Black-headed ducklings leave
their host's nest one day after hatching and fend for
themselves. Photo: B. Lyon and J. Eadie
November 29, 2004
A puzzle posed by black-headed ducks yields
to persistent biologists
By Tim Stephens
Some 100 species of birds are what scientists call "obligate
brood parasites"--instead of building nests and raising
their own young, they lay their eggs in the nests of other species
and let those birds do the hard work of parenting for them.
The black-headed duck of South America is one of these, but
it stands out from all the others in a striking way. Black-headed
ducks don't need any parental care other than incubation for
their eggs--the ducklings leave the nest one day after hatching
and paddle off into the reeds to fend for themselves.
"There doesn't seem to be much if any cost to the host
species, so you wouldn't expect there to be much pressure on
the hosts to evolve defenses against this kind of parasitism,"
said Bruce Lyon, a professor of ecology and evolutionary biology
When Lyon and John Eadie of UC Davis set out to study the
black-headed ducks, they expected to find a highly successful
brood parasite, unopposed by the antagonistic strategies that
host species deploy against more costly parasites like cuckoos
and cowbirds. Instead, they found that black-headed duck eggs
are often rejected from host nests, and it took four years of
detailed field research to figure out why. Lyon and Eadie published
their findings in the November 18 issue of the journal Nature.
The key breakthrough was their discovery that each of the
black-headed duck's two main host species--the red-gartered
coot and red-fronted coot--were busy parasitizing the nests
of their own species. The black-headed ducks were being thwarted
by defenses that had evolved as a result of brood parasitism
among the coots themselves.
The researchers carefully investigated the costs of duck parasitism
to the hosts and found few costs that could be prevented by
rejection of the duck eggs. In contrast, the intraspecific (within
the same species) brood parasitism taking place among the coots
is very costly to the hosts because they end up feeding and
raising the young of other birds at the expense of their own
Intraspecific brood parasitism in the coots also helped explain
a puzzling feature of the parasitic duck eggs. Other obligate
brood parasites lay eggs that mimic the eggs of their hosts,
but the eggs of black-headed ducks don't look anything like
coot eggs. A black-headed duck's big white egg stands out like
a sore thumb in a nest full of speckled brown coot eggs.
Apparently, the ducks have been unable to gradually evolve
egg mimicry because their hosts are already attuned to detecting
eggs laid by other birds of their own species, Lyon said. Mimicry
would clearly be an advantage, because parasitic eggs of other
coots are not rejected as often as duck eggs. But the black-headed
duck eggs differ from coot eggs in so many fundamental ways
that any incremental change in egg size or coloration would
not yield an advantage for the ducks and would not be favored
by natural selection.
The researchers confirmed this idea experimentally by adding
painted eggs to coot nests. They found that improving the degree
of mimicry had no effect on the likelihood of egg rejection.
"They're just stuck, without much wiggle room for improving
their success. There's no way for gradual evolution to bring
them into the zone of egg mimicry," Lyon said.
With different hosts, the black-headed duck's kinder, gentler
version of brood parasitism might have been a highly successful
evolutionary strategy. Instead, the species finds itself evolutionarily
stranded, caught in the cross-fire of an internecine conflict
among its hosts.
"The adaptations that have arisen in coots to deal with
the high costs of intraspecific brood parasitism have, in effect,
inoculated this system against further coevolutionary adaptations
on the part of the parasitic duck," Eadie said.
Interestingly, Lyon and Eadie's findings in South America
are very similar to the results of an experiment conducted 30
years ago by ecologist Milton Weller with American coots in
Iowa. American coots engage in brood parasitism within their
own species, but are not parasitized by other species. When
white chicken eggs were added to their nests, they rejected
some but not all of them, much like the response of the two
South American coot species to black-headed duck eggs. This
supports the idea that rejection of the black-headed duck eggs
is an incidental byproduct of the intense reproductive battles
going on within the host species.
But this raises another question: If the duck eggs are so
obvious, why aren't they all rejected by the coots? In fact,
Lyon and Eadie observed that egg rejection rates rose to 100
percent during flooding or high waves in the wetlands, when
the coots were frantically trying to build up their nests. Both
species of coots appear to be able to recognize and reject duck
eggs when they need to, but apparently do not always do so.
"It's still a mystery why the coots don't reject all
of the duck eggs. We need to know more about the cognitive processes
involved in egg recognition and rejection by the coots,"
Lyon has conducted extensive research on intraspecific brood
parasitism in American coots. He has found, for example, that
an ability to "count" the eggs in their nests plays
an important role in their rejection of parasitic eggs of their
own species. Despite his familiarity with American coots and
their parasitic habits, however, Lyon said it was not obvious
that the same thing was happening in the two South American
species. "Of all people, I should have been able to see
this," he said.
But intraspecific brood parasitism is notoriously difficult
to detect. Lyon and Eadie conducted a large-scale study of the
black-headed duck involving several different wetlands in Argentina,
observations of almost 2,000 potential host nests, and several
different experiments that involved manipulating the contents
of nests. It was only in the fourth and final year of the study
that the importance of intraspecific brood parasitism became
"For some reason it was really prevalent and noticeable
that year, and finally we had an explanation for the anomalous
results we had been finding," Lyon said.
Relatively little was known about the black-headed duck before
this study, aside from a single-year study conducted by Weller
over three decades ago. It had been thought that they exploited
a wide range of host species, but Lyon and Eadie found that
the ducks focused their parasitic efforts on the two species
of coots, only occasionally laying eggs in the nests of other
species. Thus, black-headed ducks appear to be specialist brood
parasites, relying on the two coots to eke out a living, Eadie
Obligate brood parasitism has evolved separately in five different
families of birds. The "obligate" part of the phrase
means they have no other option--they never build nests or raise
their own young, so they are obligated to parasitize others.
The black-headed duck, with its precocial young and lack of
egg mimicry, is a real oddball in this group.
Although he can only speculate about why brood parasitism
arose in the black-headed duck in the first place, Lyon said
one possibility is that the coot hosts offer good protection
of eggs from predators.
"Coots are pretty pugnacious, and their nests don't suffer
a high predation rate, so the ducks may have higher success
through parasitism despite the frequency of egg rejection,"
As for predation of the day-old ducklings after they leave
the host nest, Lyon said they must be good at hiding because
there are a lot of predators in their wetland habitat.
"We never see them after they leave the nests. We'd like
to put radio transmitters on them to find out where they go,"
The black-headed duck may be in a precarious situation considering
that adverse environmental conditions, such as flooding or high
waves, can lead to increased egg rejection by their hosts. If
environmental change, such as global warming, causes more frequent
flooding of the wetlands, the ducks could be in trouble, Lyon
Lyon and Eadie's research was funded in part by the National
Geographic Society and the British Broadcasting Company.
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