Reader Bruce Lyon, a professor of ecology and evolution at the University of Santa Cruz, has sent us another great photo-and-science post on coots, a wonderful and bizarre bird. I have to say that the phenomenon of brood parasitism, described below, is fascinating, and a great evolutionary tale.
Bruce’s notes are indented:
Another installment of the American coot [Fulica americana] soap opera from studies my students and I have done on several wetlands in central British Columbia (previous posts here, here, and here). Soap operas are often spiced up with cheats and villains, and coots are no exception. When I went to BC in 1987 to start my PhD field research, I planned to study the various aspects of family life I described in previous posts. However, within a couple of weeks of starting the field work, systematic nest checks revealed fascinating cheaters in the system and I made a snap decision to focus on that aspect for my PhD research. Female coots were laying eggs in each other’s’ nests with wild abandon, and it was clear that coots provided a great study system for investigating these reproductive cheats. Many readers may be familiar with brood parasitism across species—like the cuckoos and cowbirds that lay their eggs in the nests of other species. There are actually more species of within-species parasites, but this form of brood parasitism is less well studied, partly because it is harder to detect.
When within-species parasitism was first observed, in ducks, it was called ‘dump nesting’. It was thought to be maladaptive (‘a breakdown of maternal instinct’), perhaps because it sometimes occurred at such high frequencies that it caused lots of nest desertion and reduced hatching success. At the time, there was also no conceptual framework for making adaptive sense of the behavior, but a few key papers in the 1980’s changed that by considering the behavior in an adaptive framework; and I stumbled into the phenomenon at just the right time.
Below: A photo of a coot nest with two eggs laid by a second female coot (the parasite eggs are the darker ones).
I discovered the brood parasitism by checking nests daily and marking all new eggs. No bird in the world can lay more than one egg per day, so when nests are checked daily and there is more than one new egg since yesterday’s check it is clear that more than one female has laid eggs in the nest. Since we mark eggs as they are laid, all eggs without numbers during a visit are new ones since the last check. More than one new egg = brood parasitism! Parasitism was very common—almost half the nests were parasitized.
Below: An important first step was to make sure that these intriguing nests were not examples of cooperative group living coots—in several bird species multiple females share a territory and all lay eggs in the same nest. By marking coots with neck collars, it became clear that all territories had only one male and female, so I had discovered brood parasitism rather than cooperative breeding. Here is a photo of a territorial pair, sans collar.
Below: Coot eggs vary a lot in color and markings, as this photo of eggs from several females shows. Eggs from different females differ in shape, background color and the pattern and color of the speckling. I find these eggs quite beautiful.
Below: Eggs from five females; each column has eggs from the same female. Note that eggs vary among females in features like shape, color and speckling pattern, but a given female lays very consistent eggs. This makes it possible match eggs laid by a given female.
Below: Once a parasitized nest is detected by the unusual laying rate, we then identify which of the new eggs are the parasite’s and which are the host’s. Coots lay an egg every day, so when checking nests daily there should be at most one new host egg. Usually, one new egg looks just like the rest in the clutch (the new host egg) and the other new one(s) look different and are the new parasite egg(s). The photo below shows all of the eggs in one nest (7 host, 2 parasites) and illustrates that sometimes parasites differ strikingly from the host eggs (parasite egg P2), while at other times the difference is much more subtle (egg P1; speckles differ). Clearly two different parasites laid in this nest. A small-scale genetic study confirmed that our egg assignments were very accurate.
Below. We can also use egg features to figure out which females laid the parasitic eggs. We temporarily borrow parasitic eggs to compare them with eggs in other nests in the population. If a parasitic egg matches the owners’ own eggs in another nest it means we have found the parasite’s own nest. Most parasitism is in fact by nesting females, and these females typically parasitize a next-door neighbor. Below an intrepid field assistant risks an icy dunking (water over the top of the waders) to compare parasitic eggs (in the egg carton) to those in some nests in deep water.
Below: Nesting parasites typically laid their parasitic eggs before continuing to lay in their own nest—without skipping any days of laying. Brood parasitism by nesting females is particularly interesting because these females have their own nests: why not just lay the eggs in their own nest? Brood parasitism becomes a clutch-size problem, and I explored it in the context of David Lack’s clutch-size hypothesis. In coots, eggs are cheap but babies are expensive (because food for chicks limits family size), so most females can produce far more eggs than they can raise in their own nest. Parasitism allows them to produce extra eggs that can be fobbed off on the neighbors. Food is also limiting at the host nests, but if the parasites can lay the eggs early enough in the host’s laying cycle, they have a good chance of surviving—and it will be a host chick that starves instead. It’s basically a timing game—the early egg gets the worm.
Below: There is a second flavor of brood parasite—females without nests or territories in a given year. About a quarter of the parasitic eggs could not be matched to any nesting female in the population, and I concluded that these eggs were laid by non-nesting females. These non-breeders are apparent on some wetlands where little gangs hang out in areas that lack vegetation and therefore are not suitable as nesting habitat (as photo below shows). At our wetlands, nesting densities are very high and occupy all suitable breeding habitat, so not everybody gets to have a territory. It seems that non-nesting parasites resort to parasitism as an alternative to not reproducing at all. Richard Dawkins coined the term ‘making the best of a bad job’ to explain this type of reproductive tactic—these parasites would establish a nest if given the chance but parasitism is better than nothing.
Below: Parasitism is good for the parasite but bad for the hosts. Given the chick starvation at most nests, raising somebody else’s chicks is very costly—each successful parasite chick costs the hosts one of their own chicks. Coots have evolved some clever mechanisms to reduce the costs of parasitism. Many coots recognize and reject parasitic eggs from their nest by burying the eggs down in the nest material (where the leeches get them!). About a third of parasitic eggs are rejected. The photo below shows two eggs in the process of being rejected (the pale ones at the edge).
Whether parasite eggs are rejected depends, in part, on how similar they are to the host’s eggs in background color. I suspect that speckling pattern may also be important but I have not yet shown this. Coots also know what their own eggs look like and are not just using the simple rule of getting rid of the rare egg type that differs from the majority. When I swapped eggs among nests to make the host’s own eggs the rare type in their nest, they were not fooled and still knew their own eggs.
Within-species brood parasitism has been observed in quite a diversity of birds; I show a couple of examples below.
Below: Parasitism is common cliff swallows (Petrochelidon pyrrhonota), a colonial nesting songbird. Remarkably, a parasite swallow sometimes lays a parasitic egg in her own nest and then transfers it to a host nest in its beak! (study by Charles and Mary Brown). In the photo below (not far from my house) I wondered if the bird in flight was a brood parasite. It kept entering different nests and getting chased away by the nest owners.
Below: Brood parasitism is very common in waterfowl, including these Barrow’s goldeneyes (Bucephala islandica) at my BC site. In ducks, unlike most other birds, females return to their birth site. This can lead to cases where hosts and parasites are related, and some have suggested that kin selection may play a role in favoring brood parasitism in some species. This aspect has been studied in some detail in goldeneyes.
Below: Parasitism has also been particularly well studied in the common moorhen (Gallinula chloropus) a relative of the coots in Europe. I never observed coots in the act of parasitism but Sue McRae captured several brood parasitism events by moorhens on video camera. To lay an egg, the parasite female climbs onto the nest, usually occupied by the sitting host male, sidles up beside the male and squeezes out an egg, all while the male pecks her continuously.
I bit confused on a few points.
They can recognize and reject a parasite egg but if that egg hatches do they not recognize the chick as a parasite?
Is it only the nest owner who feeds the chicks once they hatch?
Thanks, I always enjoy the science posts even when they’re over my head.
I was deliberately confusing to create interest in the next post, which will be about whether or not hosts can recognize parasitic chick. Stay tuned.
Yes, only the nest owner feeds the chicks once they hatch (with very rare exception…. again, to be continued with the nest post.
Fascinating research. Such behavior is very puzzling from a adaptation point of view. If kin selection is important in ducks, what drives the behavior in other species?
Good stuff.
What drives the behavior is natural selection that rewards individual fitness. Parasitism allows individual females to have more total babies (gene copies in the next generation) than they would otherwise have so parasitism is favored by natural selection.
Makes sense.
But what should happen then is the genes for parasitism would spread throughout the population until every female laid eggs in another nest and would end up raising others chicks. At that point, the behavior would loose its benefit. All females would have farmed-out eggs and all females would raise other’s chicks. Equilibrium.
This is exactly right and I would have liked to make this point in the story had there been room. This is a nice example where game theory (equilibrium) makes sense. Sometimes people then ask why parasitism persists and the answer is simple: a genetic mutant that stops laying parasitic eggs has lower fitness because it still pays the price of raising parasitic chicks of others but gets none of the benefits from parasitism.
Ha! Sounds like the ultimate paradox. A strategic contradiction that cannot be undone. There must be a name for a situation like this. It’s like the tragedy of the commons, but not quite. Maybe an analogy in human experience is the tendency of drivers to purchase larger and larger cars to provide safety against the danger of all those big cars on the roads these days. Or the need to purchase guns to defend against all those people out there who are purchasing guns.
There is a name for this: Evolutionarily Stable Strategy (ESS). The key aspect is that fitness depends on what others are doing in the population. And what I found is implied by this body of theory. What is stable is not always optimal.
Just a quick guess that there are many ESS conditions including among humans.
Parasitism must be common in birds that can lay many more eggs that the parent(s) can raise. It must be a strong source of selection in favor remembering how many eggs were laid and the ability to count them. It is also another good reason to defend a territory.
Really interesting!
Love field science!
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Wow. Great pictures and an amazing study. Thanks!
Great post! Thanks for sharing such a riveting study.
The speckle patterns on those eggs is remarkable. I wonder what physical mechanism develops the pattern.
I cover a bit of this when I teach ornithology and my knowledge is only as deep as the brief treatment in the textbook I use(Gill’s Ornithology). There are shell glands in the bird’s oviduct that exude pigment towards the end of egg formation. Whether the pigment ends up as spots or as streaks and scrawls apparently depends on whether and how the egg moves during pigment deposition.
Fascinating! A question I have is whether all females show a tendency to parasitize, or is it just some females.
If it is a behavior seen in only certain females, the next question would be if the trait was inheritable.
Excellent question. My sense is with coots that the behavior is fairly plastic and that most females will be parasites if the right conditions come along. Age (older) was the best predictor of whether nesting females laid parasitically and these females had higher overall fecundity. However, it is also possible that there is some heritable variation in the the propensity to be parasitic. My colleague John Eadie (UC Davis) and I just submitted a grant proposal to try to answer this question in wood ducks.
My question is what kind of selection pressure drives the egg patterns. Distinctive patterns benefit the owner and hurt the parasite, but the roles of owner and parasite are switched early in the season. At what level would selection operate to control the degree of differentiation of the eggs?
Of course there are other forces driving the egg color, such as blending in with the nest and escaping predators who use stereotyped search images…
Not everything is adaptive, but when they are, studies like Bruce Lyon’s will inform us.
You have picked up on something really interesting and subtle. Most people assume that the variation in egg features among females is an egg signature that has been produced by natural selection but I am not so sure. My sense is that most birds can be parasitic and they can also be hosts so ON AVERAGE fitness gains from parasitism get cancelled out by fitness losses as a host. What does make sense as an adaptation to defend against parasitism is for a female to lay very consistent eggs — this benefits here as a host but has no impact on her success as a parasite. Many birds show considerable variation among females in egg patterns and some of this probably reflects non adaptive varition.
Fascinating post. I enjoyed it very much. Thank you.
As any good research it raises a lot more questions than it answers, at least in the initial stages.
Many questions were already asked above, but did I get this right: American coots do intra-species brood parasitism, while European coots do not (at least not yet known), but European moorhens do? Are there American moorhens and do they practice parasitism?
And a different note, the whole idea of the relationship of brood parasitism and kin selection makes things even more confusing and fascinating.
A fantastically mind boggling post!
I think there is now some evidence that European coots show parasitism but it has not really been studied. The North American version of the moorhen, the common gallinule, used to be considered the same species but it was split, fairly recently. Sue McRae, who studies moorhens, also studied a tropical population of gallinules in Panamaa. They are parasitic like their European relatives but there is a really interesting difference: moorhens do not recognize and reject parasitic eggs but the gallinules do.
Nifty, and field biology always looks so … involved!
So how does brood parasitism get started? Are there any examples of this happening in an observable time scale, say as a result of habitat destruction, environmental change or the appearance of new hosts? I ask this as I read Nick Davies “Cuckoo” and I didn’t really get an answer to this question.
We don’t really have any idea but in terms of time frame, this form of parasitism occurs in all of the early lineages of birds (ostrich, waterfowl, chicken type birds) so it is conceivable that the dinosaurs with parental care also had this behavior. What makes this seem likely is that most extant groups of animals with parental care also have forms of brood parasitism (insects, fish, poison frogs and tons and tons of insects).
In terms of how this first originated in birds in terms of evolutionary steps, we can only guess. My guess is that it probably first arose in contexts where some birds could not breed because of limited territories. Some birds probably get the egg laying machinery up and running (it is off for the winter) before they know whether or not they will be able to get a territory. If these birds have eggs ready to lay but nowhere to lay them then it is not too complicated a step to then find a nest to lay them in,
Thank you, the question of who was the cuckoo X has always fascinated me. Could you run another way with the question, do you think it would be possible to induce a behaviour like parasitism in an experimental situation with populations that hadn’t previously displayed this behaviour?
This has been done but in species that already show some brood parasitism. One explanation for why females lay parasitically is that they loose there own nest in the middle of egg laying and have eggs in the chute ready to lay but nowhere to lay them. Laying those physiologically committed eggs in somebody else’s nest would provide an alternative to wasting those eggs. It turns out that nest loss is not an important explanation in most species (it is important in moorhens) but several studies have been able to force specific females to engage in parasitism but removing their nests part way through laying.
This stuff is fascinating!
Engaging post thanks for posting i find it fascinating this behaviour is very ancient. Imagining doing field studies on cheating dinosaurs…
Some good Q & A and i will wait for your next for more answers.
Excellent post enhanced by Dr. Lyon’s helpful explanations in the comments. Thank you.
Yes, always nice to have the further discussion in the comments once the original post has sparked so many questions. A few of the ones I had have already been addressed in comments above. Thanks, Bruce!i Fascinating stuff!
Thank you for such a fascinating presentation. I had heard that one of the blue wren species could, at least in the Australian National Botanic Garden in Canberra, detect interspecific parasitism, chuck all the eggs out of the nest, including its own, and start again.
We have a number of rails in Australia including the dusky moorhen (Gallinula sp) and the purple fronted swamp hen. Is it known if these species indulge in intraspecific parasitism?
I know the people who do the superb fairy wren work in the Canberra garden. I do not recall the egg rejecting behavior but they did find the the wrens could recognize and reject cuckoo chicks, the first example for cuckoos.
I do not know if the rails show brood parasitism. I would be willing to bet that they do but I do not think anybody has studied it.
Fascinating.