Experimental evidence that cuckoos choose host nests following an egg matching strategy

The arms race between brood parasites and their hosts provides a classic model to study coevolution. Hosts often learn to recognize and remove the parasitic egg from their clutch. To avoid egg rejection, brood parasites should select host nests in which the colour of the eggs best matches that of their own. Although this hypothesis has received some support, direct experimental evidence is still lacking. Here, we report on a study of Daurian redstarts (Phoenicurus auroreus), a species showing a distinct egg colour dimorphism, with females laying either blue or pink eggs. Redstarts are often parasitized by common cuckoos (Cuculus canorus), which lay light blue eggs. First, we show that cuckoo eggs were more similar in spectral reflectance to the blue than to the pink redstart egg morph. Second, we report that the natural parasitism rate was higher in blue than in pink host clutches. Third, we performed a field experiment in which we presented a dummy clutch of each colour morph close to active redstart nests. In this setup, cuckoos almost always chose to parasitize a blue clutch. Our results demonstrate that cuckoos actively choose redstart nests in which the egg colour matches the colour of their own eggs. Our study thus provides direct experimental evidence in support of the egg matching hypothesis.