"What on Earth?"
By Ray Barbehenn
Photos by David Cox
Not all birds make their own nests and raise their own young. Instead, some birds sneak their eggs into the nests of other birds. They are nest (or "brood") parasites. The Brown-headed Cowbird is a relatively common nest parasite in our area. The left photo shows a pair of shiny, black males. They were photographed by Suzy Oliver in Winchester, Virginia in May of 2022. In the spring and summer, you may see them in flocks, with the males displaying and uttering squeaky calls.
The females are a more camouflaged grey-brown color from head to tail. Along the AT, Cowbirds can be seen in open areas, such as forest edges and fields. However, their females evidently search through the woods for nests away from these openings. For example, the right photo shows an inside view of an Ovenbird nest in the woods, and it had a Cowbird egg in it! The nest was found about 20 yards from an old logging road in Duke Hollow (northern Virginia near the AT) by Ray Barbehenn on June 4, 2024. Ovenbirds nest on the ground by making a little dome (the "oven") under fallen leaves. The three hungry Ovenbird chicks inside were going to be joined by a much larger nestmate. This scene is also a good example of the wide variety of nest types that Cowbirds parasitize.
How do female Cowbirds trick other birds into hatching their eggs?
A. Cowbirds lay eggs that match the coloration of the host bird's eggs.
B. Most birds do not recognize that a Cowbird egg in their nest came from another bird.
C. Cowbirds usually remove all the original eggs from a nest and replace them with their own.
The answer is at the bottom of the page!
Fun Facts and Musings
Female Brown-headed Cowbirds lay an average of about 15 eggs in a breeding season, meaning that each Cowbird impacts the reproduction of a large number of other birds. In addition, Cowbirds can successfully parasitize the nests of over 140 bird species across North America. They are able to trick all of these birds into hatching their eggs, in part, because most birds do not distinguish between their own eggs and those of another bird. This weakness in the hosts' defenses means that female Cowbirds just need to sneak into their nests for several seconds to lay one of their eggs. The success of this strategy depends on careful observation; female Cowbirds explore their areas every day and watch for birds building their nests. When these birds start to lay eggs, Cowbirds wait for a moment when the host birds are not at home. Cowbirds are stealthy intruders.
Cowbirds sometimes remove an egg from a host's nest before laying one of their own. However, they do not usually remove all the eggs from a nest, nor do they lay entire clutches of their own eggs in other birds' nests. One exception to this occurs when a Cowbird wants to get the host to restart her nest. Thus, if a host bird finds her nest empty, she will often lay a new clutch of eggs – one egg per day. This gives the Cowbird time to come back to lay her egg among the host's fresh eggs.
The destructive behaviors of Cowbirds may become extreme if a host bird removes a Cowbird egg from her nest. How would a Cowbird know this? Cowbirds continue to monitor the fate of their eggs in parasitized nests. And, if a Cowbird finds that her egg is missing, she may toss out everything from the nest – eggs and chirping chicks alike! This "Mafia behavior" is believed to be another reason why most species of birds do not fight back against Cowbirds. In other words, the ability of host birds to raise some of their own chicks can be greater when there is an extra Cowbird to feed than when the Cowbird's egg is tossed out and the host bird ends up starting over.
Not all bird species tolerate a Cowbird egg in their nest. Some birds, especially larger species, recognize a foreign egg and remove it successfully. Some recognize it and build a nest layer over it so that it never hatches. Some abandon their nests altogether and start a new nest elsewhere.
Cowbird chicks compete with their nestmates by being aggressive, noisy, and fast growing. The baby Cowbird can also imitate the calls of the host chicks, but more loudly and persistently. However, the host chicks can hear the louder Cowbird chick and have been known to turn up their own volume and repetition frequency! The nest is a very competitive place.
How do Cowbird chicks avoid learning the wrong song from their host species? Most songbirds learn to sing while they are growing up. Starting in the nest, they listen to their parents and other adults of their species. Thus, they usually become competent to sing by the start of their first breeding season. By contrast, Cowbirds forego learning to sing in the nest and wait an extra season before breeding! During their first year, young Cowbirds flock together with older Cowbirds. This is when they listen to the songs of the older Cowbirds and learn to sing. By their second spring, the males are able to effectively court females. (Females make a chattering call to communicate.)
The other birds in our area that can be nest parasites are the Yellow-billed and Black-billed Cuckoos. The Yellow-billed Cuckoo in the photo below was seen by David Cox along a canal in Fredericksburg, Virginia on July 8, 2017. Note its long, spotted tail, yellow lower beak, white front, and gray-brown back. They are secretive birds.
The reputations of our cuckoos seem to have been tarnished by the publicity surrounding the Common Cuckoo's "bad" behavior. The Common Cuckoo, which is native to Africa and Eurasia, has aggressive chicks that are often shown pushing the eggs and chicks of their hosts out of their nests. It is hard to generalize about the behaviors of all the birds in an entire species, but I am unaware of any research showing that Yellow-billed Cuckoo chicks clear out the nests of their hosts. In fact, only about half of all cuckoo species around the world are nest parasites. The other cuckoo species, including the two in our area, usually lay their eggs in their own nests. Yellow-billed Cuckoos will lay their "extra" eggs in the nests of other birds when food is abundant. But, they prefer to lay these eggs in the nests of other Yellow-billed Cuckoos and Black-billed Cuckoos!
The Common Cuckoos are also the ones that are famous for imitating the egg colors of their preferred host species. Different Common Cuckoos have different preferred hosts, which has led to a wide variety of different egg colors. By contrast, Yellow-billed Cuckoos lay only blue or blue-green eggs. Similarly, Cowbirds just lay white eggs with speckles.
Nest parasites are rare, accounting for only about one percent of all bird species. However, they get a lot of attention, sometimes because of our moralizing ("How rude!") and sometimes from their threats to endangered bird species. For example, the endangered Kirtland's Warblers have an unusually small breeding range in the Jack Pines of northern Michigan. And, the warblers were becoming even more endangered, in part from Cowbirds parasitizing their nests. To help protect the warblers, a long-term campaign was established to trap and remove female Cowbirds from the warblers' breeding areas.
The scale of the effort needed to significantly decrease Cowbird parasitism has been impressive, with roughly 4000 female Cowbirds trapped and removed from the warblers' area per year. So, should concerned citizens remove Cowbirds and their eggs whenever they are found? The experts say no. If we were to remove a few Cowbirds or their eggs it would not have any significant impact on an ecological scale. It would also be against federal laws, which require a permit. Just as importantly, our "help" at parasitized nests could have the unintended consequence of triggering a mother Cowbird's Mafia behavior!
Answer: B!
Thanks to Jeff Stone, who also sent me photos of Cowbirds! Send your photos and ideas for topics to Ray at rvb@umich.edu.
