At ground or water birds, mounting the camera/lens very low is the preferred shooting mode. The low angle gives a very unique, birdeye-level perspective of the subject, plus the BG and FG would melt into glorious bokeh that isolates the bird.
Stalking an active bird in the low light of a forest stream , while the Sigmonster is mounted very low, is not quite easy, particularly for waistline-challenged shooters like me.
It automatically would mean muddy belly and seat of pants, plus sore lower back. But it's fun nonetheless if one can get a half-decent shot at the subject...
Here's my attempt to shoot low at a Grey Wagtail yesterday - 20D + Sigmonster, ambient undercanopy low-lighting, 687 mm, f/5.6, 1/160 sec, ISO 800, manual exposure.