An Ingenious Breeding Strategy

Being a parent is hard. Raising a family takes a lot of energy and time. If you have kids, you know what I’m talking about.

I mean, I don’t have kids, but I do have some pet chickens. I imagine that’s pretty close to the same thing. 

When you’re an exhausted parent and you wish you had some time to just live your own life, you might secretly fantasize about taking your kids over to your neighbor, dropping them off, and saying, “Here. These are YOURS now. I’ve got better things to do… like sleep.”

Now I know you’d never do that, of course. But there are many birds out there that have no qualms about dumping their offspring into the care of a neighbor or even a complete stranger.

This is the phenomenon we call brood parasitism. It occurs in a number of birds, as well as some fish and insects. 

In birds, this is where a female lays an egg in the nest of another female or mated pair. The parasite female flies away and never sees her offspring. The host bird or birds are fooled into raising the foreign chick as their own.

The brood parasite seems to get a pretty sweet deal: she doesn’t have to spend energy and time raising her own chicks. She just drops her hot little eggs into other birds’ nests and then spends her days pursuing her hobbies and living her best life. She gets to pass her genes on without doing much work.

The host birds, on the other hand, get the short end of the stick. They spend enormous amounts of energy raising a baby that isn’t their own. So, from an evolutionary standpoint, this can defeat the whole purpose of building a nest and raising chicks. These host parents are less likely to pass on their own genes to the next generation.

At least 300 bird species engage in some form of brood parasitism. The most famous of these are the cuckoos and cowbirds. But there are some others you might not know about. We’ll talk about the different groups of brood parasites in a few minutes.

Anyway, this seems to be a pretty effective reproductive strategy. It’s just one type of what biologists call a breeding system. Other breeding systems in birds include monogamy, polygamy, cooperative breeding, and a few others.

As a strategy, brood parasitism has its pros and cons. The same is true for any of the breeding systems. For them to exist at all, however, the pros must outweigh the cons, on average, most of the time. But no one system is the best approach in all places and all environments.

The mathematical models of game theory have been used to examine brood parasitism in the context of evolution. It looks like being a brood parasite is a good strategy under some conditions, like when the environment varies a lot and is unpredictable. From the perspective of game theory, the brood parasite is hedging its bets, to have the best chance of passing its genes down to the next generation. 

Birds that hedge their bets this way, by being sneaky, dastardly brood parasites… well, they’re just making sure they don’t put all their eggs in one basket.

Diversity of Brood Parasitism in the Avian World

Among birds, brood parasitism rears its ugly head in more than one way. The most common form is intraspecific brood parasitism. This is where a bird lays her eggs in the nests of birds belonging to her own species. 

For example, Eared Grebes seem to regularly drop some eggs in the nests of other Eared Grebes. The same is true for Cliff Swallows and European Starlings. 

These are just a few of about 234 species that are known to dabble in some intraspecific brood parasitism. These species represent a wide diversity of avian families, from grouse and waterfowl to rails and weaverbirds.

Maybe there are many more species that have this behavior, but we just haven’t caught them in the act yet. It's not always easy for researchers to detect intraspecific brood parasitism. Because, in this situation, there are few if any physical differences in the eggs or chicks between the parasites and their hosts.

Then we have interspecific brood parasitism. This, as you have probably deduced, plays out between birds of two or more species. Intraspecific means within species, interspecific means between or among species.

Another important distinction I need to point out is the one between birds that act like parasites sometimes versus those that do it all the time. 

Facultative brood parasites routinely make their own nests but they will sometimes deposit eggs in other birds’ nests. Obligate brood parasites, on the other hand, never make their own nests and they have no choice but to have someone else raise their babies. They’ve evolved to specialize in this reproductive strategy.

So we have bird species that are facultative, intraspecific brood parasites and some that are obligate, interspecific parasites, and we have everything in between.

We will focus on the most extreme and arguably most interesting case: the obligate, interspecific brood parasites. They always lay their eggs in the nests of other species. This is a group of about 100 species, which represents approximately 1% of all bird species. Collectively, these tricksy birds parasitize over 950 host species. So this means that nearly 10% of all bird species must sometimes contend with brood parasites in their struggle to reproduce successfully. 

The 100 or so obligate brood parasites come from five distantly-related bird families. Cuckoos are in the family Cuculidae. Approximately one third of the 146 species in this family are obligate brood parasites. Cuckoos are among the most well-studied of all the brood parasites. Many of them have more than one host species. For example, the Common Cuckoo that lives across Europe and Asia is a parasite to more than 100 host species. Hosts include flycatchers, chats, warblers, pipits, wagtails, buntings, and others.

Cowbirds are the most notorious brood parasites in the Western Hemisphere. There are five cowbird species in the New World Blackbird family, Icteridae. These species are close relatives belonging to the genus Molothrus.

The Brown-headed Cowbird is one North American obligate brood parasite. These cowbirds lay their eggs in the nests of over 200 host species. The top three hosts for the Brown-headed Cowbird are the Yellow Warbler, Song Sparrow, and Red-eyed Vireo. The other North American obligate brood parasite is the Bronzed Cowbird (Molothrus aeneus) which lives in Mexico year round and breeds in some southern US States (AZ, CA, NM, TX, and LA).

Then we have the honeyguides. The 17 honeyguide species found in Africa and Southeast Asia are all obligate brood parasites. These birds make up the family Indicatoridae.

Similarly, every one of the 20 species in the family Viduidae is an obligate brood parasite. Viduids are the whydahs and indigobirds of Sub-Saharan Africa. Each of these species is paired with a single host species from the closely related waxbill family, Estrildidae.

And lastly, there is a single duck species that lives as an obligate brood parasite. The Black-headed duck lives in temperate South America. No other member of this family, Anatidae, is an obligate parasite.

There’s a lot of variation among the world’s obligate brood parasite birds—variation in the types and numbers of hosts they use, their tactics, and their virulence. By virulence, I mean the degree to which they hurt the nesting success of their unfortunate hosts. 

I’m not going to do a deep dive into all the nuanced details of how brood parasitism plays out in every pair of hosts and their parasites. Instead, let’s consider the general dynamics of brood parasitism.

Adaptations of Brood Parasites

We’re first going to take the perspective of the humble yet oft maligned brood parasite. Then we’ll consider what it’s like to be a host. We might think that the parasitic bird has it pretty easy. At least, that is, after it’s successfully placed an egg in a host nest. 

Life in the wild is rarely as easy as it looks to us. To be successful, a brood parasite has many challenges to overcome. This bird must bring to bear a whole arsenal of adaptations. These include special behaviors, physiology, and morphology. First, it needs to find a host nest, which isn’t always easy. Then it needs to figure out when and how to get into the host nest. Being caught by the host can have some dire consequences. 

Brood parasites gain access to host nests in a variety of ways. This varies across species. Some parasite species rely on stealth. They’re well-camouflaged, sneaky, and quiet. They run surveillance on the host nest and then skulk into it when the hosts are away. 

Other parasitic species use a distraction technique where the male draws the attention of the host birds. He eggs them on (no pun intended) so they leave the nest to mob him. Meanwhile, the female creeps into the nest to drop her egg.

Some cuckoos have evolved an extraordinary adaptation in their appearance that helps them access host nests. The males of these species closely mimic the shape and plumage of a bird-eating accipiter hawk or falcon. When such a mimic male swoops in close to the host nest, the host birds get spooked and leave the area. Once the coast is clear, the more cryptically-colored female cuckoo goes in and delivers the goods.

This incredible mimicry is exemplified by the 8 Hawk-Cuckoo species in the genus Hierococcyx. These birds live in various parts of Asia.

So once our parasite bird gets into the nest, she may destroy or eject one or more of the host’s eggs (video of this behavior). The latter behavior varies across species. Some cuckoos will remove one host egg and replace it with one of her own. Cowbirds, too, will often remove one or more eggs from the host nest.

After the adult flies away to lay another day, the challenges are placed on her eggs and, eventually, her babies. The eggs must not get noticed or rejected by the host birds. 

The Common Cuckoo lays eggs that are remarkably similar to those of her host, albeit a little larger. Remember that Common Cuckoos parasitize numerous host species. So how does a female cuckoo know how to match her egg to the correct host egg? She doesn’t, really. She doesn’t make any kind of decision about what kind of egg to lay. 

Female Common Cuckoos overwhelmingly select the same host species that they were raised by. If a female cuckoo was raised by Great Reed Warblers, she will seek out that species when looking for a host. She’ll lay greenish-blue eggs with dark brown spots, which mimic those of the warbler. Her ability to lay such eggs is a genetically encoded adaptation. Her daughters, too, will seek out Great Reed Warblers to parasitize. 

The end result is that there are numerous egg type lineages within the Common Cuckoo species. Egg color in these birds appears to be determined by genes on the W chromosome, which is only passed from female to female. Common Cuckoo females freely breed with males of any type, but when it comes time to choose a host… well, you know what the females do. They choose the same host that their mom chose.

Unlike these specialist cuckoos, Cowbirds are generalist brood parasites. They aren’t as particular about which host they select. Their eggs always look the same, no matter the host. 

If a brood parasite’s egg is not rejected by the host parents, it hatches and the next phase begins. Parasite eggs almost always hatch 2-4 days earlier than the host eggs. This gives the newly-hatched parasites a head start in being fed and cared for. 

Nestling parasites have a whole bunch of tricks up their sleeves for surviving in and dominating the host nest. The tactics used vary across species, of course. Mimicry is one tactic. For example, several species of Bronze-Cuckoo in the genus Chrysococcyx produce hatchling chicks with skin colors that mimic those of their hosts’ babies.

Many brood parasite chicks actively expel the host's eggs. Before their eyes are even open, they operate on instinct to push objects like eggs and other baby birds out and over the side of the nest.

And if you think that’s brutal, baby Honeyguides are even worse. These little guys are born with sharp, fang-like tips on their bills. They use these to bite their nestmates to death. Again, this happens before the parasitic chicks are even able to see. This murderous behavior is pure instinct, driven by the bird’s genes.

As they grow, brood parasite chicks of many species dominate their nests and demand the attention of their adoptive parents. They tend to be larger than the host chicks, quickly filling up the nest space as their bodies grow. They yell loudly, making begging calls that exceed the normal volumes of the hosts’ chicks. They have brightly-colored patterns on the insides of their mouths that match those of the host chicks. All of these things tug at the parents’ instincts to shove more food into the parasite chick's open gullet.

Even when a parasitic chick hasn’t killed off its nestmates, it’s large size and dramatic behavior often means the other babies in the nest become malnourished and generally neglected. Only a lucky few host chicks will survive to become fledglings under these conditions.

Adaptations in the Host Birds

The last thing host birds want to do is waste several weeks of their precious breeding season feeding and tending to a big, fat, demanding baby that isn’t their own. 

When the threat of brood parasites looms, host birds have several lines of defense. Front-line defenses might be the most important. If the hosts can keep their nests from being invaded by brood parasites, they can devote all their energy to raising their own brood. So front-line defenses, when successful, are the least costly to the hosts.

Such defenses include camouflaging your nest or placing it where it isn’t likely to be found by parasites. Or the nest itself can be constructed so that it prevents a parasitic bird from getting inside. If those tricks aren’t possible or they don’t work, you can attack any brood parasites you see near your nest. You can also stay in your nest, guarding it during the times of day when brood parasites tend to be out looking for trouble. Any combination of these tactics might work.

If, however, a parasite female gets past the first line of defense, all hope is not lost. Once the parasite has deposited her egg, the host birds may be able to use egg-stage defenses. At this point, they need to be able to recognize that something is amiss, that there’s an alien egg in the batch. Once identified, they can destroy and/or remove the egg. 

Ornithologists have identified two primary ways that host birds seem to identify parasitic eggs. The first is perceived discordancy. This is where the host birds recognize any egg that stands out. As in “one of these things is not like the others.”  This is great, except in cases where parasitic birds have replaced not just one, but most of the host's eggs with their own. Then the host egg would be the odd one out.

The other way a host might spot the imposter egg is by comparing each egg in the nest to a recognition template. This is an instinctual, mental image of what the host’s eggs look like. If an egg doesn’t match the template, it gets tossed. The perceived discordancy and recognition template mechanisms aren’t mutually exclusive, so some hosts might use them in combination to reject parasite eggs.

Hosts will sometimes just give up and abandon their nest altogether if they think it’s been infiltrated. In other situations they’ll simply add nest material to cover up all the eggs, parasitic or not, and lay another clutch on top, in the original nest.

The third line of defense is at the nestling stage. Having been duped on the front line and by the parasite's eggs, the hosts have this last chance to salvage their reproductive efforts. They need to be able to spot and reject the parasite’s nestling. 

As for host species that are reasonably good at rejecting parasitic nestlings, we have a couple interesting examples. The Superb Fairy Wren of Australia is one. One study found that 40% of host wrens abandoned their nests once parasite nestlings were present. The parasite in this case was Horsfield's Bronze-Cuckoo. Further research on the Superb Fairy Wren has suggested a fascinating mechanism by which these hosts unmask the parasite chicks. It turns out that Female Superb Fairy Wrens sing to their incubating eggs. When her babies hatch, they make begging calls that incorporate specific elements of the mother’s song. These calls are said to act like a password. A bronze-cuckoo imposter chick won’t be as good at learning the password. If mom doesn’t like the response, she may know she has a parasitic chick and she’s likely to abandon her nest.

Another bird in Australia has shown some nestling rejection behavior. The Large-billed Gerygone sometimes forcibly removes the chicks of the Little Bronze-Cuckoo. Given that these parasite chicks actually mimic the appearance of the Gerygone host, it’s not certain how the imposters are recognized.

How Brood Parasitism Evolves

Brood parasitism is an excellent example of what biologists call an evolutionary arms race. This is how it works for these birds: A brood parasite species evolves an adaptation that makes it better at getting past the defenses of its host. The host species then evolves its own counteradaptation through natural selection. Round and round it goes, escalating like an arms race between human superpowers. 

Each of these bird species has a big influence on the evolution of the other. This is the process of coevolution. The way this plays out in our avian arms race is complex and it isn’t completely understood. The evolution of brood parasitism is still an active area of research.

Over thousands or millions of years, the evolutionary progressions of the parasite and host involve numerous tradeoffs. An adaptation that provides a benefit might also come with a cost. A cost would be something that reduces the bird’s probability of survival or reduces its lifetime reproductive success.

For example, some brood parasites like cuckoos lay eggs with extra thick shells. This is an adaptation that prevents the egg from breaking when dropped hastily into the host nest and might also protect the egg from being punctured by any suspicious host parents. That’s good for the parasite—it’s an advantage.

But there’s also a cost to having a thick-shelled egg. Baby cuckoos have to be able to break out of their eggs. They tend to take a relatively long time to breach the thick shells. So a thick egg shell is advantageous only to a point. There are tradeoffs.

And here’s an example on the side of the host bird. A super useful adaptation for hosts in this arms race is the ability to identify and reject parasite eggs. But it turns out that this behavior comes with a cost for many species. In their efforts to break or push an imposter egg out of the nest, hosts sometimes destroy their own eggs by accident. Or sometimes this rejection behavior sort of short circuits, resulting in a false positive. When that happens, the hosts reject one of their own eggs. Not good.  

One of the most intriguing things here, in my opinion, is that some hosts seem to have lost the arms race. Some host species are really bad at defending their nests or rejecting parasite eggs or rejecting parasite chicks. Some don’t seem to have any defenses at all. So there are acceptor hosts and rejector hosts. Some host species always accept the eggs of brood parasites while others always reject them. And we see the whole range of intermediate behaviors too.

For many decades, biologists have been trying to unravel the reasons why some hosts are acceptors. What are the factors that determine where a host lies on the spectrum from acceptor to rejector? There’s no simple answer. And the answer is probably different for any given parasite-host species pair.

One factor is probably how long the two species have been doing their coevolution dance. Many hosts of the Brown-headed Cowbird—about 100 species—tend to be acceptors. These include the Red-winged Blackbird and the Eastern Phoebe. Perhaps these host species started to be targeted by cowbirds only recently. If it’s been only a few hundred or even a few thousand years, the hosts might not have had time to genetically adapt to the threat of the cowbirds. So maybe it's not that these acceptor hosts have lost the arms race—maybe they have not yet begun to fight. This is what biologists call the Evolutionary Lag Hypothesis. 

Another explanation for why some hosts tolerate their brood parasites is offered by the Evolutionary Equilibrium Hypothesis. This poses the idea that the degree of acceptance vs rejection that we see in a host is actually the long-term outcome of coevolution with the parasite. A host will accept the parasite eggs or nestlings if the cost of rejecting them is higher than the cost of raising them. In other words, hosts evolve behaviors that minimize the cost to their lifetime reproductive success. Natural selection over many generations shapes this behavior.

A twist on this idea is that some brood parasites retaliate when their host rejects an egg. A Brown-headed Cowbird will sometimes destroy the whole clutch of her host’s eggs if she figures out that her egg was rejected. The threat of retaliation might make it in the host’s best interest to just accept the parasite egg. Biologists call this mafia behavior. A 2007 study from the University of Florida showed that, if Brown-headed Cowbird eggs are ejected from the Prothonotary Warbler host nest, the cowbirds subsequently destroyed the host nest. This has also been observed in a species of parasitic cuckoo.  

Anyway, it’s not easy for researchers to figure out whether the evolutionary lag hypothesis or the evolutionary equilibrium hypothesis best explains a given arms race between two bird species. It’s rarely possible to get the data needed to come to a solid answer. These aren’t the only hypotheses about brood parasitism and there are plenty of other ways of looking at it. Further research will hopefully shed more light on this fascinating process.

Every organism’s goal evolutionarily is to leave behind as many copies of its genes in as many offspring as possible. A great way to do that is to drop your eggs into the care of a bunch of other families. Brood parasitism seems like an excellent strategy. Obligate brood parasitism has actually evolved independently 7 times in birds. It happened three separate times within the cuckoo family, once in New World Blackbirds, once in the duck family, once with the honeyguides, and once with the indigobird family.

But remember that there are only about 100 species of obligate brood parasites on the planet. Only 1% of birds do this. Why so few? If this reproductive strategy is so awesome, shouldn’t there be many more brood parasites out there? This is still a biological mystery. There are probably tradeoffs and challenges that make being a brood parasite a lot harder than it looks.