Your photos are ok, sharpness is good for most of the pictures. The gull is a bit soft, but this can be fixed by adding some sharpness.
You need to look for better light, better angles, better backgrounds to make your photos nicer. Try to shoot from the ground level, make pictures only when the birds has a nice angle (by example don't make pictures with the birds from behind).
300mm+MC-20 is a joy to use. The big focal length is so confortable, that you can stay away from subjects and get nice shots.
But this huge focal length is sensible to some factors :
-when the subject is more far away (+50m), the haze can affect the IQ;
-focus is less precise, hesitant in low light;
-many times to subject comes too close and will not fit into the frame.
-dificult to find the subject quickly, especially BIFs.
If I am very pragmatic, I would say that it is better to use 300mm alone, without MC-20. You will have a higher keeper rate, although not so many pixels on subject (you need to get closer). Use MC-20 only when you have a static subject and good light.