Eggshell spotting does not predict male incubation but marks thinner areas of a shorebird's shells

Abstract

Many birds lay eggs speckled with black or reddish-brown spots of protoporphyrin pigment, but the function of these spots is debated. Two recent hypotheses have received considerable attention. Under the “signaling-function hypothesis,” speckling reflects female quality and influences allocation of male parental care; under the “structural-function hypothesis,” the speckles strengthen the eggshell when calcium is scarce. The evidence for each is taxonomically uneven. For instance, evidence for the signaling-function hypothesis comes from cavity-nesting and uniparentally incubating species in which the viewing conditions and possibilities for males to assess the speckling are limited, and male response to speckling is indirect (i.e., female-feeding) or postponed (i.e., chickfeeding). We tested the predictions of both hypotheses in a biparentally incubating ground-nesting shorebird, the Northern Lapwing (Vanellus vanellus), in which the potential for a direct male response to eggshell speckling exists and visual inspection of the eggs is not hindered by light availability in a nest cavity. The speckling parameters (spot intensity, distribution, and cover) were repeatable within Northern Lapwing clutches, a requirement for a sexually selected signal, but we found no relationship between male incubation and speckling. However, the spots were associated with thinner areas of eggshell, which strongly supports the suggestion of a structural (strengthening) function. Our results do not support the signaling-function hypothesis of eggshell speckling, but extend the structural-function hypothesis and prior findings (that speckling occurs primarily at thinner areas of the shell) to other avian taxa. Furthermore, if the generally accepted view that Northern Lapwing eggs are cryptically marked is correct, our findings suggest that more than one significant function can drive the evolution of avian eggshell pigmentation.