>I have taken over 5,500 pictures with my D7000. Over the last >two weeks, all the pictures with ISO over 400 are pixelated. >This was never an issue before. Any suggestions?
Small birds are a serious challenge to photograph. They move very rapidly - even with shutter speeds faster than 1/1000 images can be blurred by motion. I've had plenty!
You were shooting in fairly poor, low contrast light at elevated ISOs and cropping severely. Any applied sharpening will cause pixelation because you're actually seeing individual pixels. The last image, although larger has the plane of focus on the tree in front of the nuthatch. This is another challenge when photographing small birds from a distance - being sure that you're focusing on the intended subject.
The only real options to get good, crisp shots of small birds are to buy a big lens and get close, or use a smaller lens and get REALLY close.
All of these were shot with a 500mm on a D7000 at 8m or closer and most of the smaller birds still involved some cropping. The starling was shot at a little over 4m. The MFD for my 500mm is 4m.
One approach to getting close would be to set your camera up on a tripod close to the feeder and trigger it with a long cable release or radio-controlled release. The birds will get used to it quickly and soon ignore it. There are also very clever low-cost tethering options. I can tether my camera to my PC, and control the camera over my home wireless network with my phone to take photos of birds in the garden as well. You could also use a hide to get you and your camera closer to the feeder.
Using an off-camera flash fired though a small umbrella would give extra light and allow you to get your ISO down to 100 as well.
I'm still a novice, but have quickly learned that photographing wildlife is very challenging, and requires patience, persistence and guile to get close to your subject. It's one of the most frustrating areas of photography, but also one of the most rewarding.