The evolution of egg colour and patterning in birds

Abstract

Avian eggs differ so much in their colour and patterning from species to species that any attempt to account for this diversity might initially seem doomed to failure. Here I present a critical review of the literature which, when combined with the results of some comparative analyses, suggests that just a few selective agents can explain much of the variation in egg appearance. Ancestrally, bird eggs were probably white and immaculate. Ancient diversification in nest location, and hence in the clutch's vulnerability to attack by predators, can explain basic differences between bird families in egg appearance. The ancestral white egg has been retained by species whose nests are safe from attack by predators, while those that have moved to a more vulnerable nest site are now more likely to lay brown eggs, covered in speckles, just as Wallace hypothesized more than a century ago. Even blue eggs might be cryptic in a subset of nests built in vegetation. It is possible that some species have subsequently turned these ancient adaptations to new functions, for example to signal female quality, to protect eggs from damaging solar radiation, or to add structural strength to shells when calcium is in short supply. The threat of predation, together with the use of varying nest sites, appears to have increased the diversity of egg colouring seen among species within families, and among clutches within species. Brood parasites and their hosts have probably secondarily influenced the diversity of egg appearance. Each drives the evolution of the other's egg colour and patterning, as hosts attempt to avoid exploitation by rejecting odd-looking eggs from their nests, and parasites attempt to outwit their hosts by laying eggs that will escape detection. This co-evolutionary arms race has increased variation in egg appearance both within and between species, in parasites and in hosts, sometimes resulting in the evolution of egg colour polymorphisms. It has also reduced variation in egg appearance within host clutches, although the benefit thus gained by hosts is not clear.

Pattern recognition algorithm reveals how birds evolve individual egg pattern signatures

“…Although one host, the dunnock, lays immaculate eggs, the remaining host species lay eggs variably covered with pattern markings. To investigate the effects of egg pattern signatures independently of egg background colour, we focused exclusively on egg patterning (or maculation), which includes the speckles, scrolls and markings formed by protoporphyrin pigment on the eggshell's surface 7 . In birds, luminance (achromatic) vision is processed independently of colour, stems from a set of receptors called double cones and is likely important for tasks related to pattern and texture perception 30 .…”

“…They can be chemical, like the hydrocarbon signatures used by Formica ants to recognize nest-mates 3 , or auditory, like the vocalizations used by Australian sea lion (Neophoca cinerea) mothers and pups to reunite in dense colonies 4 , or visual, like the facial patterns used by Polistes fuscatus paper wasps to recognize individuals in the colony 5 . In birds, egg patterns can be visual signatures of offspring identity, enabling parents to recognize their eggs in a crowded colony 6 or to distinguish their own eggs from those of a brood parasitic cheat 7 . How do signatures evolve to actively promote recognizability?…”

“…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 . Do hosts of the common cuckoo mark their eggs with recognizable signatures?…”

Extraction and analysis of colourful eggshell pigments using HPLC and HPLC/electrospray ionization tandem mass spectrometry

“…There is strong scientific, commercial and public interest in characterizing the colours of birds' eggshells, from the perspectives of evolutionary biologists and consumers (Kilner, 2006;McGraw, 2006;Zhao et al, 2006). Yet, our current knowledge of the pigments responsible for the diverse and variable coloration and patterning of birds' eggs stems from a handful of published protocols of chemical analyses that were devised prior to the development of modern analytical techniques.…”

The evolution of postpairing male mate choice

“…For example, sexual selection has been proposed to explain the evolution of egg color in birds, through the suggestion that males may adjust their parental care investment according to the colors of eggs produced by their mates (Moreno and Osorno 2003). Although this has been verified in several bird species, the conclusions are still controversial (Kilner 2006), and it is unclear why some studies have failed to find empirical support (e.g., Krist and Grim 2007; Honza et al. 2011; Johnsen et al.…”