Our Better Nature: Freeloading Feathered Fiends

The award-winning 2019 film Parasite depicts a wealthy family that gets duped into hiring four members of a ne’er-do-well family who pose as seasoned domestic help. The imposters steal from the host family and wind up laying waste to their peaceful existence. But that’s nothing compared to what happens in real life every spring.

In the avian world, there’s a group of species known as brood parasites. It seems these characters never got the memo on how to make nests, incubate eggs, and feed babies like normal birds do. Instead, the females sneak around and lay eggs, one at a time, in the nests of honest, hard-working birds. And for the most part, the ruse works – other bird-parents raise these “nest parasites” as their own, often at the expense of their biological offspring. In the U.S., at least 200 different host species are thus defrauded each nesting season.

The main culprit is the brown-headed cowbird, a small (6-8 inches long), innocuous-looking bird with a sparrow-like bill. They favor open grasslands where livestock hooves kick up bugs and worms that cowbirds can feed on, but they have adapted to urban and suburban conditions as well. Males are glossy black with a light brown head, while females are dappled brown, perfect camouflage for skulking around their neighbors’ abodes. Native to all lower 48 states, the cowbird is a type of obligate brood parasite, meaning it can only reproduce by holding other bird families hostage to its babies.

One of the reasons cowbirds get away with their theft-of-services con is that the color of their eggs, white to off-white, with brown to gray spots or streaks, is similar to that of many other species’ eggs. Size doesn’t always match, as cowbird eggs are bigger than many host species eggs. Numbers may not add up either, as female cowbirds will kick out (or eat) some of the host’s eggs if the nest is crowded. A few host species, like the yellow warbler, can recognize cowbird eggs and will eject them or build a new nest on top of them. Yellow warblers also serve as an “early-warbling system,” alerting other birds that a brown cowbird is nearby. They do this by making a ”seet” call that sends all other birds within earshot back home to protect their nests. In the end, though, host parents usually accept the odd-egg-out as one of their own.

In some cases, it may be true that hosts really don’t notice the “ugly duckling” in their nest, but there’s a darker reason so many species blithely raise the cowbirds’ cast-away brats. Cowbirds sometimes take revenge on birds that reject their eggs by destroying the would-be host’s eggs and/or nestlings while the adults are gone (this can also result when well-meaning people identify and remove cowbird eggs from songbird nests). By adopting the cowbird’s egg, and thus avoiding a Mafia-style punishment, the host parents at least have a chance to successfully rear a few of their own chicks.

Because cowbird eggs have a shorter incubation period than most songbirds’ eggs do, they’re usually the first to hatch. In addition, cowbird chicks grow faster than the young of many other species, meaning they hog the lion’s share of food their foster-parents bring home. Since cowbirds are native, and their parasitic proclivities are nothing new, they don’t seem to hurt the breeding success of host species in a significant way.

One might assume cowbirds are genetically lazy to have hatched such a breeding plan, but there’s a better explanation. It’s believed they historically followed bison herds  as they ranged across the prairies. In the disturbed soil left behind as bison passed, cowbirds would have found plenty of grass seeds and insects to eat. Over time, they may have adopted their egg-abandonment method because they had to move with the herds. On the other hand, cowbirds might have been free to follow bison due to the fact their strategy was already in play (which suggests to me they might be innate slackers).

We have two more bird species that leave eggs in other nests. Fortunately, they are facultative brood parasites, which means they sometimes offload their eggs onto others, but for the most part they make nests and raise their own young. Maybe parasitizing nests is just a hobby for them.

Found as far west as eastern Wyoming, black-billed cuckoos also breed in the Plains states, Great Lakes region, and the Northeast. They’re reclusive, preferring dense forests and brush in which to nest. You’re more likely to hear, rather than see, a cuckoo. Their song, while not ornate, is unmistakable, a low series of staccato notes that sound like “cuckoo.” No surprise there. Scrabble players might want to take note: black-billed cuckoos have zygodactyl feet, defined by 2 toes that point forward and 2 that point backward.

The yellow-billed cuckoo is our other part-time brood parasite. Its breeding range is from southern Canada down to Mexico, and from the Dakotas east to the Atlantic. They’re most abundant in Texas and Oklahoma, but also common throughout the Southeast. Yellow-billed cuckoos never learned how to say “cuckoo,” but they sound a bit like they’re trying to get the hang of it.

Yellow-billed and black-billed cuckoos are among the few birds that like to eat hairy caterpillars like the destructive tent caterpillar and spongy moth (formerly Gypsy moth) caterpillar. These pests can entirely defoliate trees, weakening and sometimes killing them. Cuckoos are often abundant during outbreaks of these caterpillars, and help to reduce their numbers.

But brown-headed cowbirds aren’t devoid of benefits. They eat insects like grasshoppers, beetles and caterpillars, many of which can damage crops, and they also eat the seeds of grasses as well as of many nuisance weeds. If we could get them to enroll in parenting classes, I’m sure we’d like them better.