nice looking goose you got here - Now I've taken time out to see how you've shot this & according to NX you shot this spot metering, f/7.1 & 1/1600s - - no E.V. compensation.
Now, as a Nikon shooter as well, I find I love my D300's matrix metering. It meters like a champ. I don't like spot metering cause it messes me up. But that's an individual thing...
To start with. I'd recommend you shoot this lens at a f/8. I would also recommend you place focus on the eye (when in the shots - - cute shot of feet you've got here...) of your subject.
Now, what you have here is a white bird. White is a color that easily over exposes, especially in sunlight. E.V. compensation to keep that to a minimum is a must. Look at your histograms & make sure you protect your highlights through them. If you're not following this, please feel free to PM me. I just went through teaching a friend how to use her D80 last week.
I like the framing of the first shot.
Looks like you're having fun & that's the most important part.
Good luck & I'll be glad to help if you'd like it.
Welcome to the forum. Good advice from Lil. #1 and 2 are very nice captures. Little bit soft but looks okay. Any consumer zoom will be little bit soft at long end. Nikon, Canon, Sigma …no difference. As Lil said one approach is try to close the aperture or focal length reduce to around 250mm.
Looking forward to your future posts.