April 7, 2003
Coots can count: Study shows
nesting behavior in common marsh birds
By Tim Stephens
Coots, the Rodney Dangerfields of the bird world, just might start to
get some respect as a result of a new study showing that these common
marsh birds are able to recognize and count their own eggs, even in
the presence of eggs laid by other birds.
An American coot (Fulica americana) swims in a
marsh in British Columbia. Photo by Bruce
The coot nest above has been "parasitized." It contains
two eggs (darker than the others) laid by an interloper hoping
to have two of her chicks raised by another female. Coot eggs,
below, vary considerably from one female to another in color and
speckle pattern. Females use these visual cues to distinguish
parasitic eggs from their own. Photos by Bruce Lyon.
The counting ability of female coots is part of a sophisticated set
of defense mechanisms used to thwart other coots who lay
eggs in their
neighbors' nests, according to Bruce Lyon, an assistant professor of
ecology and evolutionary biology. Lyon studied hundreds of coot nests
in British Columbia during a four-year investigation. His
appear in the April 3 issue of the scientific journal Nature.
"The ability of females to count only their own eggs
in a mixture
of eggs is a remarkable feat that provides a convincing, rare example
of counting in a wild animal," Lyon wrote in the
A member of the rail family, the American coot (Fulica americana)
is a slate-gray bird with a white beak, about the size of a small duck.
It inhabits lakes, ponds, and marshes, often in large numbers. Coots
are ungainly on land, reluctant to fly, and not very impressive in the
water, either. Use of the word coot to mean a stupid person or simpleton
reflects prevailing attitudes toward these rather comical birds. But
this perception is belied by Lyon's discovery of their impressive cognitive
"I was shocked. At first, I didn't believe the results,"
Lyon originally set out to study how coot parents care for
But the focus of his research changed when he discovered
levels of "brood parasitism," the practice of laying eggs
in other birds' nests. Most studies of brood parasitism have looked
at birds like cuckoos, which lay their eggs in other species' nests
and thereby avoid the trouble of raising their own chicks. But brood
parasitism also occurs within species, and Lyon's research on coots
is shedding new light on this little-studied phenomenon.
Lyon and his field assistants tracked the fate of every egg in more
than 400 coot nests, checking the nests every day. Because
no bird can
lay more than one egg per day, parasitism was readily
detected. In addition,
coot eggs laid by different parents can often be distinguished on the
basis of color and speckle pattern.
"The egg pattern is like a fingerprint," Lyon
turns out that in most cases, the parasitism is done by females who
have their own nests, and often it's the next-door
neighbor, so by comparing
the eggs in nearby nests we can tell who's doing it."
Parasitism was rampant, affecting 41 percent of the nests
for 13 percent of all the eggs laid in the study population.
Furthermore, the cost of parasitism to the hosts was quite
"There's not enough food for all the eggs that get
laid, so there
is a massive die-off of chicks. Typically about half the chicks in a
nest starve to death," Lyon said. "That explains both the
cost of parasitism to the host and the benefit to the
Female coots, Lyon found, are quite good at recognizing
parasitic eggs. Rejected eggs were buried deep in the
and never hatched. In some cases, parasitic eggs were not
but were banished to inferior incubation positions,
resulting in delayed
hatching and decreased likelihood of survival for the
"They may use that strategy to deal with eggs they
are less certain
of," Lyon said. "It's subtle, but it has an
effect on reproductive
success, because the ones that hatch later are more likely
Evidence that coots are able to count their eggs was
Lyon wanted to look at how the presence of parasitic eggs
size. Coots are indeterminate layers, meaning that they'll
eggs until an external cue tells them they're done.
Speculation about the nature of the cue has long favored the sense
of touch--when the female feels like she's sitting on the
of eggs, the development of new egg follicles stops. (Follicles that
have already started developing continue to develop, so
there is a delay
between the cue and the cessation of egg laying.)
If the birds use a touch cue, the presence of parasitic eggs should
cause a female to lay fewer of her own eggs. But Lyon found that when
female coots recognized the parasitic eggs in their nests, eventually
rejecting them, they did not reduce their clutch size. In contrast,
females that failed to recognize parasitic eggs laid one
fewer of their
own eggs for each parasitic egg they received.
"Rejection takes a long time, so the clutch-size
decision is made
while the parasitic eggs are still in the nest," Lyon
means they are not using a touch cue. These birds are
looking at their
nests and counting only those eggs they recognize as their
own to make
a clutch-size decision."
Lyon noted that the meaning of "counting" in animals is a
highly contentious issue among cognitive psychologists and
experts. What he means by counting is that the birds are
based on the number of eggs in their nests.
"That's pretty amazing for a 'stupid' bird like a coot,"
Lyon said. "It's very satisfying to rescue a study animal from
a bad rap."
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