Pick the Odd Man Out

Why don't we try our hand at the age-old game: Pick the odd one out.

A series of words will be given out of which you need to pick up the odd one and say why it is so.

Whenever the previous question has been answered correclty, a new series will be given. Other members are also welcome to participate in asking questions:

So, let me start with a couple of easy ones:

1...Jan, April, March, August, May
2.. biology, chronology, opthalmology,ornithology, physiology