Egg colour mimicry in the common cuckoo Cuculus canorus as revealed by modelling host retinal function

Abstract

Some parasite cuckoo species lay eggs that, to the human eye, appear to mimic the appearance of the eggs of their favourite hosts, which hinders discrimination and removal of their eggs by host species. Hitherto, perception of cuckoo-host egg mimicry has been estimated based on human vision or spectrophotometry, which does not account for what the receivers' eye (i.e. hosts) actually discriminates. Using a discrimination model approach that reproduces host retinal functioning, and museum egg collections collected in the south of Finland, where at least six different races of the European cuckoo (Cuculus canorus) coexist, I first assess whether the colour design of cuckoo eggs of different races maximizes matching for two favourite avian hosts, viz. the redstart (Phoenicurus phoenicurus) and the pied wagtail (Motacilla alba). Second, I assess the role of nest luminosity on host perception of mimicry by the same two hosts. Phoenicurus-cuckoo eggs showed a better chromatic matching with the redstart-host eggs than other cuckoo races, and in most cases can not be discriminated. Sylvia-cuckoo eggs, however, showed better achromatic matching with redstart-host eggs than Phoenicurus-cuckoo eggs. Also, Motacilla-cuckoo eggs showed poorer chromatic and achromatic matching with pied wagtail-host eggs than Sylvia-cuckoo eggs. Nest luminosity affected chromatic and achromatic differences between cuckoo and host eggs, although only minimally affected the proportion of cuckoo eggs discriminated by chromatic signals. These results reveal that cuckoo races as assessed by humans do not entirely match with host perception of matching and that achromatic mechanisms could play a main role in the discrimination of cuckoo eggs at low-light levels.

Figure 1

Mean values of (a,c) chromatic and (b,d) achromatic matching between eggs assessed by humans as belonging to six different cuckoo gentes in southern Finland and average values per clutch of redstart (n=39 clutches) and pied wagtail (n=22 clutches) host eggs. Bars are standard deviations. Values of matching are sorted by size. Number of host clutches that had a value of chromatic contrast below the threshold value for discrimination ΔS of 1.0 JND are displayed above the bars. (a,b) Phoenicurus phoenicurus and (c,d) Motacilla alba.

Figure 2

Efficiency in cuckoo egg detection in relation to nest luminosity. Points represent the value of the difference in (a,c) chromatic and (b,d) achromatic matching between cuckoo eggs of the Phoenicurus and Motacilla gentes and those of their corresponding hosts when viewed under high- and low-light regimes at the nests. Grey areas indicate where mismatching had greater values when nest luminosity was low (n=39 redstarts and 22 pied wagtail clutches).