group by range of values

Hello,

Here's my table:

db=# select * from tester order by birth_year; birth_year | salary
------------+--------       1946 |  78000       1949 |  61000       1951 |  58000       1953 |  56000       1958 |
52000      1962 |  50000       1965 |  45000       1967 |  60000       1968 |  57000       1970 |  47000       1972 |
32000      1973 |  42000
 
(12 rows)

How can I display the average salary grouped by decade of birth year?
That is, is it possible to display the average salary of those born in 
the 1940's, the average salary of those born in the 1950's, average 
salary of those born in the 1960's, and those born in the 1970's, all in 
one result table?
Something like:

decade | average(salary)
-------+-----------------  1940 |  69500  1950 |  53333.33  1960 |  53000  1970 |  40333.33

Thanks in advance,
C