Critical Periods and Causes of Death in Avian Embryonic Development

Abstract

ARIAN embryonic mortality has long been a subject of biological interest. It is also a problem ooe obvious economic importance. The total embryonic mortality of domestic fowl, Gallus gallus, has been progressively increasing for some time. Less than 30 years ago, when flocks were small and kept at large, the embryonic death rate rarely exceeded i0 per cent. As simple and primitive methods of poultry management have been replaced by such efficient modern practices as intensive feeding, breeding in confinement, and large-scale artificial incubation, embryonic mortality has frequently risen to 25 per cent or more. Numerous investigations have attempted to determine the causes of death during the incubation period, and to explain the rising embryonic death rate. Studies have been made not only of chickens, but also of a number of other domestic birds and some game birds raised in captivity. DISTRIBUTION OF MORTALITY In 1919, Payne pointed out a fundamental fact regarding embryonic mortality in the fowl when he published his data on the distribution of deaths throughout the period of incubation. His observations of 2,142 dead embryos indicated that mortality is especially high between the third and fifth days, and again on approximately the nineteenth day. He also found that artificially incubated embryos were much more likely to die during the later period of susceptibility than embryos incubated under the hen. This discovery immediately suggested that environmental factors were at fault, an inference that has subsequently been borne out by the results of many studies. Later, Riddle (1930) extended Payhe's findings on embryonic mortality to include doves, $treptopelia risoria, and pigeons, Columba livia. Although the incubation periods of doves, pigeons, and chickens are of different lengths, the curves of embryonic mortality are nevertheless similar for all three species. Two peaks are seen at essentially equivalent points, one early in embryonic life, the other shortly before the hatching date. In fact, arian development in general is characterized by these two periods of increased mortality. Data on turkeys, Meleagris gallopavo, ducks, Anas platyrhynchos, pheasants, Phasianus torquatus, grouse, Bonasa umbellus, and quail, Colinus virginianus (Romanoff, 1934 and unpublished ms.; Romanoff, Bump, and Holm,