In the spring of 2015, a male House Wren and his mate built their nest inside a nestbox near a honeysuckle. His mate laid her eggs and dutifully incubated them. Then, one morning— cheep! cheep! High-pitched calls and gaping red mouths cried hungry, daddy! and the male wren was off in a paternal tizzy, collecting bugs and delivering them to his new offspring.

It was, maybe, odd that his new offspring weren’t in the nest that he had built. It was, maybe, odd that other, larger birds were also feeding his babies. It might even have been called odd that his mate was still sitting in their nest, atop whole and silent eggs. But— cheep! No time for that! The chicks were hungry!

What this male House Wren was doing, no doubt to the profound irritation of his mate, was feeding the offspring of a pair of Northern Cardinals who had nested in the honeysuckle near his nestbox.

Admittedly, young songbirds pretty much all look the same—pink skin, a few tufts of fluff, red mouths—but it’s still a surprising mistake to make. The baby cardinals were in an open-cup nest: the kind of nest you picture as the prototypical bird nest, open to the sky. House Wrens nest in cavities, all hidden and dark. Too, the cardinal parents were also feeding their offspring, and the wren knew it: he would wait until they left the nest to feed the chicks.

After a few days of this, the wren chicks hatched, at which point you might have expected the male wren either to A) stick by “his” cardinal babies, or B) realize his mistake and transfer his attentions to his true offspring. No: the wren chose option C), feed both sets of nestlings simultaneously. As soon as the cardinal chicks left the nest, however, he ignored them, devoting himself entirely—finally—to his baby wrens.

This is all quite unusual. Birds do occasionally “adopt” other species, but that is generally either in cases where the chicks are mobile, and so the chick might have conceivably wandered off (e.g., ducklings), or in cases where there is a nest with immobile chicks and the adopted chick has managed to end up in the nest. It’s well known that birds with nests identify which chicks are theirs by looking at which chicks are in their nest: this is why sneaky brood parasites can get away with slipping an egg into a nest and leaving it for the parents to raise, and why sneaky biologists can swap chicks among nests in a method called “cross-fostering” to examine the relative roles of genetics and environment. A bird with a nest, who then transfers his affections to a different nest, but not so entirely that he doesn’t also feed his own chicks when they hatch, is… a puzzle.

So I am very happy that this strange male wren decided to build his nest and pursue his confused paternal obligations in the backyard of Rae Spencer, who pays a lot of attention to the birds in her yard. There are surely lots of oddball animals who engage in strange behaviors in places where no one sees, or—worse—someone sees but doesn’t notice, and that information is lost. Rae noticed, and documented, and then contacted me through this blog to ask, “Is this normal?” And I got really excited, and declared that the world needed to know, and we published a short note about the wren in the Wilson Journal of Ornithology.

Alone, that note is just one strange anecdote about a confused wren. But the great thing about publishing in science—especially now that publications can be searched online—is that it allows data to be found, and put together with future or past data. There were two previous accounts of very confused House Wrens in the scientific literature: one fed a brood of Northern Flickers (which do nest in cavities, but don’t remotely look like baby House Wrens); the other fed a brood of Black-headed Grosbeaks, then a brood of House Sparrows. The more we document these strange things, the more we can begin to put them together into something approaching an understanding of this unusual behavior. Our weird wren is a puzzle, but he’s also potentially a piece of that puzzle.

To see photos of this confused wren and his cardinal neighbors, head over to Rae Spencer’s blog post about it. My favorite is the first photo in her post: a wren, seen through sunlight-kissed green leaves, leaning over to feed a gaping chick. If you didn’t know wren nesting ecology, it would look perfectly natural. Only by knowing that wrens nest in dark, hidden holes—and therefore that these cannot be the wren’s chicks—can you see the photo for the fascinating weirdness that it is.

Reference:

LaBarbera K, Spencer R. 2016. House Wren (Troglodytes aedon) provisions nestlings of Northern Cardinal (Cardinalis cardinalis). The Wilson Journal of Ornithology 128(3):676-678.