Development during the first seven days post-hatching.

The neonate has basic needs that must be satisfied at the time of hatching if survivability and maximum potential are achieved. Some of these basic needs are fresh air, clean water, proper feed, and heat. The developmental state at hatching of the neonate differs among all avian species (Nice, 1962). The post-hatching period for altricial neonates is more critical than for precocial birds because they hatch in a less mature state. Differing amounts of maternal care are therefore essential for each species (Nice, 1962). Although embryonic growth among species is very similar, no two physiological systems seem to mature at the same rate (Ricklefs and Starck, 1998). In addition, maturity may be a function of the egg conductance constant (Ar and Rahn, 1978), which is determined by egg mass, eggshell conductance (or functional properties) and the length of the incubation period (see Box 1 later), all of which may constrain neonatal maturity. When viewed energetically, the difference between maturity types resides in the different water concentrations in eggs and hatchlings, in the density of chemical potential energy in the dry matter of true hatchlings, and in the different amounts of energy transferred from the egg to the spare yolk (Ar et al., 1987). On the basis of species comparisons of posthatching growth of all birds (Lilja, 1983; Ricklefs, 1987), it has been suggested that the rate of growth after hatching is at least partially determined by the pattern of organ growth. It appears that a high rate of growth is correlated to early growth of ‘‘supply organs’’ (oesophagus, proventriculus, gizzard, intestines, heart and liver) at the expense of ‘‘demand organs’’ (breast, wings, legs and feathers). These changes begin very early in development (Lilja and Olsson, 1987), and Schmalhausen (1930) hypothesised that growth and organ function come into conflict when growth occurs too slowly or too rapidly. Even under the most optimum conditions, a newly hatched bird is not free from stress. This is impossible because the absence of stress is death (Selye, 1951). However, hatchlings possess abilities to cope with stress via adrenal cortical hormones (Davis and Siopes, 1989). Hatchlings have different blueprints for growth and maturation that must occur within a predetermined time frame. Many times this blueprint does not include adjustments that need to occur in maturational and growth processes in an imperfect environment. The objective of this paper is to define a ‘‘physiologically normal’’ hatchling and to describe the principles involved in the maturation and growth of several organ systems during the initial stages of life. At least six physiological systems exist that require maturation during the last week of incubation or, in the case of altricial species, the initial days of life outside the shell. These systems are: (1) the circulatory system (heart and blood); (2) the kidney and body fluids system; (3) the digestive system; (4) body temperature regulation; (5) the respiratory system; and (6) the immune system. These systems are discussed here using published data to help clarify important points.