Chapter II.--Difficulties in Constructing Life Tables for Birds 
In theory a life table ought to start with the zygote. In 
practice, such an ideal is virtually impossible to attain at this time, 
especially in viviparous and ovoviviparous animals. In our most 
studied species, man, fertilized ova less than ten days old are still 
unknown to science (Arey 194, p. 139). Taussig (1936) in her classic 
treatise on abortion demonstrated the unreliability of published data 
on intra-uterine mortality. She assumed that in United States cities 
one abortion (premeditated or unpremeditated) occurred to every 2.5 
deliveries, and that in rural districts this ratio was 1 to S. More 
recently, Arey (194, pp. 133-134) has estimated that about one 
pregnancy in is unsuccessful, largely because the embryos are not 
vigorous enough to reach birth as living individuals. Perry (19)5) 
in his careful study of brown rats (Rattus norvegicus) found intra- 
uterine mortality to amount to about 10 per cent. It is obvious that, 
from a purely biological point of view, mammalian life tables, includ- 
ing the modern actuarial tables for man, are still incomplete. The 
research of ornithologists on early mortality in each species is 
clearly less complicated, although still beset with difficulties. 
Real versus theoretical populations 
The construction of an avian life table depends on special 
and diverse research opportunities that are not always given to the 
same investigator. Mortality in the egg and nestling stages of the 
caspian tern can, for instance, be investigated by a single person 
at one or more colinies. In this species, however, a low rate of 
recovery of banded birds requires such a large-scale banding program 
that compilation of mortality data on the adult stages of life almost 
certainly will require the field work of many banders over a long 
period. Life tables for birds ideally should result from single well- 
coordinated research projects, but their present scarcity encourages 
the construction of synthetic tables derived from a mosaic of reliable 
data obtained from diverse sources. These synthetic tables of course 
represent theoretical populations. A theoretical population, however, 
is the one most frequently encountered in all life-table work. When- 
ever an investigator computes an average, as of the per cent of eggs 
hatching in a 3-year study period, or of adults returning in a 5-year 
trapping program, this statistic departs from reality. It is not the 
product of what occurred in a given year but the composite product 
of what happened in several years. In effect, it holds for a theoretical 
population rather than for a real one. Failure to appreciate this 
point has frequently confused students with whom I have discussed life- 
table results. This confusion is particularly associated with mortality 
rates for adult birds. 
Life tables by Kraak, et al. (190), Lack (1943a, 1943b) and 
others emphasize that a constant annual mortality rate exists throughout 
the adult birds life. One is tempted to ask: Do not severe and atypical 
winters bring about wide fluctuations in adult mortality rates? The 
answer is that they do; but they operate on real populations, not 
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