct their own nests. Instead, females lay their eggs within the nests of other North American birds. Due to this parasitic lifestyle, many passerine species have suffered population declines.
You can deter cowbirds from your backyard by planting more trees, changing bird food to smaller seeds or using bird feeders that are specific to the bird you want to attract, such as tubular feeders.
