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 con - trasting 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 Spot- ted 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 intra- clutch 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 appear - ance, rather than a decrease, might be advantageous when discriminating against non-mimetic Great Spotted Cuckoo eggs.