Intra-clutch variation in egg appearance assessed by human vision does not relate to rejection of parasite eggs in Iberian Azure-winged Magpies

The coevolutionary arms race between cuckoos and their hosts predicts that low variation in egg features within a host clutch would facilitate discrimination of mimetic parasite eggs. Here, we experimentally examine this prediction by using artificial and natural parasite eggs showing contrasting level of matching with host eggs. We quantified, based on human assessment, intra-clutch variation in egg appearance and egg discrimination in the Iberian Azure-winged Magpie (Cyanopica cyanus), a presumed former host of the Great Spotted Cuckoo (Clamator glandarius). Azure-winged Magpies rejected parasitic eggs in relation to their degree of dissimilarity with own eggs: Great Spotted Cuckoo model eggs were relatively more often rejected (73.7%) than real Great Spotted Cuckoo eggs (44.4%) and the less contrasting conspecific eggs (35.5%). Contrary to our predictions, we found that, irrespective of mimicry level of parasitic eggs, intra-clutch variation in host egg appearance did not differ significantly between rejecters and acceptors. We found, however, that individuals with higher variation in egg-size were almost significantly more prone to be rejecters than individuals showing lower variation in egg size. Our results do not support the hypothesis that the extent of intraclutch variation in egg discrimination varied with parasite egg mimicry in this particular system, and add to previous findings suggesting that perhaps an increase in intra-clutch variation in egg appearance, rather than a decrease, might be advantageous when discriminating against non-mimetic Great Spotted Cuckoo eggs.