theoretical ones. The winter of 1933-3), for instance, was so severe 
in the northeastern part of the United States that Lake Ontario and 
Long Island Sound both froze over. Mortality rates in a cohort of 
herring gulls banded in, say 1928, would be governed by this disastrous 
weather (table 3). But in a sample of birds banded from 1925 to 19h1, 
the effects of this winter would be masked. Other winters, both 
severe and "open," would help create an average picture of mortality 
from 1925 to 1941, and the mortality rates would tend to become the 
product of all conditions. The final figures for adult mortality rates 
over a 20- or 25-year period would thus be representative of a purely 
theoretical population. 
Table 3.—-Variation in Herring Gull Mortality During Specific Years 
Birds banded as nestlings and later found dead. Each year starts 
September 1. Notice the higher mortality reported in 1929-30 and 
1933-3) e 
Year Year Birds Were Reported Dead 
Banded '2h '25 '26 '27 '28 '29 130 '31 132 133 '3) '35 9% 37 98 89 owl 
192k 21 1 1 1 1 2 BY 1 Oo 4 103100000 
1925 - 32 6 2 3 iT 2 0 0 3 10000 0 0 0 
1926 - =- 61 lt 2 6 1 2 3 112120032100 0 0 
1927 - = - 6 3 6 &¥§ kL 7 2 OVO 13120011 
1928 - = = = Wh 20 7 8 2 6 21011%i110 
TOTAL 
Dead 23 38 16 15 12 165123311221 
Alive at start 135 112 7h 59 bh 32 161110 8 § k 3 1 
Mortality rates 17 3h 20 25 27 50 31 ~---+ 2 ---- 
#Percent per annum 
Under such circumstances, it seems to me proper at this time to 
combine the results of nesting studies (which yield survival data on 
eggs and young) with the results of banding analyses of subadult and 
adult survival-~even though the field data may originate in different 
regions and from the work of different investigators. 
Determination of fledgling survival 
The early steps of an avian life table can only come from 
intensive studies of nesting populations. When one considers the 
large number of species available, surprisingly few of these studies 
have been published up to the present. One serious difficulty in 
working with many species is the frequent impossibility of following 
the young birds after they leave the nest. This is unfortunately com 
plicated by the rapid breaking down of family ties among most of the 
smaller birds, and by the dispersal of the young from study areas 
(Sherman 192h; Nice 1937, p. 133). It is also paralleled by an important 
failure of the bird-banding technique to record accurately the mortality 
of these very young birds from predation, starvation, or the weather. 
