Firstly we reestablished the characteristics that define each class.
What make a bird a bird? beak, two legs, lays eggs etc.
What makes a mammal a mammal? skin/fur, warm blooded, has live babies and gives them milk.
What makes a reptile a reptile? cold blooded, lays eggs, etc.
All the ideas/answers for this came from the kids memory - they didn't even need to go and read their books.
Next we talked a lot about where each animal went and why. For example, the snake is a reptile because it has scales, lays eggs etc. The lion is a mammal because it has fur/skin, has live babies and feeds them milk etc. We used the technical words we had learnt from the reading in context, in new sentences and related them back to a concept the kids understand (animals). Talking aloud and discussing why each animal went where was really effective in getting the kids to use their new words and some technical words as well (E.g. mammary glands).
We had to move some of the pictures around into the right places, but the talk was awesome.
I highly recommend this as an oral language activity, as it is very effective in getting kids to review the ideas and vocabulary they had learnt. They are so focused on physically moving around the pictures they don't even realise the rich conversations they are having.