Increase in membrane thickness during development compensates for eggshell thinning due to calcium uptake by the embryo in falcons

Abstract

We compared membrane thickness of fully developed eggs with those of non-developed eggs in different endangered falcon taxa. To our knowledge, membrane thickness variation during development has never been examined before in falcons or any other wild bird. Yet, the egg membrane constitutes an important protective barrier for the developing embryo. Because eggshell thinning is a general process that occurs during bird development, caused by calcium uptake by the embryo, eggs are expected to be less protected and vulnerable to breakage near the end of development. Thus, egg membranes could play an important protective role in the later stages of development by getting relatively thicker. We used linear mixed models to explore the variation in membrane thickness (n = 378 eggs) in relation to developmental stage, taxon, female age, mass and identity (73 females), egg-laying sequence (105 clutches) and the study zone. Our results are consistent with the prediction that egg membranes are thicker in fully developed eggs than in non-developed eggs, suggesting that the increase in membrane thickness during development may compensate for eggshell thinning. In addition, our data shown that thicker membranes are associated with larger, heavier and relatively wider eggs, as well as with eggs that had thinner eggshells. Egg-laying sequence, female age and the study zone did not explain the observed variation of membrane thickness in the falcon taxa studied. As we provide quantitative data on membrane thickness variation during development in falcons not subjected to contamination or food limitation (i.e. bred under captive conditions), our data may be used as a reference for studies on eggs from natural populations. Considering the large variation in membrane thickness and the multiple factors affecting on it and its importance in the protection of the embryo, we encourage other researchers to include measurements on membranes in studies exploring eggshell thickness variation.

Speckles of cryptic black‐headed gull eggs show no mechanical or conductance structural function

“…We consider this an unlikely explanation, because of the small time (c. 20 days that would have passed between the laying of undeveloped and freshly laid eggs and the probable individual variation in egg production capacity. Finally, in support of developmental shell thinning in black-headed gulls we observe that our values of eggshell thinning are well within the range of those found for other non-passerines (c. 7-15%, Finnlund et al, 1985;Castilla et al, 2007;Castilla et al, 2010). The significant interaction between stage and membrane effects highlights the dynamic changes the calcium shell, the inner membrane and possibly the cuticula undergo in the course of embryonic development, which we discuss in more detail elsewhere (Maurer et al, 2011).…”

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

“…, Castilla et al . , ). Latitudinal variation in shell thickness is potentially biologically relevant as it could determine areas of greatest gas conductance and shell breaking strength or the major source of calcium for embryonic growth and development.…”

The effect of developmental stage on eggshell thickness variation in endangered falcons

“…All measurements were conducted without the membrane, since membranes are rather thick in falcon eggs (Castilla et al, 2009b). The outer thick eggshell membrane and the thin vascularized chorioallantoic membrane were removed from the eggshells after immersing them in water (see Castilla et al, 2010).…”