nonbreeding. It would therefore follow that on August 1 in our hyvo- 
thetical population of herons we had 160 young per 100 females. Under 
these conditions, 32 females out of 100 were unsuccessful or nonbreed~ 
ing. The turnover in this population follows the life table from this 
point: About 50 per cent of the young alive on August 1 die in the 
next 5 months; by July 31 additional ‘young die equivalent to about 
10 per cent of those alive on the preceding August 1. The surviving 
40 per cent replace 30 per cent of the adults, and the population 
remains stable. ; 
_ The following will perhaps clarify what would take place 
under such circumstances; 

Age Ratios 
. Population Alive Dead 
Alive on August 1 160 young 200 adults (0.8~1) 
Semonth mortality 80 young 20 adults | (4-1) 
Alive on Jamary 1 80 young 180 adults (0.45~1) 
7=-month mortality 20 young 40 adults (0.5=1) 
Alive on July 31 60 yearlings 10 adults (0.1) 
When we say that 10 per cent of the young alive on August 1 die dur- 
ing the last 7 months of the year, this is the same as saying that 
25 per cent of the young alive on January 1 die during this period. 
This is apyrox‘mately the same mortality rate suffered by the adult 
birds (0+180=22 per cent). 
Summary 
Banding studies of the blackecrowned night heron in North 
America reveal it to be a highly mizgratory species, the eastern popu- 
lation (South Dakota to Massachusetts) moving south to Florida, Cuba, 
and the West Indies, the western population wintering in Mexico and 
Guatemala. Of 11 banded young alive on August 1, 52 per cent were 
later reported found dead or shot in the first 4 months. The mean 
adult mortality rate for the survivors was 30 per cent per year, the 
last bird in this sample dying in its thirteenth year of life. 
A comparison of these data with the results of a nesting 
study by Gross (1923) carries no hint that the banding data are 
biased and suggests that about 30 per cent of the females one year 
of age or older were either unsuccessful or nonbreeding. 
61 
