Criteria for host selection in a brood parasite vary depending on parasitism rate

Avian brood parasites leave parental care of their offspring to unrelated foster parents; therefore, their fitness will closely relate to their efficiency in selecting the best host. Empirical studies have shown that parasitism is not random, although it is yet unclear if this results from strategic choices of better quality hosts by brood parasites or can be a consequence of hosts and/or nests characteristics that make them more exposed to parasitism. Here, we hypothesize that the optimal strategy of host use for a brood parasite may switch depending on fluctuations in the relative abundance of parasites with respect to hosts and predict that parasites should more likely select phenotypic traits indicating host quality when this is relatively low (i.e., many available host nests for each parasite at a given time). In an 8-year study of magpie Pica pica hosts parasitized by great spotted cuckoos Clamator glandarius, we have found that in years with lower than average parasitism bigger magpie nests were more likely parasitized than smaller ones, whereas in years with higher than average parasitism big and small magpie nests had a similar chance of being parasitized. Because nest size is a postmating sexually selected signal revealing parental abilities in magpies that can be used as a target to choose profitable hosts by cuckoos, our findings support the hypothesis that active selection will become a more prevalent great spotted cuckoo laying strategy when cuckoos have a larger availability of host nests to choose among.