London: Using field experiments in Africa
and a new computer model that gives them a bird`s eye view of
the world, Cambridge scientists have discovered how a bird
decides whether or not a cuckoo has laid an egg in its nest.
Only seven groups of birds in the world have evolved
as brood parasites, laying their eggs in other birds` nests,
and ecologists have long been fascinated by this behaviour as
an example of evolution in action.
Dr Claire Spottiswoode and Dr Martin Stevens of the
University of Cambridge worked on two tropical African
species, the parasitic Cuckoo Finch and one of its hosts, the
Until recently, most work on cuckoos has been done in
temperate regions - Europe and North America - where species
are relatively young in evolutionary terms.
In the tropics, however, the Cuckoo Finch and Prinia
could have been locked together in an evolutionary arms race
for up to 20 million years.
As parasites have evolved ever better manipulation of
their hosts, hosts have responded with ever more refined
defences to evade parasitism.
As a result, the Cuckoo Finch`s mimicry of host eggs
is extraordinary, as is the Prinias` ability to spot the
parasite`s eggs According to Dr Spottiswoode: "Prinias lay
probably the most diverse range of eggs of any bird in the
world, and this is likely to be an outcome of the long
co-evolutionary battle with the Cuckoo Finch."
"The eggs are analogous to a bank note, in terms of
the variety and complexity of markings, perhaps to make them
very hard to forge by the parasite."
To find out exactly how Prinias detect the foreign
eggs, Spottiswoode and Stevens set up more than 100 rejection
experiments in southern Zambia, putting one Prinia egg into
another`s nest and waiting to see if the egg was rejected.
They also collected data to feed into a computer model
to give them a bird`s eye view of the world, using a
spectrophotometer to measure egg colours and a digital camera
to analyse the eggs` complex patterns.
The finding offers unique insights into a 20
million-year-old evolutionary arms race, a University of
Cambridge release said.
In the past, this kind of analysis was tackled by
humans comparing eggs by eye, but human vision differs hugely
from that of a bird.
Birds can see ultraviolet light and because they have
four types of cone in their eyes, compared with three in
humans, they see a greater diversity of colour and pattern.
Spottiswoode and Stevens found that Prinias are
amazingly good at rejecting foreign eggs, and that they use
colour and several aspects of pattern to spot the parasite`s
Mysteriously, however, they do not seem to use the
scribbles that uniquely occur only on the Prinias` eggs.
The specific traits used to distinguish foreign eggs
were exactly those found to differ most between host eggs and
real parasitic eggs.
This suggests that natural selection is currently
acting to make Cuckoo Finch eggs better mimics of their
hosts`, and also that Prinias use the most reliable
information available in making rejection decisions.