I do a lot of shooting from a blind under a big Oak in my yard. It is dark under there, but the birds like it, and the seed the squirrels dump from the feeder. The built in flash is often just not enough. I use 2 units that are triggered by photocells, I can set them out where I predict I'll need the light. One is an inexpensive unit, OK, just a bit fragile, the other a Vivitar 283 with a photocell trigger added.
A note on the Vivitar and other older flash units, they often have trigger voltages high enough to fry a digital camera, when in doubt, DON'T use them in your cameras hot shoe. The little Phoenix unit is digital safe, the Vivitar isn't, although newer Vivitar units work just fine.