Today’s post continues the Story of the American Coot (Fulica americana) , a tale in three parts with a lot of cool science. Story and photos are by Bruce Lyon, a biologist at the University of California at Santa Cruz. Part I was posted six days ago, this is Part II, and we’ll have Part III soon. I’m fascinated by these birds, and used to watch them at the pond on the campus of the University of California at Davis. They have weird feet and the most bizarre-looking chicks of any North American bird I know. Bruce’s notes are indented:
Coot Soap Opera, Part II
At first blush, American coot family life seems downright crazy, but with closer inspection it mostly makes sense. In the previous post on coots. I mentioned that coots forgot to read the textbooks on how territorial fights are supposed to work. In contrast, though, coots are textbook experts in some aspects of family life. Specifically, the species provides a really nice illustration of David Lack’s ideas on clutch size evolution in birds.
Lack, director of the Edward Grey Institute of Ornithology at Oxford from 1945-73, suggested influential ideas about the evolution of clutch size. These ideas were couched in terms of individual selection—why a female benefits from laying a particular clutch size—and they played a role in the big debate at the time over whether natural selection acted on the level of individuals or on groups/populations. Individual-level selection won the day.
To study coot family life, we had to find coot nests and follow the fates of each egg from laying, through to hatching and then independence from parents at the end of the breeding season.
Below: A student volunteer checks a nest to see how many eggs are present and, with a Sharpie marker, she labels any new eggs. Coots lay an egg a day and in some years we checked nests daily so that we could know the exact laying order of each egg in a nest.
Below: a nest showing eggs with their individual numbers.
Below: For some research questions we needed know which egg each chick hatched from, so I used a technique called pip-marking, long used by waterfowl biologists. When an egg starts to hatch, the chick pokes a little hole near the top of the egg with its beak. Once that happens, the exact position of the chick’s foot inside the egg is known because chicks always orient their bodies in the same way. Then a small hole in the shell can be made at the foot location and a toe can be extracted. Duck biologists attach a tiny numbered metal tag to the duckling’s web; coots lack webbed feet so I just dulled a toe nail slightly by clipping the tip; and after hatch I searched for the chick with the dull toenail. I do not have a good photo of this technique for coots, but here is a photo showing a goldeneye duckling being pip-marked in the egg with a web-tag.
Coots lay quite large clutches (average 9 eggs, range 4-15 eggs). Most of these eggs hatch, but only about 50% of the hatchlings survive in a population. At the family level, some parents raise almost all of their chicks while others raise only one or two. The general term for this chick mortality is ‘brood reduction’.
Below: When chicks are young they have to be fed by their parents or they starve. This parent offers a chick the tiniest morsel.
Why do so many coot chicks die? I suspected that starvation was the culprit, for during brood observations some chicks were rarely or even never fed by their parents. I carefully surveyed several focal territories daily to see if I could find any bodies of the chicks that were disappearing and it did not take long to confirm starvation. The territories were littered with carcasses of dead chicks and the chicks were all underweight based on predictions from their leg size.
Below: The ugly face of starvation— a sample of some of the chicks I found dead on their parents’ breeding territories.
Whether a coot chick lives or dies is strongly linked to hatch order. Coots show extreme ‘hatching asynchrony’, whereby hatching is very staggered. Parents achieve this hatch pattern by starting to incubate the eggs well before all eggs in the clutch have been laid, which causes the earlest-laid eggs to hatch before later-laid eggs. The average hatching spread from the first-hatched to last-hatched chick in a brood is five days, but in extreme cases the age difference can be eight or nine days. It is the younger, later-hatched chicks that tend to starve in a brood, and the relation between risk of starvation and hatching order is very strong.
Below: I don’t have a decent photos of hatching asynchrony in coots, but the photo below of short-eared owls, a ground nesting owl, shows the pattern nicely (photo taken near Kinston Ontario). Note the size difference among the chicks—and one egg is yet to hatch! Many of these owlets had already left the nest and were hiding nearby in the grass so they had to be rounded up for their family portrait.
All of the death and destruction in coot families seems wasteful and unnecessary—why lay so many eggs that cannot be raised? Once again it was David Lack who provided key insights on brood reduction. Building on his idea that clutch size is determined by the amount of food parents can provide to their family, Lack realized that this food supply may not always be predictable when the eggs are laid. A solution would be to lay an optimistic clutch size for the best possible conditions and then, if needed, let brood reduction align the family size with the actual food supply. [For Monty Python fans, note the resemblance here to the The Meaning of Life skit on Catholic family size: life is unpredictable, Dad suddenly loses his job at the mill, and the huge family of kids must then be sold off for scientific experiments.]
Below: Two fortunate full-grown coot chicks that made it through the childhood gauntlet. Note that the chicks lack the ‘frontal shield’ of their parent (middle bird)—the extension of the beak up into the forehead area. In some rails, the frontal shield is a badge of status that indicates fighting ability—birds with bigger shields are socially dominant. This may also apply to coots.
Coots seem to fit Lack’s brood reduction idea pretty well. That some parents raise one chick while others raise ten chicks certainly suggests that food supply for the kids varies among families. And the observation that the birds with small families still tend to lay nine or ten eggs suggests the coots are not very good at predicting food supply for chicks when they lay their eggs. I can think of a couple of possible explanations for why coot parents cannot predict food supply, but these are pure speculation. First, family size correlates with territory size, so perhaps coots cannot count on their territory borders remaining constant through the season. Second, my study of marked coots (neck collars) revealed an unusual pattern for birds—almost none of the adults I tagged ever returned to breed on the same wetlands the following year. Coots clearly live for more than one year, so this means that they must breed in different locations each year. Changing breeding locations would mean that coots would not have prior information on how productive a wetland is in terms of food supply for chicks.
Below: Random fact—despite high rates of chick starvation, coot families still spend considerable time loafing about and preening. Kids schmids—a parent needs a little down time.
JAC: LOOK AT THOSE CHICKS!!
Back to the hatching asynchrony. David Lack proposed that the primary function of hatching asynchrony is to facilitate brood reduction; if food is limiting, the size differences among chicks create competitive differences that permit rapid, efficient culling of surplus chicks. The fact the chick survival is so strongly linked to hatch order in coots is consistent with this idea, and I was long convinced that this was the function of hatching asynchrony. However, when my student Dai and I experimentally created synchronously hatching broods, we found that brood reduction was just as efficiently at synchronous broods. Hatching asynchrony must have some other explanation (there are something like 16 hypotheses for hatching asynchrony, so there are lots of possibilities to consider).
TO BE CONTINUED…..
Thanks for this.
I’m loving this series of posts on coots. I’m look forward to Part III.
Very fascinating-
Thanks so much!
+1!
+1
+ 3!
Once again, a great and interesting post. I too look forward to part III.
Coots are so interesting! Who knew?
“Dad suddenly loses his job at the mill, and the huge family of kids must then be sold off for scientific experiments.”
https://www.youtube.com/watch?v=AsWxkU0g9Z4
This is utterly fascinating. Thank you, Bruce, and thank you, Jerry, for sharing this with us. I was completely unaware such a creature existed before now, which makes their story that much more engaging.
Woonderful!
Matthew 6:26-27
Look at the birds of the air, for they neither sow nor reap nor gather into barns; yet your heavenly Father feeds them. Are you not of more value than they? Which of you by worrying can add one cubit to his stature?
Oops… Apparently does not apply to some baby coots.
Psalm 147:9
He giveth to the beast his food, and to the young ravens which cry.
Oops … most young ravens starve.
I am impressed with toe marking technique.
Awesome photos and information. This is a great series!
What’s the metric of efficiency here?
Suppose we have two broods of the same size, let’s say eight chicks, one synchronous and one asynchronous. Suppose further that the optimal brood size is six for both families. Even if both families succeed in reducing their broods to that size, it’s still likely that the synchronous family has expended more resources on chicks that got culled, because they died, on average, at an older age than the culled asynchronous chicks.
Great question. We tried to look at various measures, including feeding rates. We have not written the paper up yet but I think there might have been some subtle patterns in terms of wasted parental effort. So yes, I probably made too strong a claim in my post.
Great post! I wonder if nest predation could also be a factor in hatching asynchrony. Because synchronous hatching is achieved by delaying the start of incubation until the full clutch is laid the effect is to extend the period of time between the laying of the the first egg and the fledging of the first chick and hence the period of time over which clutch and brood are at risk of predation. By beginning incubation more or less immediately after the first egg is raised the parent reduces the time until at least one chick fledges. If predators are likely to remove the entire contents of the nest this could be a significant factor perhaps?
Yes, this seems likely and a colleague of mine who did an independent study on hatching asynchrony in coots concluded that this is probably one of several factors that explains the asynchrony. We have evidence that hatching asynchrony also works as a defense against brood parasitic coots (females who lay eggs in other coot nests). There are two ways that hatching asynchrony is beneficial to hosts who have brood parasites in their nests. Brood parasitism will be the topic of another post, perhaps in a month or so.
Thank you for these posts. My students will enjoy learning about these studies in my animal behavior course next spring.
My website publication page has pdfs available for all of the papers on this work, if that would be helpful to provide more details.
Yes, thank you. I’ve been reading some pubs I missed. The PDF links are very helpful. And these posts are providing valuable perspective.
Too many questions
Why does the American coot not return to the same breeding locale? What about the ones who don’t migrate [Pacific coast, SW USA & Mexico] – do they stick to the same patch year on year or select new local breeding grounds each year?
From reading Wiki I note that the breeding season is only a small part of the year so for the rest of the season do coots wander willy nilly from feeding ground to ground without territories? territories are only for the breeding season? Also I noted the females will ‘parasitise’ other occupied coot nests.
Conflict seems to be the coot watchword! How do you get near the eggs? 🙂
Good questions: I can only speculate as to why birds do not return. It might be that in some parts of the range wetlands are ephemeral and it is better to stop at the first sure thing rather than wait and find out that last years wetland is dry. That actually happened at my site and like the coots, I had to change study wetlands because my 1987 ponds dried up and most had no coots.
Yes I suspect that permanent residents remain on the same wetland and even many migratory populations may be faithful to site.
I have no information on what birds do in winter but there do seem to be stable flocks in many places.
I studied the brood parasitism for my PhD and in a few weeks will prepare a couple of posts. Saving the best for last! And yes, these guys are all about conflict!
We get to the nests with waders—fortunately most of our nests are in shallow enough water that we can walk up to them. Sometimes it is touch and go and we get a wader full of water, which can be bracing to say the least in early spring when the ice has only recently melted.
Thanks for the wonderful reports Bruce, and these great comments. It sounds like a fascinating way to make a living actually. I’d gladly take a wader full of ice water for the chance to work on these fascinating creatures.
Very interesting. Reminds me of Shoebills. They will lay 1-3 eggs per season but they only ever raise one chick – they starve or sometimes outright kill the younger chicks. Evidently the younger chicks serve the purpose of ‘backups’. If the oldest chick dies quickly, the Shoebill will raise the younger chick. But if everything goes right with the older chick, the Shoebill kills the backups.
Thus the expression – “Mom always liked you best!” 8-(
Funnily enough, if you Google “mom always liked you best” the first few hits are descriptions of our coot study (coot chick colors).
Hmmm…that’s interesting. I get the Smother’s Brothers quote. I suspect Google is keeping a folder on each of our “preferences”. 😎
Yeah Barry – it’s Smothers Bros. all the way down for us normal geezers!
Mr. Google [& thus NSA, GCHQ etc.] know our search propensities backwards & forwards. As a cootologist you really should be using DuckDuckGo for searches 🙂
https://duckduckgo.com/
It claims to not mess with your privacy & it may well be spooky agency proof. It’s pretty good.
Scary. I did not realize how much Google had become Big Brother.
Interesting theory of brood reduction. Coot question: Wouldn’t clutch size also be determined by food availability for females that need the protein to be able to lay 4-15 eggs? If food was scarce, I could imagine a clutch of say < 8 eggs rather than around 13 and let infanticide take over. As you said, many factors. One might be that protein sources (insect larvae hatches, minnows, tadpoles, etc.) are themselves temporally staggered throughout the laying season and restrict a female's ability to feed herself, grow eggs, feed young, and the young to feed themselves except in batches. Maybe an insect hatch allows egg laying but is over when fledglings have hatched, leading to increased chick starvation and poor predictability of food supply as you say.
Perhaps for a few females. For most females eggs seem very cheap (I have had several females lay 20-30 eggs in a continuous sequence when forced to renest several times. Also, the parents and kids eat different things. Parents are mostly vegetarian and seem to have lots of food for themselves. All of the evidence I have suggests that for most females, eggs are cheap and babies are expensive.
Cool, thanks!
Lots of info here. Looking forward to your next post.