Differences in egg size, shell thickness, pore density, pore diameter and water vapour conductance between first and second eggs of Snares Penguins Eudyptes robustus and their influence on hatching asynchrony

Abstract

Brood reduction in birds is frequently induced by hatching asynchrony. Crested penguins (genus Eudyptes) are obligate brood reducers, but in contrast to most other birds, first‐laid eggs are considerably smaller in size than second‐laid eggs; furthermore, first‐laid eggs hatch after their siblings. The mechanisms underlying this reversal in size and hatching order remain unclear. In this study, we tested whether the second‐laid eggs of Snares Penguins Eudyptes robustus have a higher eggshell porosity allowing them to maintain a higher metabolic rate throughout incubation and to hatch before their first‐laid siblings. We investigated differences in egg size, shell thickness, pore density, pore diameter and water vapour conductance between first and second eggs within clutches and examined the influence of these shell characteristics on hatching asynchrony. First‐laid eggs of Snares Penguins were approximately 78% of the size of the larger second eggs. Second‐laid eggs had considerably thicker shells and more pores per cm2 than first eggs, whereas pore diameter did not differ between eggs. Water vapour conductance was greater in second‐ (16.8 mg/day/torr) than in first‐laid eggs (14.9 mg/day/torr). The difference in water vapour conductance between first‐ and second‐laid eggs within clutches was related to hatching patterns. In nests where second eggs hatched before first‐laid eggs, second eggs had a considerably greater water conductance than their sibling, whereas in nests where both eggs hatched on the same day, the difference in water conductance between eggs was very small, and in a few nests where small first eggs hatched before their larger sibling, they had a greater water conductance than their larger second‐laid nestmate. Surprisingly few studies have investigated differences in shell characteristics between eggs within clutches and associated effects on hatching asynchrony. This study has demonstrated that such differences exist between eggs within clutches and that they can influence hatching patterns.

Shell thickness and pore density in relation to shell colouration, female characteristics, and environmental factors in the Collared Flycatcher Ficedula albicollis

“…Since later laying has often been associated with lower reproductive success as a result of declining food availability during the period of parental care (Norris 1993;Wiggins et al 1994), there may be a reproductive advantage for females breeding later in the season to reduce incubation time. It has been shown in numerous studies that total pore area of the eggshell affects water vapour conductance, and both of them are positively related to the growth rate of embryos and hatchability of eggs (Burton and Tullett 1983;Ar and Rahn 1985;Soliman et al 1994;Massaro and Davis 2005;Zimmermann and Hipfner 2007). In the Yellow-eyed Penguin, detected an increase in pore density with laying date which may allow embryos to develop more rapidly than earlier laid eggs.…”

Determinants of eggshell strength in endangered raptors

“…Previous results from poultry and wild birds have indicated that eggshell breaking strength and/or eggshell thickness are influenced by the egg size and color (Kennedy and Vevers,'73), the location of the egg where these parameters are measured (Gosler et al, 2005), egg developmental stage (Vanderstoep and Richards, '70;Bunck et al, '85;Bennett, '95;Castilla et al, 2007), shell microstructure (Massaro and Davis, 2005), female age, body mass and health status, the time the eggs spend in the uterus, the length of incubation period, egg laying sequence and clutch number (Ar et al, '79;Massaro et al, 2002;Massaro and Davis, 2004;Castilla et al, 2009), the type of diet (Connor and Arnold,'72) and genetics (Francesch et al,'97).…”

Geographical variation in egg size dimorphism in rockhopper penguins

“…Among all populations of rockhopper penguins (Table 1), we similarly noted that the least dimorphic eggs were those from the Falkland Island population. In this study, the A-egg averaged 0.79 of the B-egg volume, a value between the Snares penguin and the Fiordland penguin species (0.77 and 0.85, respectively, Warham 1974a, b;Massaro and Davis 2005). This population has, therefore, the second least dimorphic eggs of all crested penguins after the Fiordland penguin.…”