I do hunt ducks, but I am more and more shooting them with the camera.
you'd be amazed at the number of people that end up making that same transition
As for the shot some comments:
1) your certainly going about things the right way - getting low down with the birds. With wildilfe its often best to try and shoot as close to or lower than their persepective - a low down shot is a sight that most people don't see (since we tend to watch whilst standing) and so makes for a more interesting shot
2) I tend to find that shooting in the daytime with a clear sky and bright sun that its best to use exposure compensation to underexpose the shot (set it to a negative value up to -1 should be fine) and thus preserve the highlights from blowing out -whites are always a pain though.
2) mode and settings wise I would recomend looking at aperture priority and shutter priority. Each of those modes will let you set the setting for the priority in that mode as well as the ISO, whilst the other setting is auto set by the camera to fit the meter (which will also respond to exposure compensation). The advantage is that these modes will respond to lighting changes faster than you can which makes them very popular with action and wildlife.
Normally I recomend starting in aperture priority since aperture defines the depth of field of a shot (area of a shot in focus) however more and more I am starting to shift to shutter priority (especially when the lighting is poorer) since whilst one can save a darker shot, one cannot edit out motion blur from an animal. Fast speeds are always needed - I think 1/1000 or faster is a birding speed (very very rough guess that).
3) 150mm is definatly pushing your skills at getting close to the subject, if wildlife and birds are your thing look to investing in a longer lens. I would recomend a prime (single focal length lens) since they tend to have bigger max apertures (that means more light gets into the lens when set at a larger aperture (smaller f number) which improves performance in low light conditions; and they also tend to have better image quality than zooms.
A 300mm lens is often the wildlife minimum focal length with 400mm and 500mm being recomended as birding lengths (Because birds tend to be very very small and flightly subjects).