[Rondec]

I think if you could shoot at f8, get closer to your birds, you would have better results. Use as low iso as you can while at the same time maintaining decent shutter speed. Long lenses are tough.

I really do think the biggest thing with birding is getting close to the birds. A long lens just isn't a substitute...

[Lowell Goudge]

Lets consider a few things here

First of all, the bird you are shooting is about. 125mm (5 inches for the metric impaired) assuming shot at 1/500 you are getting about 1/8 of the 16mm frame height on the sensor, therefore the image size is 2mm. Considering image size = subject size x focal length / distance and re-arranging to calculate shooting distance you are apparently 31.25 meters or 100 feet away

MOVE CLOSER the finch could be approachable enough to fill the frame, they are pretty tame creatures. (tomG can fill the frame of a 300mm with one) Also consider that when you crop in by a linear dimension of 8x the blur from any vibration is magnified by that amount, so the recommendation of shooting at 1/focal length is a little optimistic.

Do not get overly concerned about maximum DOF, shoot at about wide open minus 1 or 2 stops. I find shooting in Tav mode with auto ISO, and a shutter of 1/000 and aperture as suggested, perhaps F8 will give you good results. The lens is at its best at those apertures.