unreported mortality at this level will obviously exaggerate 
the productivity required to keep such a life-table pom lation 
stable. These tendencies may well enter the banding work here 
analyzed but the distortion they produce (if any) is unknown 
and seems to be a minor one in the sample studied. 
On the evidence available at this writing, there is ne hint 
that the banding data on survival are biased. The 0 per cent mortality 
mentioned above for nestlings seems to be too high to be accertable. 
The alternative hypotheses are that not all female night herons are 
successful in raising young or that not all adults breed at the end 
of one year. There is a strong probability that this latter vohencomenon 
coverns the situation encountered here. 
Pepulation dynamics 
Alexander (19), 1945, 1946) has shown that an asymptote in 
the common heron population is rapidly regained following low densities 
induced by severe winters. Errington (195, 1946) has also emphasized 
that such lows are followed by high rates of reproductive gain, and 
Elder (1945) has pointed out that this inversity principle seems to 
be illustrated by the common heron. Although clutch size appears to 
be genetically fixed (Lack 1947), we can expect that the production 
of young will periodically vary in a given species. With only a 
Single nesting study to furnish the important statistics on produc- 
tivity in black-crowned night herons, and with only a small sample 
of banded birds to give us data on subadult and adult survival, no 
final picture of the population dynamics of this species can be given 
at this time. 
The pieces of information available at the moment can, how- 
ever, be fitted into an hypothesis, the outline of which is very 
instructive. The mosaic to be constructed requires information on 
the mortality suffered by young birds in the month after they leave 
the nest ard on the percentage of unsuccessful and nonbreeding females. 
If about 52 per cent of the young die between August 1 and December 31, 
an estimate of 15 or 16 per cent mortality in July seems fairly con- 
servative. 
When 2 heron population with 2.7 young per successful female 
on July 1 is subject to juvenile mortality of about 16 per cent in 
July, and 2.5 per cent mortality for adults at the same time, the 
age ratios change as follows: 
July 1 190 young per 70 successful females 
(2.7=1) 
August 1 160 young per 68 successful females 
(2.h4-1) 
Now from table 23 we inferred that the ratio was 1.6 young 
for each female in the population, successful, unsuccessful, and 
60 
