| Abstract: | The intensity of selection exerted by brood parasites on their hosts depends on the proportion of nests that are parasitized and the fitness costs of parasitism. Nest detection by brood parasites influences the probability of parasitism, and we propose that the difficulty faced by brood parasites of finding nests on the ground may make ground‐nesting species subject to lower levels of parasitism, causing a reduction in levels of defence compared with species breeding in shrubs, trees and elsewhere above the ground. We tested the prediction that the rejection rate of Common Cuckoo Cuculus canorus eggs by hosts is inversely related to the frequency with which they build nests on the ground, both at local and at continental scales. First, we used estimates of the rejection rate of non‐mimetic model eggs experimentally introduced into the nests of 26 potential host species breeding in the Sierra Nevada Mountains of southern Spain. Most species tested in the Sierra Nevada showed high rejection rates of both mimetic and non‐mimetic eggs, whereas the European Robin Erithacus rubecula, with a low rejection rate, was the only species that was regularly parasitized. At the continental scale we used all available published information on rejection rates of non‐mimetic models by European hosts of the Common Cuckoo. The frequency of ground‐nesting explained interspecific variation in rejection rate of non‐mimetic model eggs both for the species tested in the Sierra Nevada and for all European hosts after controlling for all other life‐history variables known to affect rejection rates. An effect of the abundance of trees in a particular habitat, previously shown to affect parasitism by the Common Cuckoo, was only apparent from analyses of continental‐scale data and not from the Sierra Nevada mountains, suggesting that particular properties of mountainous areas affect Common Cuckoo parasitism. Ground‐nesting species showed lower rejection rates than species breeding in bushes or trees. These results suggest that species nesting on the ground may have suffered lower parasitism pressures in their historical coevolutionary interactions with the Common Cuckoo. |
