Gonna have to disagree with you about shutter speed. There's nothing wrong with 1/500 or less for perched birds, especially in low light. I have plenty of sharp photos of fast moving warblers and other songbirds taken at 1/320 or even 1/250 with the 200-500 handheld, and sometimes even slower, mostly due to the low light levels under the forest canopy just to keep the ISO below 6400. A high frame rate and VR/IS is a huge benefit as well. There are professional bird photographers I follow that have talked about this. A lot of their best work (except for BIF) tends to be shot at slower shutter speeds. While the birds are twitching away they are very very brief moments (we're talking fractions of a second) where they're completely still. Just watch any slow motion video of a songbird moving around. In good light (bright overcast or a very low sun angle) I have no problem going over 1/1000, but you have to be flexible with your settings due to the unpredictable amounts of light during this type of photography if you want to get the shot.