By Robyn Bailey, NestWatch Project Leader
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.
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!
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.