Birds that lay eggs in other bird's nests and trick them to raise their young display a behaviour which has fascinated biologists for many years. Once hatched, a intruder chick (L) will quickly grow and out-compete the host sibling (R) causing its death.
Soon the nest contains just one giant chick that has cheated its way into the family. Pictures released show how researchers are trying to understand how birds prevent this and detect 'egg cheats' in the nest.
The cuckoo finch (male top, female bottom) lives in tropical Africa and tricks warbler-like birds called tawny-flanked prinias and cisticolas to raise its young by laying eggs that mimic their own.
These are all eggs from tawny-flanked prinias, the commonest host species for the cuckoo finch. The extraordinary variation in appearance makes it hard for the cuckoo finch to mimic the eggs well enough to avoid being detected and rejected from the nest.
The researchers from the University of Cambridge want to find out how hosts tell the difference between their own eggs and those of the cuckoo finch. Eggs in the left column are hosts' eggs, and those in the right column are cuckoo finch eggs.
The team were interested how the host and parasite have evolved together. Unlike species in evolutionary young temperate regions, the cuckoo finch and prinia in the tropics may have been locked in an evolutionary arms race for up to 20 million years.
New methods allow analysis of what birds see and respond to, rather than what a human perceives. Digital photographs of the eggs allowed the complex markings, colour and pattern of the egg shells to be analysed, and fed into a computer model.
Using this computer model Dr Spottiswoode and Dr Martin Stevens were able to get a 'bird's eye view' of the eggs. One of the model's parameters is the amount of light in the host nest, here being measured by Collins Moya using a spectrophotometer.
To test how prinias detect a foreign egg, Dr Spottiswoode set up over a hundred rejection experiments, putting one prinia egg into the nest of another prinia and waiting to see whether or not the egg was rejected.
The experiments showed that prinias are amazingly good at rejecting foreign eggs (the right-hand egg in each clutch pictured) that do not perfectly match pattern and colour. Of this selection, only the foreign egg on the top right (102) was accepted.
Only the best-matching cuckoo finch eggs have a chance of surviving to hatching. In this parasitised nest the cuckoo finch's eggs (blue) are a striking mismatch with the prinia's eggs (red), and were quickly rejected.
However, sometimes the cuckoo finch exhibit brilliant acts of deception and match the prinia's eggs perfectly as this picture shows (the cuckoo finch egg is the one on the far right).
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