Photo ©
Jeff Baldani
By Robyn Bailey, NestWatch Project Leader
Spirited Away
In medieval European folklore, fairies were said to occasionally kidnap human children and replace them with their own unruly offspring. Human parents were on their guard against such "changelings."
In the Realm of Fairies
In pre-industrial European folklore, a “changeling” was said to be a fairy child that had been swapped for a human child, fairies being very covetous of hearty human children. The changeling would initially resemble the original child, but over time it would reveal its dubious nature through some fault (for example, a bad temper or physical weakness). Changelings were a way to explain why something had gone wrong with the child’s health or temperament, while simultaneously allowing parents to distance themselves emotionally. Being suspected of being a changeling often had devastating consequences for human children—leading to actual abuse or even infanticide. Belief in changelings persisted until at least the 1890s, but where did this peculiar folklore originate?
If you study nesting birds, you almost have to wonder if the changeling mythology had its origins in a reproductive strategy known as “brood parasitism”. Some birds have evolved to lay their eggs in the nests of other birds, leaving the unsuspecting hosts to rear the imposter. The brood parasite “swaps” her own egg for one of the host’s eggs, generally one that closely matches her own egg coloration. Unlike the fairies, the parasitic mother does not reciprocate parental care, but rather discards the unfortunate host egg. Upon hatching, the imposter nestling tends to beg more loudly, grow more quickly, and may even push nest mates to their death. Obligate brood parasites are species that are obliged to breed this way; they make no nest of their own, and this is their only way of reproducing. Examples include the cowbirds of the Americas and the Old World cuckoos.
No Family Resemblance
A Eurasian Reed Warbler feeds a Common Cuckoo nestling which has outgrown the nest. The cuckoo hatched from an egg that was deposited in the warbler's nest by a parasitic female Common Cuckoo.
However, brood parasitism can also happen casually and opportunistically, as in the case of many cavity-nesting ducks in North America. A NestWatcher monitoring a large nest box in a swampy forest is likely to find more eggs in a Wood Duck nest than a single female could lay on her own. This is called conspecific or “same species” brood parasitism. You may also find eggs of different colors and textures mixed in from a Hooded Merganser or other cavity-nesting duck that shares its habitat; interspecific or “between species” brood parasitism is at play here. This opportunistic nest parasitism is not obligatory, and the parasitic mother may go on to raise her own young in her own nest. She typically does not remove the eggs of the host bird. She’s increasing the odds of passing along her DNA in the event her own nest is destroyed. Read more about facultative (i.e., non-obligatory) brood parasitism in Sneaky Ducks and Scrambled Eggs.
One can easily imagine a bird being interpreted as cunning and otherworldly, and the observation of brood parasitism by some early person could have served as inspiration for, or reinforcement of, the changeling superstition.
More Than Just Cowbirds
Scientists are gradually uncovering novel instances of facultative brood parasitism, which is perhaps only rarely undertaken by some species. It may go completely undetected by researchers unless a species has undergone an exhaustive study of parentage and mating strategies; however, collecting and analyzing DNA from eggs and parents is costly. New methods could bring the cost of discovery down.
A great example comes from a 2016 study which incidentally documented a Wild Turkey parasitizing a nest. While pursuing other questions, researchers planted artificial ground nests filled with chicken eggs in an Arkansas forest and placed cameras on them to record predators. What they didn’t expect to see, however, was a female Wild Turkey taking advantage of a found nest and laying an egg in it. The most intriguing finding is that she didn’t do this right away, or even the following day. She first poked around a bit, moving the eggs around. She returned the next day and visited the nest. Finally, two days after discovering the nest, she laid an egg in it and never returned again. Having found the nest, and checked that the coast was clear for a day or so, she made up her mind to parasitize it and then she never looked back!
Look Closely
In this White-eyed Vireo nest, a Brown-headed Cowbird nestling (right) dwarfs the vireo's own young (left).
A Malicious Habit
Upon learning about the habits of Brown-headed Cowbirds, many NestWatchers take an immediate dislike to them. And while you may vilify cowbirds for duping unwitting couples into accepting their changeling child, keep in mind that 80 species worldwide are known obligate brood parasites, and at least another 236 species also occasionally parasitize, including the beloved Eastern and Western Bluebirds, Purple Martin, and Tree Swallow. Ornithologists generally find brood parasites fascinating study organisms because of their ability to shed light on topics such as co-evolution, nest defense, song learning, and mimicry (to name a few).
The Irish poet W. B. Yeats, writing in 1892, describes fairies as “on the whole good” apart from their “malicious habit” of stealing children. In trying to reconcile the “malicious habits” of birds with our expectations and human values, perhaps we should accept their nature as people once accepted fairies—not as black-and-white villains, but part of a rich tapestry of beings whose ways are different from our own.
As with all things rare and hard to detect, citizen scientists can help separate myth from reality! Report your unusual sightings, take pictures, and document what you see in the nests around you. You may just find something that hasn’t been reported yet.
I watched little song sparrow feeding three large cowbird chicks against one it’s own.
It’s hard to accept it as a nature.
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Thank you, Robyn. Very interesting. Among other things, I didn’t know for sure that Eastern Bluebirds and Tree Swallows will occasionally parasitize — on my birdhouse trails, that might help explain when I occasionally find a blue egg in a TS nest and, once in a while, a whitish, smaller egg in a Bluebird nest. I’ll photograph those next time.
Hi Tom, yes that could very well be happening between species. But, it can also happen when a bluebird or tree swallow nest suddenly has an extra egg (of the same species) in it, and you know it’s not from the original female. It’s interesting how they might take advantage of an opportunity like that. Always good to take pictures whenever you see it. Thanks for reading.
I observed a cowbird chick begging to be fed by its song sparrow mother on the ground behind my house where I throw bird seed for the quail and other birds. It was loudly twittering and fluttering its wings. It finally got more attention than it wanted; a cooper’s hawk swooped down and flew away with the chick. I guess everyone must eat.
Two birds were screaming in tbe front of my house. Then an hour later i look on the cement patio and a perfect white oval egg is there. No birds around. No place for a nest
What do i do ? Leave it?
Hi Cheri, When you find an egg out of a nest, it’s usually too late to replace it in the nest unless you watched it land. Because of this, and because you do not know where the host nest is, it’s best to leave it. Please let us know if you have any other questions by emailing nestwatch@cornell.edu.
It said up their that they lay in the nest of a bird that has similar egg coloring but I found their eggs in the nest of a eastern phobe. the eggs of the phobe are all white , while cowbird eggs are speckled dark brown.
Great article. thanks
Interesting article. I am fascinated by cowbirds and all of the other obligate brood parasites. I want to learn everything I can about them. This strategy can provide so many insights into evoloution and the intelligence of birds in general. Unfortunately I live in a major urban area and will probably never have the opportunity to observe this behavior in nature.
I have Cowbirds~ a male and female for the first time ever in my yard. 4-16-23 I know they are protected and have looked at tips to discourage them. Like putting out food they don’t care for ex. safflower seeds, peanuts etc Taking down the platform feeder for a week. I’m concerned for my house finch nestlings. Any tips? thanks.
Hi Paula,
House Finches are vegetarian and actually make poor foster parents of cowbirds, so cowbird young don’t usually survive in a House Finch nest. That means that the House Finches should be able to successfully raise their own young even if a cowbird finds the nest. So we just recommend letting nature take its course.