Abstract
Brood parasitism generally has detrimental effect on host fitness. To avoid the negative consequences of brood parasitism, hosts have often evolved an effective counter-adaptation-recognition and rejection of parasitic eggs.
Because eggshells of the Eurasian reed warbler (Acrocephalus scirpaceus) and its brood parasite, the common cuckoo (Cuculus canorus), differ significantly in their ultraviolet (UV) reflectance, one would expect that the ability of the hosts to see UV light will play an important role in recognition and rejection of parasitic eggs. To test this assumption, we performed 3 sets of experiments differing in the level of mimicry of the parasitic egg.
In the first 2 experiments, we parasitized host nests by conspecific eggs (either own or from other females with perfect and good mimicry, respectively) coated either with a UV blocker (completely reducing UV reflectance of the eggshell) or with Vaseline as a control treatment. Although the UV blocker significantly decreased eggshell UV reflectance, hosts accepted most of these conspecific eggs and the type of coating did not significantly affect the probability of rejection.
In the third experiment, we parasitized clutches with 2 types of non-mimetic eggs differing only in UV reflectance (UV- and UV+). We found that hosts rejected UV+ eggs at a significantly higher rate than UV-eggs, probably due to their lower mimicry in the UV spectrum.
Here, we demonstrated that hosts use UV signals during egg recognition. Moreover, we suggest that such signals may play a more important role when the parasitic egg is non-mimetic rather than mimetic, when hosts can use additional cues, such as spotting pattern.