how to handle a nest box?

Near the office where I work is a small park area where there's a number of nest boxes hanging from various trees, usually about 3 meters above ground level I've spotted four of these with inhabitants near a infrequently used path. All of these boxes are occupied by either great tit or blue tit.

I've found that the parents don't mind it if I stand at about 5 meters from the opening of the box and don't move much.

If I'm lucky with the light I can shoot something like this:
http://farm5.static.flickr.com/4040/...66a3aba903.jpg
big:

but with the harsh sunlight of the last few days I'm more likely to end up with:

http://farm4.static.flickr.com/3311/...29a6de980f.jpg
big:

As you can see there's a lot of foliage and therefore not all that much light.

Now I was curious to see if I could get anything nice from a bird leaving or entering the nest box but what I've managed to shoot so far is either a slightly unsharp head or rear end sticking from the nest box opening or a blur next to the box that's only just recognizable as a bird.

I would like to know if anyone has any ideas on how I can shoot some nice shots in these circumstances without scaring the bird poo out of these little birds and without rushing to the shop to part with many thousands of euros.

I own:
Nikon d300
Nikon 70-300 VR
sb-600 flash
monopod
old velbon tripod.

Anyone any ideas?