By Emma Brennand
Earth News reporter
Imitation egg patterns allow cuckoo eggs to infiltrate unsuspecting nests.
Cuckoos' egg forgery skills are increasingly being put to the test, as host birds evolve better defences, say scientists.
These brood parasites, as they are called, are master deceivers - hiding their eggs in other species' nests.
To avoid detection, cuckoos have evolved to mimic colour and pattern of their favoured host birds' eggs.
But researchers have developed "bird's-eye view" models to find out how the hosts see the intruders' copycat eggs.
If host birds do not reject cuckoo eggs, the newly hatched cuckoo chick ejects other eggs from the nest by hoisting them onto its back and dumping them over the edge.
This study revealed details about the "evolutionary arms race" in which cuckoos are embroiled; as they evolve better mimicry, their hosts evolve the skills to spot these damaging intruders.
Mary Caswell Stoddard and Martin Stevens from the Department of Zoology at the University of Cambridge, UK, published their findings in the journal Evolution.
Previous egg pattern research has focused on assessing differences between colour and markings based on human visual inspection."But birds have better colour vision than humans do," Ms Stoddard told BBC News.
"Birds have four [colour-sensitive cells] known as cones in their retinas, while humans only have three."
Eggs laid by the various cuckoos (top row) and their hosts (bottom row)
"This additional cone in birds is sensitive to ultraviolet wavelengths [of light]. As a result, birds can see a wider range of colours than humans can."
The team used a technique called spectroscopy to measure the amount of light reflected from the different coloured egg shells.
They modelled these colour values to work out how the egg patterns appeared from a bird's perspective.
Cuckoos have target hosts. For example, a cuckoo that lays eggs in a redstart nest lays a blue egg. To the human eye, this is identical to the redstart egg.
However, the cuckoo that targets a dunnock nest lays a white egg with brown speckling, visibly different from the dunnock's immaculately blue egg. Yet despite this obvious colour mismatch, dunnocks readily accept the foreign eggs, whereas redstarts are much more likely to eject the cuckoo's egg.
Other nest invaders
The American coot is a brood parasite and invades nests within its own species
A recent study by Dr Daizaburo Shizuka, University of California, found that egg recognition is based on visual characteristics with over 40% of 'foreign' eggs being rejected.
Rejected eggs are often buried in the nest lining or abandoned at the edge of the nest delaying their hatching rate.
To investigate this optical conundrum, the team used their technique to study cuckoo and host bird eggs from 248 invaded nests held in the Natural History Museum in Tring, Hertfordshire.
They found that redstarts and their invading cuckoos' eggs had a higher degree of colour mimicry, so the cuckoo egg was a good copy.
The scientists think the cuckoos have been forced to evolve this high degree of mimicry because redstarts are better at identifying these alien eggs.
Even seen with this bird's-eye view, the cuckoos that targeted dunnocks' nests showed no colour overlap, so the forgeries were poor replicas.
The fact that the dunnock usually accepted these forgeries suggested that it lacked the defensive skills the redstart had evolved.
Exactly why many hosts accept such obviously alien eggs continues to baffle biologists.
Researchers think that naive hosts, like the dunnock, are still at early stages of the evolutionary arms race and, "they accept alien eggs, because they have not yet evolved defences against parasitism," explains Ms Stoddard.
"Another hypothesis is that tolerating cuckoo eggs may be the most stable strategy for some hosts."
So, for birds that do not often suffer cuckoo invasions, the overall "cost" of mistakenly ejecting their own eggs might be higher than the cost of tolerating the occasional parasite.