counting unique factors in r

I would like to know the number of unique dams which gave birth on each of the birth dates recorded. My data frame is similar to this one:

dam <- c("2A11","2A11","2A12","2A12","2A12","4D23","4D23","1X23")
bdate <- c("2009-10-01","2009-10-01","2009-10-01","2009-10-01",
           "2009-10-01","2009-10-03","2009-10-03","2009-10-03")
mydf <- data.frame(dam,bdate)
mydf
#    dam      bdate
# 1 2A11 2009-10-01
# 2 2A11 2009-10-01
# 3 2A12 2009-10-01
# 4 2A12 2009-10-01
# 5 2A12 2009-10-01
# 6 4D23 2009-10-03
# 7 4D23 2009-10-03
# 8 1X23 2009-10-03

I used aggregate(dam ~ bdate, data=mydf, FUN=length) but it counts all the dams that gave birth on a particular date

bdate dam
1 2009-10-01   5
2 2009-10-03   3

Instead, I need to have something like this:

mydf2
  bdate      dam
1 2009-10-01  2
2 2009-10-03  2

Your help is very much appreciated!