I wouldn't rule out the careful use of flash if things are getting too dark for available light photography. I've seen some absolute belters taken in pitch dark, in fact my favourite wildlife photo ever was the one at the World Wildlife Photographer of the Year 2007; a photo of a bat fishing over a river that'd been taken using a strobe flash. Every time the flash strobed it created an image of the bat on the picture so the final pic was a composite that looked like several bats in various stages of catching the fish. Awesome, just awesome. Didn't win much to my displeasure
