Differential predation by mammals and birds: implications for egg-colour polymorphism in a nomadic breeding seabird

Abstract

Selection for crypsis in varying environments has long been established as the main evolutionary force promoting the huge variation in avian egg coloration. In several avian species, variation in egg coloration exists, but available information available on the relative success of these different colour morphs against predation is scarce. We investigated the value of eggshell coloration against mammal and avian predators in the South American Tern, Sterna hirundinacea. We found evidence of a relationship between particular eggshell ground coloration and success against predation, in different tern colonies, where strong selection was caused by single avian and mammalian predator species. Survival to hatching of eggs with greenish ground coloration was greater than in eggs of the remaining colours when a mammalian carnivore was present. This implies that the human visual system does not accurately represent predator perception but that, viewed through the predator's eyes, the conspicuous greenish eggs are well concealed. The rate of artificial nest predation by visually searching gulls was higher for eggs more conspicuous to the human eye than for eggs more closely resembling the nest substrate. The evolution of polymorphisms in eggshell ground colour may have resulted from differences in the type of predator present, and differences in choice of breeding site varying in the background substrate. The nomadic breeding behaviour of terns may imply that females differing in the frequency of alleles expressing particular egg coloration, selected for in particular environments, may eventually gather in some colonies, thus producing the observed intracolony variation in egg coloration. We hypothesise that egg colour variation could be maintained in the population by shifting peaks of predation impact in the different locations where colonies form, e.g. islands without mammalian predators vs. mainland sites.

Implications of nest‐site limitation on density‐dependent nest predation at variable spatial scales in a cavity‐nesting bird

“…In contrast, breeding density at the scale of a 2 km radius arose again as the main determinant of nest predation. This indicates that density at this particular spatial scale may influence the variation found in the impact of predators, as a likely consequence of their identity or particular features of their activity, distribution or abundance (Blanco and Bertellotti 2002).…”

“…Predation may have important consequences not only on breeding performance but also on population dynamics of spatially structured populations depending on the strength of this kind of density dependence (Serrano et al 2005), which in turn may depend on habitat features affecting the prey but also the spatially variable guild of predators (Blanco and Bertellotti 2002). This may be especially true in choughs due to the tenacity of breeders to maintain traditional nest-sites from year to year despite recurrent nest failure due to predation in particular nests, although predators may be different and have variable impacts in different areas (Zuñiga 1989, Blanco andTella 1997).…”

Why are birds’ eggs speckled?

“…Although evidence from a few taxa supports a signalling role for the pigments on avian eggshells, either in crypsis (effectively a dishonest ÔsignalÕ) from predators (Bakken et al 1978;Blanco & Bertellotti 2002;Sanchez et al 2004), for eggpattern mimicry in the case of brood-parasites (Davies & Brooke 1989a,b) or in signalling female condition in the case of biliverdin-based pigments (Moreno & Osorno 2003;Moreno et al 2004), explanations based on a signalling function cannot account for the eggshell-pigmentation of most species (Newton 1896;Lack 1968;Verbeek 1990;Weidinger 2001). First, all the species in most non-passerine orders lay white, essentially unpigmented, eggs.…”

Temperature drives the evolution and global distribution of avian eggshell colour

“…2). Our use of avian perceived colour and luminance is important for future studies involving conspecific signalling 21 , brood parasitism 39 , and avian predation 40 . Our calculated avian perceived colour and luminance relate to a measure of blue-green chroma (the sum of the reflectance within 450 and 550nm divided by the sum total reflectance within 300 and 700nm) and brightness (the average reflectance within 300 to 700nm) 41 ( colour : β = −0.97, CI 0.95 = −0.99 to −0.95, p < 0.0001; luminance : β = 0.98, CI 0.95 = 0.98 to 0.99, p < 0.0001), respectively.…”