Back in graduate school, I studied grassland birds and how they were affected by habitat fragmentation. While tramping around the prairie looking for birds, I’d periodically come across their nests as well. Often, there would be two very different kinds of eggs in a nest – one set belonging to the bird that built the nest and another set deposited by one or several brown-headed cowbirds. Since I knew that cowbird ‘brood parasitism’ could be really hard on the host species I was studying, I had a choice to make when I came across cowbird eggs in someone else’s nest. Toss the cowbird eggs out and help the host? Or let things play out without interfering?

For those not familiar with brown-headed cowbirds and their strategy, the summary is that they basically bypass parental care of their young. Female cowbirds stalk birds of other species and follow them back to their nests. They wait for the adults to leave and then sneak in and lay one of their own eggs in the nest. Sometimes, they’ll remove one of the host’s eggs to make room. Upon returning, the host parent might reject that new egg and get rid of it (some host species are much more likely than others to do this). More often, the cowbird egg gets treated like part of the family and the host will work overtime to feed the cowbird chick once it hatches – usually at the expense of the host’s own babies.

It seems like a terrific strategy for cowbirds, who can avoid doing all the work to find and feed baby birds – though finding nests of other birds is no easy task either. The host birds who end up feeding those cowbird babies, though, usually fare poorly, often raising only the more aggressive – and often much larger – cowbird babies while their own chicks starve or are killed by cowbirds. Does that make cowbirds bad? Or have they just evolved a ‘clever’ strategy?

Brown-headed cowbirds aren’t the only birds that have developed this parenting (lack-of-parenting?) approach. In fact, about 1% of bird species are known to utilize a similar game plan. That includes other species of cowbird and some cuckoos, honeyguides, finches, and ducks. In fact, it’s thought that the strategy has evolved independently seven times among birds. In other words, we know of seven unrelated examples of a bird species becoming a brood parasite. That’s pretty incredible, but also a testament to how advantageous the strategy can be.

Even if you’re familiar with cowbirds, cuckoos or other avian brood parasites, you might not know how common brood parasites are in other taxonomic groups of animals. At least one species of fish (the cuckoo catfish) is a brood parasite, along with a number of invertebrate species, including some butterflies, true bugs, beetles, ants, wasps, and bees. In fact, it’s likely the approach is much more common than we know because of how little we understand about the reproductive strategies of many invertebrates.

Within insects, there are at least two different categories of brood parasitism we’re aware of. The first is similar to the cowbird scheme, such as when a ‘cuckoo bee’ sneaks into the nest of a solitary bee species. (Solitary bees are those in which a single mom builds and provisions a nest by herself – as opposed to social bees that have a queen, workers, etc.) The intruder deposits her eggs and quickly exits the nest, leaving her offspring to hatch and eat the food intended for the host’s babies. Sometimes the intruder’s young eat the host’s eggs too.

Within the second category of insect brood parasites, the raider invades a colony of another species and takes it over by usurping the queen and tricking the colony to work for its new boss. This often plays out like an action movie, with a heavily armored invader trying to fool and/or fight guards as she makes her way to the queen’s bedroom.

This second, and more aggressive kind of brood parasitism is a lot more dangerous than just entering a solitary bee’s nest after making sure the only resident adult has left. On the other hand, the reward is that you get a whole colony of insects to pamper you and parent the many children you produce as the new queen. To the bold go the spoils.

You might be wondering how this crazy, but effective, brood parasitism strategy has evolved so many times within such a range of unrelated animal species. It must be pretty advantageous if birds, catfish, bees and butterflies all ‘figured it out’, evolutionarily speaking. In fact, when you think about it that way, the real question isn’t “why so many animals have developed brood parasitism?” The better question is “why is it so rare?”

One answer, of course, is that if everyone started relying on others to parent their kids, nobody would be parenting any kids. That would work out poorly. Another answer is that it might be a lot more common than we think. We’re pretty ignorant about the parental strategies of a lot of the world’s animals – especially the tiny ones and the ones living where relatively few biologists are able to watch them closely. It wouldn’t be a surprise to learn that lots more animals are following the brood parasitism lifestyle.

Scientists have been trying to understand the origins of brood parasitism for a long time. I’m no expert in the field, so I’m certainly not going to try to represent the state of scientific knowledge of the topic. From what I can tell, there are lots of ideas, some evidence, and still a lot of exploration to do. It’s unlikely that there are just one or two main reasons for the evolution of brood parasitism. The reasons may be as diverse as the brood parasite species themselves.

It does seem, though, that we can debunk one common story you might have heard about why brown-headed cowbirds became nest parasites. It was hypothesized that cowbirds in North America followed herds of bison across the landscape, feeding on insects on and around them. The hypothesis further conjectured that the birds’ nomadic lifestyle made it too difficult to establish a static nest location, leading female cowbirds to start dumping eggs into whatever nests they came across.

It’s a pretty good story but almost surely not true. There are a couple main problems with it. First, it’s thought that North American cowbirds descended from cowbirds on other continents (most likely, South America) where there weren’t big roaming herds of bison or similar large animals. Second, cowbirds are actually pretty territorial and not very nomadic. Males defend defined territories and any females they can attract. Meanwhile, females probably need to stay in one place so they can get to know the habits and, eventually, the nest locations of the local birds whose broods they’re trying to parasitize.

I’d love to understand more about how brood parasitism came about, but in the meantime, I’m happy to simply admire the strategy. You might call it ‘cheating’ or you might call it ‘creative problem solving’. Either way, it’s pretty amazing.

Also, if you do think these species are cheaters, they’re certainly not the only cheaters in nature. There are countless ways organisms ‘cheat the system’. As one quick example, many flowers produce lots of nectar as a way to bribe pollinators to visit and carry away pollen. That works pretty well most of the time. However, there are also little ‘cheaters’ who will just chew through the outside of the flower and steal the nectar without ever touching the pollen. Those ‘cheaters’ might not fit inside the flower or have tongues long enough to otherwise reach the nectar but nectar is really tasty and nutritious so they found a workaround.

Now, while I think brood parasitism in general is pretty fantastic, there can sometimes be serious negative consequences for hosts (victims?). It’s never cool to be compelled to take care of someone else’s offspring at the expense of your own. If you’re already dealing with habitat loss or other big issues, however, the added stress of another creature’s babies stealing from your own can be enough to drive you nuts. Or drive you to extinction.

In situations like that, it’s easy to see why conservationists might choose to step in and fight back against brood parasites. There have been significant efforts to control populations of brown-headed cowbirds, for example, to help save rare birds like least Bell’s vireos in California or Kirtland’s warblers in Michigan. Similar programs have been discussed or implemented in many other situations as well.

It’s not the fault of brown-headed cowbirds, of course, that habitat loss and other stresses have driven their hosts to the point where brood parasitism can be the last straw. Similarly, cuckoo bees that steal into nests of other bees aren’t responsible for our pollinator crisis. They’re just doing what they’ve done for a very long time.

As is always the case in nature, the topic of brood parasitism comes with a lot of complexity and nuance. Despite that – or maybe because of it – it’s immensely fascinating to me. I hope it’s fascinating to you as well.

For the record, I didn’t toss those cowbird eggs out of nests when I came across them. Right or wrong, it felt like something the birds needed to work out between them. (Also, removing cowbird eggs from a nest without a permit is illegal…) Instead, I’ve dedicated much of my career to improving the habitat conditions those birds (and many other species) rely upon. When habitat conditions are good, brood parasitism becomes less of a deadly last straw and more of an intriguing and wonderful adaptation.

If you want to read more about brood parasitism, here are a couple options:

A special journal publication on the topic

More detail about brown-headed cowbirds and the myth of following bison