Admiration for ‘Cheaters’

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.

A female brown-headed cowbird with three males. Have you ever wondered how cowbirds know they’re cowbirds? They’re raised by other species. How do they know how to sing cowbird songs or that they’re supposed to mate with cowbirds?

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.

Epeolus sp. A brood parasite bee that roosts overnight that clamps itself to plants with its mandibles. This is apparently common among ‘cuckoo’ bees.

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.

A cuckoo bee (Epeolini sp) feeding on ironweed (Vernonia baldwinii).

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.

A juvenile cowbird flies up from the ground in front of a bison at The Niobrara Valley Preserve. Cowbirds tend to feed on insects around bison and cattle, which contributed to the erroneous story about how they developed their brood parasitic ways.

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.

Two dickcissel eggs (blue) and two cowbird eggs (speckled) in a dickcissel nest at our family prairie.

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

Photos of the Week – January 21, 2023

We finally got measurable snow at the Platte River Prairies! It’s been a long brown winter so far and it was great to see those big flakes coming down and transforming the landscape earlier this week. After a very busy work week, I went out Friday morning to tromp through the snow with my camera and see what I could find.

On the way to the prairie, I had to stop and photograph some Canada geese relaxing on the ice along open channels of the Platte River. Most of the geese were sleeping – or close – while standing on ice or in very shallow icy water. Others were keeping an eye on the situation. With the frosted trees in the background, it was a scene I couldn’t pass up.

Canada geese on the Central Platte River. Sigma 100-400mm lens @100mm. ISO 320, f/4.5, 1/4000 sec.

Once in the prairie, I trudged uphill through snow that was 6-10″ deep, depending on where I stepped. I was a little late to the party because I had to finish up some other work before I left, but the frost on all the prairie plants was still hanging on, despite the sun getting higher. Everything, everywhere was gorgeous and it was really hard to decide where to point my camera and what lens to use. I spent most of my time lying down or getting up – all while trying to keep snow off the front of my lens.

Canada wildrye (Elymus canadensis) with frost in snowy prairie. Nikon 10.5mm fisheye lens. ISO 400, f/22, 1/320 sec.
Canada wildrye (Elymus canadensis) with frost in snowy prairie. Nikon 10.5mm fisheye lens. ISO 400, f/22, 1/500 sec.

Canada wildrye covered in frost seemed like an easy first subject, so I stuck a fisheye lens on and got right up next to the seed heads, which were waving around in a gentle breeze. Because they were moving, I had to take a lot of photos to get a few sharp ones. It was yet another reminder of how digital photography has changed the game. I never would have been able to afford those risky shots when I was shooting slides.

(For those of you youngsters out there, slides were film that, when developed through a magical and expensive process and put into little white frames, turned into tiny photographs. Once you got a bunch of them, you could put them in a machine that shone light through them onto a big screen in a darkened room. People largely used slides to put their relatives to sleep by showing them lots of photos of recent vacations.)

Sand lovegrass (Eragrostis trichodes) and snow. The bottom ‘branch’ of the seed head was touching the front of my lens. Nikon 10.5 fisheye lens. ISO 400, f/22, 1/500 sec.
Sand lovegrass (Eragrostis trichodes) and frost. Nikon 105mm macro lens. ISO 400, f/13, 1/1600 sec.
Green sage (Artemisia campestre) with snow piled up behind it. Nikon 10.5mm fisheye lens. ISO 400, f/22, 1/500 sec.
Cell phone photo of my camera/tripod setup for the below photo.

I walked around for an hour or so before light started getting too bright to do much with. Before I walked back to the truck, though, I put my macro lens on and clipped a magnifier to the front of it. I wanted to photograph the individual frost crystals that were everywhere. I concentrated on the little ridges created by wind on the snow’s surface because the crystals had accumulated on the ridge and there were shadows behind the to create contrast. I got some nice images, including the one below.

Frost on snow. Nikon 105mm macro lens with Raynox 250 magnifier. ISO 400, f/22, 1/500 sec.

I’m not sure how long this snow will stick around – it started melting shortly after I left the prairie yesterday. We had the chance for a few flurries today, but nothing much materialized. I’m really glad I took the chance to go out when I could!

If you like snow, I hope you’ve gotten some recently. If you don’t like snow, I get that too, but I’d respectfully suggest trying out a snowy prairie to see if that changes your mind.