Order by birthdate

I'm trying to generate a birthday list ordered by month, then by day. 
This is what I thought would work...

SELECT name,birthdate FROM table WHERE birthdate is not null ORDER BY
date_part('month',birthdate)

but psql returns:

ERROR:  parser: parse error at or near "("

What's the problem?

--
Tim J Trowbridge                         trowbrid@writeme.com
Interested in my genealogy?          http://www.execpc.com/~trowbrid