What pattern do you observe in beak length from 1973 to 1978

In 1973,

The maximum length for the beak of birds was obserbed to be less than 12mm in length

While, at 1978, it was observed that some birds had beak length at 12mm.

From 1973 to 1978, it can be observed that short beaks bird was more in population than 1978.