for all living things, a macro with not too short focal length is the best ... your models will be scared if you have to get too close to them and might fly away
Well if you have the money you could get yourself a proper macro lens. That's a link to Pentax's own range of autofocus macro lenses. But those are fairly expensive. Tamron's 90mm f/2.8 macro lens probably costs less and is generally very well regarded. Sigma and Tokina also offer macro lenses. Another option is older, manual focus macro lenses.
Probably the cheapest option however is simply to buy some extension tubes and use a normal lens (preferably a prime) on the end.