Egg colour matching in an African cuckoo, as revealed by ultraviolet-visible reflectance spectrophotometry

Abstract

Despite major di¡erences between human and avian colour vision, previous studies of cuckoo egg mimicry have used human colour vision (or standards based thereon) to assess colour matching. Using ultraviolet-visible re£ectance spectrophotometry (300^700 nm), we measured museum collections of eggs of the red-chested cuckoo and its hosts. The ¢rst three principal components explained more than 99% of the variance in spectra, and measures of cuckoo^host egg similarity derived from these transformations were compared with measures of cuckoo^host egg similarity estimated by human observers unaware of the hypotheses we were testing. Monte Carlo methods were used to simulate laying of cuckoo eggs at random in nests. Results showed that host and cuckoo eggs were very highly matched for an ultraviolet versus greenness component, which was not detected by humans. Furthermore, whereas cuckoo and host were dissimilar in achromatic brightness, humans did not detect this di¡erence. Our study thus reveals aspects of cuckoo^host egg colour matching which have hitherto not been described. These results suggest subtleties and complexities in the evolution of host^cuckoo egg mimicry that were not previously suspected. Our results also have the potential to explain the longstanding paradox that some host species accept cuckoo eggs that are non-mimetic to the human eye.

Dark nests and egg colour in birds: a possible functional role of ultraviolet reflectance in egg detectability

“…134 species belonging to 16 families, according to Sibley & Ahlquist (1990)). The egg coloration was estimated by using UV-vis spectrophotometry, which is a technique providing highly repeatable measures of egg colour even for species with spotted eggs (Cherry & Bennett 2001;Langmore et al 2003;Avilés et al 2004;Soler et al 2005), and allowing the recording of UV information to which humans are blind (Endler 1990). The reflectance spectra in the range 300-700 nm were obtained from all eggs using a spectroradiometer with a deuterium and a halogen light source (DH 2000, Ocean Optics Europe).…”

Egg colour mimicry in the common cuckooCuculus canorusas revealed by modelling host retinal function

“…Colour was measured in each of these four bands. Previous studies have shown that this technique provides highly repeatable measures of egg colour even for species with spotted eggs (Cherry & Bennett 2001;Langmore et al 2003;Avilés et al , 2006aStarling et al 2006). Reflectance was measured with the probe placed at a constant distance at an angle of 458.…”

“…The four measurements from each egg were averaged to give a mean spectrum per egg, and a mean host spectrum for each clutch was calculated (e.g. Cherry & Bennett 2001;Avilés et al 2006a).…”

“…Bennett et al 1996;Bowmaker et al 1997;Cuthill et al 1999Cuthill et al , 2000. Indeed more recent application of spectrophotometry that accounts for UV wavelength has revealed the existence of differences hidden to humans in the coloration of cuckoo eggs laid in different hosts in the red-chested cuckoo Cuculus solitarius (Cherry & Bennett 2001), the common cuckoo and the pallid cuckoo Cuculus pallidus (Starling et al 2006). A limitation to these studies is that matching was measured as the difference in reflectance between cuckoo and host eggs, which does not account for what the host's eyes and its sensory systems actually process and discriminate Cuthill et al 2000;Endler et al 2005).…”

Cuckoos, cowbirds and hosts: adaptations, trade-offs and constraints

“…In some brood parasites such as honeyguides and parasitic finches, similarities between parasitic egg and host egg are best explained by common ancestry rather than coevolution (Davies 2000), but the selection pressure that hosts impose on brood parasites to lay mimetic eggs has been recognized for almost a century (Baker 1913) and there is good evidence that hosts are more likely to accept mimetic eggs and to reject non-mimetic ones Lotem et al 1995;Stokke et al 1999;Lahti & Lahti 2002). While the generalist brood parasitic cowbird species do not exhibit egg mimicry for the great majority of their hosts, many cuckoo species lay mimetic eggs, sometimes indistinguishable from the host egg for the human eye (Langmore et al 2003) and even a good match for the host egg in the UV spectrum (Cherry & Bennett 2001;Langmore et al 2003). The significance of the UV mimicry is currently not clear.…”