per cent (Marshall 1947; Paludan 1951; and this paper). The mere 
agreement of these last three on this point does not prove our 
respective life tables are reliable; each may be subject to the 
same kind(s) of bias. Both band loss and/or vulnerability to 
shooting could made the first-year mortality rate calculations too 
high. It is my feeling that, whatever the reason, this exaggera- 
tion is present in table 0. 
Common Tern 
The life span and age structure of Sterna hirundo popula- 
tions on Cape Cod, Mass., have been analyzed In considerable detail 
by O. L. Austin (1938, 1942, 1945, 1947a, 19)47>, 1949) in a series 
of reports that are still in progress. In the last of these, Austin 
mentions a "high" mortality rate of 17.2 per cent per year appar- 
ently obtained from a preliminary statistical analysis that is to 
be discussed in a future contribution. 
Austin is now wrking up the results of what is the most 
extensive population study ever undertaken in ornithology. Up to 
the present, he has published only on his retrapping data. The life 
span and age structure of his populations are invariably presented 
on a time-specific basis, such as those found in the population for 
the year 1938, or the years 1938-1. The reliability of this pic- 
ture depends not only on absence of band loss (which Austin has 
worked hard to overcome) but also on sampling errors (which he shows 
are small) and on corrections for variations in the numbers origin- 
ally banded (a point that I feel he has overlooked). The nature of 
these variations is evident in column (2) of table 1, 
Austin (1942, 1945, 1947a) has mentioned the life span 
of the common tern as being 10 years, with a small number persisting 
even later in the population. "Life span" like the "turnover period" 
is a term that always should be carefully defined. “On a graph made 
of the percentages of birds banded as chicks which return each year 
of their age," writes Austin (1945), "the points for all more than 
twelve years are below a base line indicating the presence of none." 
I find it hard to reconcile this statement with Austin's raw data 
that are reproduced as columns Ly and L', in table 1 and especially 
with column 1", when it is transposed to semilogarithmic paper 
{figure 11). It appears to be contradicted further by other state- 
ments in this same paper (Austin 1945) that 1 out of every 200 
adults is 13 or more years old and that the recording of ages past 
18 years can be expected in subsequent retrapping operations. 
It is perhaps worth re-emphasizing here that the recording 
of the disappearance of the last individual in any population is 
governed not only by biological laws but by statistical ones as 
well, The biological evidence now available points to no marked 
changes in annual mortality rate throughout most of a bird's adult 
life. Under such conditions, and with a given mortality rate, the 
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