Egg colour mimicry in the common cuckooCuculus canorusas 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.

Visual modeling shows that avian host parents use multiple visual cues in rejecting parasitic eggs

“…In the light of recent advances in analyzing color differences as seen by nonhuman animals (6, 7), three studies have used advanced perceptual models of avian discrimination to investigate cuckoo and host egg coloration (8)(9)(10). These confirmed that quantification of visual signals used by hosts to detect parasitism provides much greater insights when analyzed with models of bird vision than does subjective human assessment (5).…”

“…These confirmed that quantification of visual signals used by hosts to detect parasitism provides much greater insights when analyzed with models of bird vision than does subjective human assessment (5). However, although valuable, these studies were not directly linked to rejection experiments (8), involved species that were rarely parasitized and artificial eggs that were not a close match to host egg colors (9), or involved systems in which host rejection of foreign eggs does not occur (10).…”

Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures

“…Cuckoo females belong to different genetic races, each of which selectively targets a host species. To sneak their eggs into host nests, many cuckoo host-races have evolved remarkable egg mimicry [11][12][13][14] . Hosts can then defend themselves by evolving better discrimination abilities or by evolving more recognizable egg pattern signatures that facilitate detection of an imposter egg 9,15,16 .…”

“…Hosts can then defend themselves by evolving better discrimination abilities or by evolving more recognizable egg pattern signatures that facilitate detection of an imposter egg 9,15,16 . Evidence for enhanced discrimination by hosts comes from many studies demonstrating that host discrimination of foreign eggs is at its most refined in species that are parasitized by common cuckoo host-races with nearperfect egg mimicry [11][12][13][14] . However, evidence for the evolution of recognizable egg pattern signatures by common cuckoo hosts is more mixed 7 .…”

Egg phenotype matching by cuckoos in relation to discrimination by hosts and climatic conditions

“…It is therefore important to use models of avian vision that account for what the receiver's (i.e. host's) eye actually can perceive as colour matching [52,53]. Here, we ran (Vorobyev -Osorio) physiological models [54] with AVICOL software v. 3 [55] that reproduce host retinal functioning and that account for nest luminosity and bird sensitivity for estimating colour (i.e.…”