Avian eggshell thickness in relation to egg morphometrics, embryonic development, and mercury contamination

Abstract

Eggshell thickness is important for physiological, ecological, and ecotoxicological studies on birds; however, empirical eggshell thickness measurements for many species and regions are limited. We measured eggshell thickness at the equator and the egg poles for 12 avian species and related eggshell thickness to egg morphometrics, embryonic development, egg status, and mercury contamination. Within an egg, eggshells were approximately 5.1% thicker at the equator than the sharp pole of the egg, although this difference varied among species (0.6%–9.8%). Within Forster's tern (Sterna forsteri), where eggshell thickness was measured at 5 equally spaced positions along the longitude of the egg, eggshell thickness changed more rapidly near the sharp pole of the egg compared to near the blunt pole of the egg. Within species, eggshell thickness was related to egg width and egg volume for six of the 12 species but was not related to egg length for any species. Among species, mean eggshell thickness was strongly related to species mean egg width, egg length, egg volume, and bird body mass, although species mean body mass was the strongest predictor of species mean eggshell thickness. Using three species (American avocet [Recurvirostra americana], black‐necked stilt [Himantopus mexicanus], and Forster's tern), whose nests were carefully monitored, eggshell thickness (including the eggshell membrane) did not differ among viable, naturally abandoned, dead, or failed‐to‐hatch eggs; was not related to total mercury concentrations of the egg content; and did not decrease with embryonic age. Our study also provides a review of all existing eggshell thickness data for these 12 species.

Shifts in eggshell thickness are related to changes in locomotor ecology in dinosaurs

“…2017; Peterson et al. 2020), which might limit the overall variation of mean eggshell thickness for a given egg mass.…”

“…1974; Deeming 2006; Birchard and Deeming 2009; Peterson et al. 2020). Flight in extant birds has also been proposed to influence or constraint physiological and life history traits (Prange et al.…”

Thicker eggshells are not predicted by host egg ejection behaviour in four species of Australian cuckoo

“…We used a precision industrial wall-thickness measuring tool to directly measure eggshell thickness via Hall-effect magnetic-inference, in a similar approach to Peterson et al 75 . However, unlike this previous study we did not cut or damage the eggshell to take measurements.…”

“…To investigate whether cuckoo eggshell thickness deviates from this general relationship, we calculated the mean eggshell thickness in a total of 78 species, comprising 12 avian orders and 34 families (total N = 3134 eggs) (Table 2 ). Our analysis included previously published data for 12 species 75 . We used a phylogenetic generalised least squares regression (PGLS) in the R package caper 78 , and estimated the relationship between eggshell length and two measures of thickness (apex and meridian).…”

“…Within a species, outliers in the distribution of mean thicknesses (as defined as above) were removed. To account for inter-specific differences in egg size (which is correlated with eggshell thickness) ‘normalised thickness’ was calculated for each sample by dividing the eggshell thickness by egg length 75 . This approach is expected to successfully normalise the data because egg length explains a large proportion of the inter-species variation in eggshell thickness (Fig.…”