Table 23.--An Abridged Life Table for Black-crowmed Night Herons: ° 
Based on nestlings banded from 1926 to 193 inclusive. Age intervals» 
are in years and start August 1. : 
ge ow Recovere ve a or y 
Interval epor otal Start of Per Cent 
in Years Dead Shot Mortality Year (1,) Per Year (qx) 
0-1 h2 Li 86 Wy1 61 
1-2 5 8 13 55 : 
2=3 5 13 18 2 
34hs he 1 5 2h 
hes h 2 6 19 
5-6 1 L 5 13 31 
b6=7 1 1 2 8 
7<8 = 1 1 6 
8-9 7 1 1 5 
9-10 3 ~ 3 b 
10~11 - - ~ 1 | 
11-12 = = - 1 
12-13 1 - 1 1 a4, 
Total , 320 ee, 
Juv. /Ad. 1.7 lek 1.6 0.8 
Juv. 64 59 61 bid 
i/ mean for entire sample 
Age ratios and productivity 
If we bear in mind Gross's (1923) statistic of 2.7 young 
herons per nest, the bias in table 23 might be examined under several 
contingencies: 
(1) If all young herons of this species breed at the end of one year 
and each pair raises 1.6 young to August 1, the mortality rates 
of this table (23) would make for a stable population. The valve 
of 1.6 potentially represents about 0 per cent mortality opera- 
ting on the nestling average of 2.7 furnished by Gross. This 
mortality would have to occur between June or July and August l-- 
the start of the life table. 
(2) It seems to me wise to explore the assumption that all night 
herons begin to breed at 2 years of age. Under such conditions, 
table 23 would result in annual population decreases of about 
10 per cent. While this situation probably did not prevail in 
the population from which this sample was taken, it is well to 
realize that the sample itself is small] and there is a good 
possibility that some individuals might attain breeding condi- 
tion at one year, and others only at 2 years of age. 
(3) Any tendency for (a) the older birds to lose their bands or 
(bo) the deaths reported for first-year birds to be atypical of 
59 
