Birds targeted by cuckoos develop sophisticated egg patterns to help them recognise the fakes.
Cuckoos infamously lay their eggs in the nests of other birds, forcing them to care for their young. But species that are repeatedly targeted by cuckoos have developed a range of complex egg patterns to help distinguish the invader's eggs and kick them out of the nest.
Cuckoos will hatch before their nest-mates and throw them out, so that the parent birds will only raise the cuckoo. When a host bird recognises a cuckoo egg, they may puncture it or throw it out the nest before it hatches.
Scientists from Harvard and Cambridge Universities developed a pattern-recognition software called NaturePatternMatch that mimics the way birds see the world. Using the extensive egg collections of the Natural History Museum at Tring, they then analysed the eggs of eight bird species commonly targeted by cuckoos.
They found that where cuckoos had developed accurate copies of host-bird eggs, the host birds had in turn developed more complex egg patterns. 'The egg patterns on cuckoo and host eggs reveal an evolutionary arms race,' said lead author Dr Mary Caswell Stoddard from Harvard University.
Samples of cuckoo's eggs in host bird clutches from the Museum's collections. Cuckoo's eggs are often bigger than their host's.
'In many cases, cuckoos have evolved excellent egg mimicry in order to trick host birds into accepting foreign eggs. In these instances, host birds have evolved excellent egg pattern signatures on their own eggs as a defence.'
The host birds studied used different techniques to set their eggs apart. Some females of the same species, such as the great reed warbler, laid very different eggs to each other, but very similar eggs within their own nest. Others, such as the brambling, lay eggs with unevenly spaced, sparse markings, making a good signature.
Collections and tools
Museum egg and nest curator Douglas Russell said he was 'fascinated by the insights [this paper] provides into the particular mechanisms that appear to be involved.' The extensive bird collections, which cover 95% of all bird species alive today, are involved in numerous studies.
'They can contribute to research as diverse as understanding migratory routes and investigating chemical contamination of the environment over time.'
The NaturePatternMatch software may also be used in a range of projects. 'How do animals recognize their neighbours, enemies and kin? How have visual signals evolved to maximize distinctiveness? Computer vision tools like NaturePatternMatch will help us answer these important evolutionary questions,' said Dr Stoddard.