The importance of avian egg components in the determination of hatchling size and quality has yet to be fully evaluated. In the first experiment, 20% of the albumen and/or the yolk was removed from chicken eggs to determine the impact of each egg component on metabolism and various size measures in near-term embryos. Results show that metabolic rate, dry body mass, and internal organ mass are largely independent of egg composition. Removal of albumen resulted in a decrease in wet body mass corresponding to decreases in water content in the body and the yolk sac, and decreased tibiotarsus length. Removal of yolk resulted in no change in body mass, but decreases in both wet and dry yolk sac mass. In a second experiment, removal of 15% of either egg component led to reductions in hatchling mass similar to those observed in whole near-term embryos. Albumen, as the primary source of water in the egg, is the primary determinant of hatchling size and may influence hatchling success through size-related limiting factors. Differences in yolk content may influence neonatal quality as a nutritional supplement, but seem not to result in greater tissue formation during embryonic development.

From data in the literature an attempt is made to provide a general overview of the composition of bird eggs and how individual components are related to the relative yolk content, which varies from about 15% in some Sulidae to 69% in the Kiwi (Apteryx). Regression equations are given for the relative water content of the fresh egg, albumen, and yolk, the solid and lipid fractions of egg content, and the calculated caloric density of egg content as a function of relative yolk content. As relative yolk content of eggs increases among 127 species representing 44 families, relative water content decreases. These changes are related to a progressive increase in hatchling maturity from altricial to precocial birds according to the classification of Nice (1962). Certain exceptions to this rule are discussed, namely, eggs of the pelagic feeding Procellariiformes and offshore-feeding terns. Both groups have reduced their clutch to a single egg, have unusually long incubation and fledging periods, and larger relative yolk content than predicted. A general model of egg components is presented which relates the ash, carbohydrate, protein, lipid, water, and caloric content to the relative amount of yolk in the freshly laid egg. Lipids make up 58% of all yolk solids, which have an energy content of 33 kJ.g-I; total solids in egg contents have an average energy content of 29 kJ.g-I. An overview shows how the initial solids and caloric contents are redistributed in the yolk-free neonate and the yolk reserve as well as estimates of the solids and caloric loss during development; energy cost of development is similar in all birds and amounts to ca. 36% of the initial energy content of fresh eggs.

Clutches of leatherback turtles, Dermochelys coriacea, have lower hatching success than those of other sea turtles, but causes of high embryonic mortality are unknown. We measured characteristics of clutches along with spatial and temporal changes in PO(2) and temperature during incubation to determine the extent to which they affected the developmental environment of leatherback embryos. Minimum PO(2) in nests decreased as both the total number and mass of metabolizing embryos increased. Increases in both the number and mass of metabolizing embryos caused an increase in maximum nest temperature. However, neither PO(2) nor temperature was correlated with hatching success. Our measurements of relatively high nest PO(2) (lowest 17.1 kPa or 16.9% O(2)) indicate that hypoxia apparently does not cause the low hatching success of leatherback clutches. Oxygen partial pressure increased and temperature decreased from the center toward the periphery of leatherback nests. We inferred from these measurements that positions of eggs within nests vary in quality and potentially affect overall developmental success of entire clutches. The large metabolic mass of leatherback clutches and limits to gas flux imposed by the sand create a situation in which leatherback embryos collectively affect their own environment.

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001-2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.

Attaining endothermic homeothermy occurs at different times posthatching in birds and is associated with maturation of metabolic and aerobic capacity. Simultaneous measurements at the organism, organ and cellular levels during the transition to endothermy reveal means by which this change in phenotype occurs. We examined development of endothermy in precocial Pekin ducks (Anas platyrhynchos domestica) by measuring whole-animal O 2 consumption (V O2 ) as animals cooled from 35 to 15°C. We measured heart ventricle mass, an indicator of O 2 delivery capacity, and mitochondrial respiration in permeabilized skeletal and cardiac muscle to elucidate associated changes in mitochondrial capacities at the cellular level. We examined animals on day 24 of incubation through 7 days post-hatching. V O2 of embryos decreased when cooling from 35 to 15°C; V O2 of hatchlings, beginning on day 0 posthatching, increased during cooling with a lower critical temperature of 32°C. Yolk-free body mass did not change between internal pipping and hatching, but the heart and thigh skeletal muscle grew at faster rates than the rest of the body as the animals transitioned from an externally pipped paranate to a hatchling. Large changes in oxidative phosphorylation capacity occurred during ontogeny in both thigh muscles, the primary site of shivering, and cardiac ventricles. Thus, increased metabolic capacity necessary to attain endothermy was associated with augmented metabolic capacity of the tissue and augmented increasing O 2 delivery capacity, both of which were attained rapidly at hatching.