Attached is an image of my Canon 7D and 580 EX II Flash settings. I am currently using a 400 mm f/5.6 telephoto. I have three custom mode settings selected which I use for bird photography, stationary, birds in flight, etc. I prefer to shoot in Tv mode (1/400 sec) to ensure relative stability when handholding on stationary subjects. And I employ Auto ISO, so that at 1/400th and f/5.6, if there is still not enough light, the ISO will theoretically ramp up automatically. The flash is set to HSS and Manual mode and I simply adjust the power for varying degrees of darkness.
Most of the time this works very well. But occasionally when in dense foliage, my exposures are made at less than 1/400 sec and are consequently unsatisfactory due to blur. For instance, I just captured a chestnut-sided fledgling at 1/160 sec, f/5.6, and ISO 400. In fact, all of my shots with flash and Beamer seem to be at ISO 400.
So, the first question is why, in Tv mode, would my shutter speed ever drop below that which I have chosen? And second, why are my flash/Beamer shots at ISO 400 when Auto ISO has been selected?