A comparison of indices and measured values of eggshell thickness of different shell regions using museum eggs of 230 European bird species

Abstract

The thickness of avian eggshells is used to assess shell quality in wild and domestic species, as an indicator of environmental pollution and as an adaptive explanation for shell maculation. Both direct measurements and calculated eggshell thickness indices (ETI) are used in such research, yet this is the first study to quantify, across a large spectrum of bird families (and thus egg shapes), the correlation between measured thicknesses and ETI. Furthermore, few studies have quantified thickness variation across the entire length of the shell, although this variation may influence both gas transfer and embryonic development. We measured the thickness of 942 eggshells of 230 European bird species from the Class II material at the Natural History Museum, Tring, UK, both in the conventional manner, at the equator through the blowhole and, uniquely, after a single longitudinal, cut at its equator at the blunt and pointed ends. Over half of the samples revealed shell defects, cautioning against the indiscriminate use of museum specimens. Strong positive associations were found between species‐specific means of shell thickness with each other and also with ETI, especially those derived from Schönwetter's ‘Handbuch der Oologie’ method, validating the interspecific comparative use of ETI. Thickness measurements and ETI factors are provided for all 230 species. Eggshells were usually thinner at the blunt end (the location of the air sac) than at the equator, but of equal thickness in passerine eggs. This difference was greatest in species producing elongate eggs and suggests that there is a functional significance of shell thickness variation among species that requires further investigation.

First light for avian embryos: eggshell thickness and pigmentation mediate variation in development and UV exposure in wild bird eggs

“…a). However, it must be noted that our shell samples were derived from museum‐sourced eggs, which were mostly collected during the early incubation stage (as indicated by small blowholes made for the preparation of the specimens; Maurer, Portugal & Cassey ()). Because eggshells are thin during development, as the embryo mobilizes the eggshell matrix to source calcium for its skeletal development (Richards & Packard ), our measurements, and estimates, may not be fully representative of the light transmission experienced by older embryos.…”

Rapid development of brood‐parasitic cuckoo embryos cannot be explained by increased gas exchange through the eggshell

“…Here, because of the smaller size of the eggshells, we instead use whole eggs. Eggshell thickness ( μ m) was measured on the sampled eggs following conductance measurements, according to a previously published protocol (Maurer, Portugal & Cassey, ). Briefly, length and width of eggshell and blowhole diameters were measured to 0.1 mm using Mitutoyo ABS Digimatic Callipers CD‐6” C (Mitutoyo Coventry, Wickmans Drive Warwickshire, Coventry, Warwickshire, UK).…”

“…Both anvils of the micrometer had been capped with an aluminium pin with a diameter of 1.35 mm (radius 0.35 mm). The repeatability (Lessells & Boag, ) of thickness measurements with the Series 207 micrometer was assessed previously on a sample of 20 helmeted guineafowl Numida meleagris eggs measured 10 times at the same location of the equator (Maurer et al ., ). Measurements showed a highly consistent repeatability (intra‐class correlation coefficient r > 0.99, n 0 = 10, a = 20, see Maurer et al ., for full details).…”

Embryonic eggshell thickness erosion: A literature survey re-assessing embryo-induced eggshell thinning in birds

“…This means that shell fragments from various parts of the egg (differing in their thickness, e.g., Maurer et al, 2011aMaurer et al, , 2012see Results) were analysed together (cf. Falk et al, 2006), which could have produced an overall artifact regarding the causes of eggshell thinning (discussed in Snyder and Meretsky, 2003).…”

“…Overall, eggshell thickness seems to be influenced by a variety of factors: genetics, egg colour, maculation and size, the time the eggs spend in the uterus, female characteristics (condition, age, stress, health status), female diet or infection (Snyder and Meretsky, 2003;reviewed in Castilla et al, 2010;Maurer et al, 2012Maurer et al, , 2011a. More importantly, despite the considerable body of research devoted to the erosion of embryonic shell thickness, the effect of developmental stage on eggshell thickness or strength remains unknown for most wild species (cf.…”