Your focus appears to pretty much spot on. I downloaded the image and zoomed in and the bird was pretty sharp. One suggestion that goes with what Linda said about depth of field especially with a long lens; try closing down the aperture to say f16 or so. That should bring the rest of the tree behind the nest into sharp focus. Depending on light, you may need to adjust your ISO a bit. Doesn't look like you had too much to work with in the sky that day. A trick someone taught me is to take the highlights down all the way and bring the shadows up all the way and start to work from there. If there is any detail in the sky, it should start to appear. Nothing worse than a day with a pure grey or blue sky with no contrasting clouds. Last suggestion; get Lightroom to do your post processing. I have both lightroom and PS and the process is much easier in Lightroom. Unless you are an absolute expert in PS, try lightroom. I think you will see a difference in your processing of images.

Keep on keeping on with your shooting. It's how all of us improve.

Digital1022

You mentioned that that you had to crop significantly, so perhaps the distance worked against your getting an accurate spot metering reading.

Regarding color of sky: unless it was very dark blue, you are likely to end up with whiter like shown here because of the relative darker color of the birds. If you shoot in raw and are learning processing, you can probably glean a bit more color from the sky, especially since you indicate the shots are slightly under-exposed. Personally, I think the light sky here is an eye-catching contrast to the subjects and that cool tree

Very appealing series! The type of tree (some sort of nut?) adds interest.

The distance between subject and its background does affect the depth of field. As you can see, some of the more distant elements (such as in #2) are more out of focus than ones closer to the birds.

A handy site:

http://www.dofmaster.com/dofjs.html

(I have never used a teleconverter, so will leave that question for someone else)

Would you elaborate on what you meant in your comment, "My sky was still pretty white in spite of my focusing on the bird" so that we can discuss exposure further? Thanks!

The best opportunity for good bird images is when you have the sun behind you. Practice doing that and you will be surprised how tour images will improve. Also, keep you shutter speeds high with this long lense, 1000 or more, to begin with. As your shooting techniques improve you can start going lower for stationary subjects. But remember, if the bird moves quickly, you will never get it at 1/250. And ff coarse, getting more bird in the frame is better. Chasing birds can work on some subjects, but figuring out where the bird is going, it's habits, will help and you patiently wait. For instance, if I am chasing birds in the field, I may be out of luck. If I find where a pair are nesting, I set up, with the light behind me, and wait. Both birds will show up in a bit. Good luck!

If you want to try to improve your chances hand holding try a pistol grip ($15 or so on Amazon/Ebay). I have a Tamron 150-600 that I used to shoot hand held with a grip. Now I mostly use it on a tripod with gimbal head and my hand held lens is a Canon 100-400L mark II. see attached image for one of my cameras set up with pistol grip and Red Dot sight for finding and following Birds In Flight. When hand holding high shutter speed, image stabilization and props like fences, poles, trees, window sills etc are your friends.
Hand held or tripod, keep the shutter speed up, even if the bird is sitting they make a lot of little darting motions etc and if it does take off you have a chance at a BIF shot that a slow shutter speed will never handle. Carefully set your first shot and use short bursts, esp if the bird is moving around, it will give you a choice of poses to pick from. To get the high SS use higher ISO, your D750 should handle 1000, 2000 or 3000 well in good light, esp with a good noise reduction app in post processing.

Google "Backyard Bird Photography" or just "Bird Photography tips" and you will get lots of things to read that can help. There is a magazine "Birds and Blooms" on planting a yard to attract birds and they have articles on photographing the birds also.

Once you get a few techniques learned, practice, practice, practice. With digital it is easy, back in the film days it got expensive to do a lot of practice at only 36 shots a roll of film plus developing, the wait to see how it worked etc.
Digital, big memory card(s), extra batteries, a good cataloging and sorting program like Light Room and cull/delete the mistakes after you have learned from them.

Keep it up, happy practice and enjoy.

Fran, the camera work is fine - good exposure, you posted images that did not have any movement, etc. Good stuff. However, a little post processing could make these special. Saturation is fine, but revealing a little more detail in the head, revealing detail in the plumage, softening the background and foreground, etc may help. Also the white and color balance is similar for 1 and 3 but different for #2.

There is no need to go to 1/500 sec and give away image quality to noise if you have a steady hand (which is obvious by what you posted). Best practice is to shoot at the slowest shutter speed that gets you what you want, and then adjust the ISO to give you a good exposure.

I do have one question - your metadata shows the camera to be a D750, but the lens is a Sigma 50mm F2.8 Macro, so you may have selected the wrong lens profile. Also, the focal length is identified as 600mm, but the effective focal lenght in 35mm equivalent is 900mm, suggesting that somehow your software is also thinking that you are using a crop sensor camera. Lastly, the metadata is showing a .02 and a .03 circle of confusion - so it is totally confused.

Don't stop, keep experimenting, and keep up the good work!

Fran, the camera work is fine - good exposure, you posted images that did not have any movement, etc. Good stuff. However, a little post processing could make these special. Saturation is fine, but revealing a little more detail in the head, revealing detail in the plumage, softening the background and foreground, etc may help. Also the white and color balance is similar for 1 and 3 but different for #2.

There is no need to go to 1/500 sec and give away image quality to noise if you have a steady hand (which is obvious by what you posted). Best practice is to shoot at the slowest shutter speed that gets you what you want, and then adjust the ISO to give you a good exposure.

I do have one question - your metadata shows the camera to be a D750, but the lens is a Sigma 50mm F2.8 Macro, so you may have selected the wrong lens profile. Also, the focal length is identified as 600mm, but the effective focal lenght in 35mm equivalent is 900mm, suggesting that somehow your software is also thinking that you are using a crop sensor camera. Lastly, the metadata is showing a .02 and a .03 circle of confusion - so it is totally confused.

Don't stop, keep experimenting, and keep up the good work!