Characteristics of Eggs and Hatchlings of the Altricial Double-crested Cormorant (Phalacrocorax auritus) and the Precocial Ring-billed Gull (Larus delawarensis)
Reed, Wendy L.
Two general developmental patterns are used to classify birds; altricial birds hatch at a relatively early stage of development, and precocial birds hatch at an advanced stage of development. Along with developmental differences between these two bird types there are intrinsic differences between natural allotment of yolk, albumen and shell to these eggs and the use of these components throughout incubation, and in the construction of the egg shell, which prescribes how much water is lost to the environment. I conducted a study of egg shell characteristics, egg components and hatchling components of the Ring-billed Gull (Larus delawarensis) and the Double-crested Cormorant (Phalacrocorax auritus), which are representative of semi-precocial and altricial birds respectively. Freshly laid gull eggs are allotted considerably more yolk and less albumen than fresh cormorant eggs, however at hatching a gull has used a greater percentage of its yolk during incubation resulting in a yolk sac that is much smaller than that of a cormorant. Both embryo types, however, use a similar fraction of the total energy available at the start of incubation. Both eggs lose similar amounts of mass due to evaporative water loss and at the end of incubation cormorants have a greater percentage of water in the yolk sac and relatively more water in their tissues when compared to gull hatchlings. Metabolism of an altricial cormorant is notably less than that of a gull, but conductances for the two types of eggs differ sufficiently such that the predicted air cell gas tensions for these birds are similar. Thus, it has been demonstrated with these species that characteristics of eggs, embryos, and incubation are coordinated in such a fashion to produce interspecific similarities in the presence of interspecific differences in their patterns of development.
vi, 34 p.
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