Imperfectly Camouflaged Avian Eggs: Artefact or Adaptation?

Abstract

For many birds, the fight for survival begins at the egg stage: avoiding predation is paramount. At a broad phylogenetic level, selection by predators for egg camouflage appears to be the primary driver of variation in egg colouration and patterning. Despite this, experiments at fine-scale taxonomic levels have largely failed to find support for adaptively camouflaged egg appearance. How are we to resolve this baffling contradiction? Here we present and evaluate five explanations which are not mutually exclusive and which may explain why eggs appear imperfectly camouflaged at the genus or species level. First, imperfect camouflage may be an artefact of imperfect measurements. In studies of egg camouflage, researchers have consistently neglected to account for predator vision, and only rarely have egg appearance and camouflage been quantified carefully and objectively. Recalibrating our assessment of egg camouflage may answer many questions, but it is unlikely to wholly explain why many eggs do not appear seamlessly cryptic. Instead, imperfect camouflage may stem from mechanistic constraints on pigment production in some avian lineages or may be a consequence of selection for anti-microbial protection. Or perhaps the manifold demands of the egg and selection for functions other than protective concealment have tugged the egg phenotype away from an ideally cryptic appearance. Finally, imperfect egg camouflage may occur if other forms of defence are simply more effective, or if a diverse brigade of predators imposes different selective pressures on egg appearance. A combination of these factors is likely involved. Avian eggs provide an excellent model system for investigating animal camouflage, and recent advances in numerous fields make this area particularly ripe for future research.

Eggshell Conspicuousness in Ground Nesting Birds: Do Conspicuous Eggshells Signal Nest Location to Conspecifics?

“…The role of predator pressure is often cited as an important role in the evolution of eggshell colouration and speckling (Wallace, 1889;Abercrombie, 1931;Stoddard et al, 2011a). The ancestral eggshell colour is believed to be plain white (Wallace, 1889;Packard et al, 1980;Kilner, 2006) because bird's closest living relatives have unpigmented eggshells (Packard et al, 1980).…”

“…Despite this, many species lay strikingly conspicuous eggshells that seem counterintuitive considering the predation risk facing eggs (Götmark, 1992;Blanco et al, 2002;Svagelj et al, 2003;Avilés et al, 2006;Castilla et al, 2007;Westmoreland, 2008). Over the past 200 years, researchers have been interested in why birds lay eggs that are conspicuous despite threats posed by visually oriented predators (reviewed in Stoddard et al, 2011a). Hypotheses include inter-and intra-specific signalling (Moreno et al, 2003;Hanley et al, 2010), selection by environmental factors (McAldowie, 1886;Maurer et al, 2011), and tradeoffs between these competing selection mechanisms (Magige et al, 2008).…”

Effects of environmental conditions on the egg mass, yolk antioxidant level, eggshell thickness and eggshell spotting patterns of Great Tits (Parus major)

“…Protoporphyrinbased eggshell pigmentation may help to provide camouflage (Stoddard et al 2011), it may enhance egg mimicry or allow individual egg recognition in species exposed to brood parasites (Stokke et al 1999;Moskát et al 2008), it may reinforce eggshells (Gosler et al 2005;García-Navas et al 2010;Hargitai et al 2013), or it may signal the quality of the female and the egg to the male in order to stimulate more intense paternal care (Moreno and Osorno 2003;Sanz and García-Navas 2009;Stoddard et al 2012;Hargitai et al 2016b). The accumulation of protoporphyrin has been reported to induce oxidative stress, leading to an increase in the activity of antioxidant enzymes to avoid oxidative damage (Afonso et al 1999;Pimentel et al 2013).…”

Behaviorally Induced Camouflage: A New Mechanism of Avian Egg Protection

“…Pigmentation can camouflage eggs by background matching (Stevens and Merilaita 2009a) or disruption of the egg outline (Stevens and Merilaita 2009b), as well as by creating within-clutch color variation that is visually confusing to a potential predator (Hockey 1982;Lloyd et al 2000). Nevertheless, pigmented egg color and patterning is typically relatively consistent within each female (Kilner 2006), which means that optimal camouflage may be achieved only through a genetic correlation with the microhabitat or through optimal choice of a nest site (Stoddard et al 2011;Lovell et al 2013).…”

“…We use objective techniques to quantify the extent of camouflage and field experiments to assess its adaptive value. Egg coloration is a widespread avian strategy for defending eggs against visually hunting predators, especially in ground-nesting species (Tinbergen et al 1962;Underwood and Sealy 2002;Nguyen et al 2003;Kilner 2006), and it is commonly achieved through the deposition of bilirubin or biliverdin in the eggshell (Kilner 2006;Stoddard et al 2011). Pigmentation can camouflage eggs by background matching (Stevens and Merilaita 2009a) or disruption of the egg outline (Stevens and Merilaita 2009b), as well as by creating within-clutch color variation that is visually confusing to a potential predator (Hockey 1982;Lloyd et al 2000).…”