IT IS one of nature鈥檚 most ruthless tricks. African village weaver birds, like many other species, can be fooled into raising cuckoo chicks at the expense of their own. But unlike the others they fight back, evolving egg markings that help them resist being parasitised, while the cuckoos evolve better ways to trick them.
Now a biologist has shown how weaver birds respond once the pressure is off. David Lahti at the University of Massachusetts in Amherst compared populations of weaver birds in South Africa, which are parasitised by Diederik cuckoos, with those on the islands of Mauritius in the Indian Ocean and Hispaniola in the Caribbean, where there are no cuckoos. The weaver birds were introduced to the islands 100 and 200 years ago respectively.
In South Africa, weaver birds lay eggs with family-specific patterns and colours. Because all the eggs in a clutch have the same pattern, a cuckoo鈥檚 egg will stand out. But it takes energy to manufacture the pigments needed to make eggs with complicated patterns, and weaver birds on Hispaniola and Mauritius have stopped doing it (Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0508930102).
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In the Hispaniola birds, which have been isolated for longer, the anti-cuckoo patterns have faded further. 鈥淭he work demonstrates the operation of natural selection very well,鈥� says Lahti.
