Embryonic Growth and Development

Abstract Over the past century, avian biologists have developed a conventional wisdom concerning the relationship among egg size, incubation period, embryo growth, and the developmental state of the neonate. Precocial birds lay larger eggs relative to adult body size than do altricial species (Rahn et al. 1975; Sotherland and Rahn 1987), and they take longer to hatch (Portmann 1955). It seems reasonable that precocial offspring should benefit more than altricial offspring from larger neonatal size (e.g., Nisbet 1978; Ankney 1980; Williams et al. 1993) and that the more mature state of the precocial neonate should take longer to achieve. Portmann (1955) argued that altricial and precocial species progress through the same developmental stages but that altricial chicks hatch relatively earlier in this progression and precocial chicks later. Thus, a portion of the maturation achieved by altricial nestlings is accomplished by precocial embryos before they hatch. Increased self-reliance of precocial neonates results in reduced postnatal growth potential, apparently because of a trade-off between functional maturity and the growth rate of tissues (Ricklefs 1979; see chapter 11). By the same argument, altricial chicks maintain high growth rates well after hatching. Because postnatal growth is strikingly influenced by the functional state of the neonate, it is reasonable to ask about the extent to which variation in the neonate condition is reflected in growth of the embryo.