Connecting the dots: avian eggshell pigmentation, female condition and paternal provisioning effort

Abstract

Differences in avian eggshell pigmentation could be an honest signal of female quality that males use to inform their nestling provisioning effort. We investigated whether among-individual variation in protoporphyrin-based eggshell pigmentation in house wrens (Troglodytes aedon) reflects female fitness-associated traits and whether males use that information. Females laying lighter clutches were older and larger than females laying darker clutches. Nestlings hatching from lighter clutches had greater size-corrected mass on post-hatch day 11, a measure that strongly predicts survival and recruitment to the breeding population. To test whether male provisioning effort responds to clutch pigmentation, we used a reciprocal clutch cross-fostering design, swapping dark with light clutches and light with dark; in controls, we swapped light with light clutches and dark with dark. Shortly before hatching, clutches were returned to their original nest to avoid confounding effects of nestling quality on male provisioning. Contrary to the sexual selection hypothesis, clutch pigmentation had no effect on male provisioning. Males were probably able to observe eggshell pigmentation and thus had information about female quality, but they did not use this information to modulate their nestling provisioning. This may be because of constraints on species-specific reproductive opportunities, or because variation in eggshell protoporphyrin serves other functions.

Avian eggshell coloration predicts shell-matrix protoporphyrin content

“…Eggshell pigmentation is an extended phenotypic trait of female birds, one that may provide information about a female's underlying physiological condition (Moreno and Osorno 2003;Moreno et al 2006;Hodges et al 2020) as well as serve as an adaptation for camouflaging eggs (Poulton 1890: 61ff. ;Lack 1958;Weidinger 2001;Westmoreland and Kilti 2007), strengthening eggshells (Underwood and Sealy 2002;Gosler et al 2005), and, for the embryo, mediating light exposure and modulating the thermal environment (Bakken et al 1978;Mauer et al 2015;Lahti and Ardia 2016).…”

“…In house wrens (Troglodytes aedon Vieillot, 1809), the ground (base) color of the eggshell varies from white to light tan or pink, and it is either sparsely or densely maculated with D r a f t 4 small spots or larger blotches of reddish brown (Johnson 2020;Hodges et al 2020). In a recent study relating protoporphyrin-based eggshell pigmentation to female condition and levels of male parental provisioning in our study population, we found that females laying eggs with more sparsely maculated and redder shells were older and produced heavier offspring than females laying more densely maculated and browner eggs (Hodges et al 2020). Thus, the physically quantified differences in the coloration of house wren eggshells (Fig.…”

“…1), which vary greatly among females (see Fig. 1 in Hodges et al 2020) and are highly repeatable within clutches (Hodges et al 2020), provide visual information about a female's condition and, potentially, her protoporphyrin load and her state of oxidative balance at the time of egg-laying. What is not clear, however, is whether the differences in egg-surface coloration accurately reflect the amount of protoporphyrin deposited into the shell matrix, an important assumption when testing the different hypotheses that have been proposed to explain among-female variation in eggshell pigmentation (Wegmann et al 2015).…”

How much calcium to shell out? Eggshell calcium carbonate content is greater in birds with thinner shells, larger clutches and longer lifespans

“…Egg production provides a critical example of life-history theory in action as the investment into an egg and/or clutch will greatly influence the quality of that offspring, but, conversely, will reduce the parent's resources for both immediate self-maintenance and future reproductive investment [ 7 ]. This trade-off has been explored in the context of egg contents [ 8 ], such as androgen deposition in the yolk [ 9 , 10 ] and pigment deposition in the shell matrix [ 11 ], yet the production of the eggshell itself and its composition have not been considered within the same framework.…”

“…of egg contents [8], such as androgen deposition in the yolk [9,10] and pigment deposition in the shell matrix [11], yet the production of the eggshell itself and its composition have not been considered within the same framework.…”