Interspecific brood parasitism represents a prime example of the coevolutionary arms race where each party has evolved strategies in response to the other. Here, we investigated whether common cuckoos (Cuculus canorus) actively select nests within a host population to match the egg appearance of a particular host clutch. To achieve this goal, we quantified the degree of egg matching using the avian vision modelling approach. Randomization tests revealed that cuckoo eggs in naturally parasitized nests showed lower chromatic contrast to host eggs than those assigned randomly to other nests with egg-laying date similar to naturally parasitized clutches. Moreover, egg matching in terms of chromaticity was better in naturally parasitized nests than it would be in the nests of the nearest active non-parasitized neighbour. However, there was no indication of matching in achromatic spectral characteristics whatsoever. Thus, our results clearly indicate that cuckoos select certain host nests to increase matching of their own eggs with host clutches, but only in chromatic characteristics. Our results suggest that the ability of cuckoos to actively choose host nests based on the eggshell appearance imposes a strong selection pressure on host egg recognition.

In recent decades, numerous studies have examined factors affecting risk of host nest parasitism in well‐known avian host–parasite systems; however, little attention has been paid to the role of host nest availability. In accordance with other studies, we found that nest visibility, reed density and timing of breeding predicted brood parasitism of Great Reed Warblers Acrocephalus arundinaceus by the Common Cuckoo Cuculus canorus. More interestingly, hosts had a greater chance of escaping brood parasitism if nesting was synchronized. Cuckoo nest searching was governed primarily by nest visibility at high host‐nest density. However, even well‐concealed nests were likely to be parasitized during periods when just a few hosts were laying eggs, suggesting that Cuckoos adjust their nest‐searching strategy in relation to the availability of host nests. Our results demonstrate that host vulnerability to brood parasitism varies temporally and that Cuckoo females are able to optimize their nest‐searching strategy. Moreover, our study indicated that Cuckoos always manage to find at least some nests to parasitize. Thus, in this case, the co‐evolutionary arms race should take place mainly in the form of parasitic egg rejection rather than via frontline pre‐parasitism defence.

Discrimination of foreign eggs is one of the most studied aspects of host defences against avian brood parasites. Although many factors affecting host egg‐recognition processes have already been evaluated, only a few attempts have been made to test the importance of light conditions in microhabitats of host nests. Here, we examined whether the objectively measured nest light environment affects great reed warbler (Acrocephalus arundinaceus) responses towards real common cuckoo (Cuculus canorus) eggs. More specifically, we predicted that parasitic eggs will be rejected with a lower frequency from nests placed in darker conditions than those in lighter conditions. However, we found no effect of the ambient light on egg‐rejection behaviour alone, but the photosynthetically active radiation exhibited a positive interactive effect with chromatic contrast between cuckoo and host eggs. Most rejection events were accomplished when cuckoo eggs of poor mimicry were laid in well‐lit nests. Our study suggests that this phenomenon may have important implications for the evolution of egg mimicry and host egg discrimination. We encourage further testing of the light environment hypothesis in other host species breeding in variable nest microhabitats and light conditions.

Avian brood parasites pose a serious threat to hosts, substantially reducing their fitness, which selects for the evolution of host defences. A classic example of a host frontline defence is mobbing, which frequently includes contact attacking of brood parasites. Here, we investigated how the nest defence of a very aggressive host, the Great Reed Warbler Acrocephalus arundinaceus, influences the speed of egg‐laying and egg‐removing behaviour of its brood parasite – the Common Cuckoo Cuculus canorus. We video‐recorded 168 brood parasitic events at 102 active host nests and found that Cuckoos avoided host mobbing in 62% of cases. If hosts spotted the Cuckoo at their nests, they almost always attacked it (91 of 104 cases; 88%); however, such attacks only rarely and temporarily prevented Cuckoos from parasitizing (11 additional cases). When attacked, Cuckoos parasitized host nests significantly faster and left them immediately after laying. However, when not attacked, Cuckoos frequently stayed at or near the nest, suggesting that host aggression, rather than the risk of being spotted, influences the speed of brood parasitism in this species. Further, we found that Cuckoos performed egg‐removing behaviour in all parasitic events without regard to host aggression. As a result, Cuckoos removed at least one egg during all brood parasitism events except those when the egg slipped from their beaks when they were attacked by hosts and, thus, remained in the nest (in nine of 82 cases when they were attacked; 10.9%). This indicates that egg‐removing behaviour is not costly for the Common Cuckoo and is an essential part of its parasitism strategy, widening our understanding of this elusive behaviour.