Question: I'm making bird houses and I have been screwing the joints together. I have been drilling a pilot hole (2mm bit) and using 3mm X 30mm screws. 

I hung one in my garden and noticed that a couple of joints appear to be coming apart slightly after around a month or so. The houses are finished with Cuprinol Shades outdoor paint and the wood is bare before I paint (no primer etc).

Is there a more effective way of joining end grain to prevent warping/movement? Would gluing the joints, using larger screws, a smaller pilot hole or priming the wood first help to prevent this? The joints are 30 and 45 degrees.

Answer: 
I don't know how much effort you want to put into a birdhouse, but consider some type of woodworking joint. Depending on the grain orientation at the joint, a glued spline joint might be easy enough, and give the strength you need.

Screws in end grain typically don't hold very well.

Here are some good illustrations of a variety of mitered joints, including a splined miter: <http://www.craftsmanspace.com/knowledge/mitered-woodworking-joints.html>
the joint is coming apart because the wood has warped due to moisture. A miter joint will "work" if both pieces are sufficiently stabilized. You can achieve this using cross-grain battens screwed in on the underside.