Maybe you looked into a bird’s nest and saw one egg that was a different size and color than the rest. Or you noticed a chick that was larger, louder, and more aggressive than the others. Maybe you saw a parent bird feeding a chick larger than itself, and different in color and shape. What you saw was a brood parasite, an animal that fools animals of other species into caring for its offspring, usually at the expense of the host species’ own young.
We have known about brood parasites since at least the days of Aristotle, who wrote “The cuckoo… does not build a nest. Sometimes it lays its eggs in the nest of a smaller bird after first devouring the eggs of this bird.” He was presumably writing about the common cuckoo, a species widespread across Europe and Asia. The cuckoo family includes more brood parasites than any other group of birds, though not in our part of the world: Two species of cuckoos live here in Massachusetts, the black-billed and yellow-billed, both of which build nests and raise their own young.
The brood parasites of American birds are not cuckoos, but cowbirds. Cowbirds are a genus of blackbird, and often flock together with other blackbird species like red-winged blackbirds and common grackles. They also frequently follow herds of grazing animals, hence the name “cowbird.” Perhaps “bisonbird” would be more authentic, reflecting the species’ original association before European settlers hunted bison to near-extinction. The grazers create areas of short grass and bare dirt where the cowbirds find food much more easily than in tall grass or other vegetation. Some will also eat insects found on or around the mammals.
Not all brood parasites are birds, not even close. Quite a few are insects. In the world of slave-making ants, aka Amazon ants, a queen takes over the nest of another ant species and chemically induces the workers to raise her young.
A tiny metallic green wasp creeping on wood is usually a cuckoo-wasp, one of several brood parasite species named for their famous avian counterpart, searching for nests of thread-waisted wasps in which to lay their eggs. Cuckoo bees mostly fly in the early spring and target ground-nesting miner bees. There are numerous other bee brood parasites including cuckoo leafcutters, cuckoo bumblebees and cuckoo sweat-bees (aka “blood bees”), who lay their eggs in the nests of leafcutter bees, bumblebees and sweat bees, respectively. Massachusetts is home to species from all of these groups, but not to one of the strangest brood parasites, the cuckoo catfish, endemic to Africa’s Lake Tanganyika, where it parasitizes mouth-brooding cichlids (including tilapia, a popular imported fish that is now farm-raised here in the U.S.).
Most of us have a negative attitude toward brood parasites. How can anyone feel otherwise while watching a baby bird die of starvation because an interloper is hogging all of the food? Or die of exposure after being pushed out of its nest? How can we feel positive toward a bird that cracks open the eggs of other birds, or pecks others’ chicks to death, to make way for its own? And, magnifying their bad reputation, brood parasites also pose a challenge for conservation of rare bird species. Our efforts to protect and restore populations of Kirtland’s warbler, golden-cheeked Warbler, black-capped vireo, least Bell’s vireo, California gnatcatcher, and southwestern willow flycatcher have all involved trapping and killing cowbirds in order to improve the rare species’ breeding success.
However, our attitude is more negative than it should be. The deaths of host species’ chicks are overemphasized in nature documentaries, made to appear more common and more dramatic than they actually are, in order to attract viewers and hold their attention. The cuckoos and cowbirds of such shows may as well have black hats and handlebar mustaches, with the helpless warblers and flycatchers tied down on railroad tracks. Host species in reality often capably defend their families, breaking or ejecting parasite eggs, or abandoning parasitized nests and building new ones, sometimes stacking new nests directly on top of parasitized ones. Some brood parasites may even benefit their hosts: The chicks of some caciques and oropendolas, Central American birds related to our orioles, grow up healthier with a giant cowbird chick sharing their nest than without, thanks to the young cowbirds eating botfly larvae that would otherwise infest the host chicks.
Cowbirds coexisted with warblers, vireos and flycatchers for thousands of years before we came along. Brood parasitism was no threat until the host species’ populations were greatly reduced by human-caused habitat destruction. The same habitat changes that threatened the host birds also supported explosive growth of cowbird populations, as cowbirds find food much more easily on lawns, golf courses, and plowed fields than in wooded or shrubby landscapes. Cowbird trapping may have stopped the population decline of Kirtland’s warbler, but the decline did not reverse until we started restoring the warbler’s habitat. As usual in conservation, we see the real villain when we look in the mirror. When we recognize and correct our mistreatment of birds’ habitat, it lets us admire brood parasites for the innovative survivors that they are.