1. Your first sample shot appears to be entirely soft. Even though you used a reasonably fast shutter speed (1/1250), that could be due to camera shake blur.

2. The second shot of the three pelicans is actually reasonably sharp and in focus. It may be about as good as you can expect from a lens that age.

3. Third shot appears to be reasonably sharp closer (look at the closest birds in the water), but soft in the middle distance to infinity. It simply appears that you weren't focused to infinity. This also could be an atmospheric effect, which may have made matters worse, or might even nothing to do with the lens.

You should try a more formal test of the lens. Put everything on a tripod, aim at a detailed, flat subject like a weathered fence or brick wall, line up so that the camera's sensor plane is as parallel to the flat target as possible. Now lock the mirror up or use Live View (which also raises the mirror), focus very carefully and take one or two test shots. Use a remote release (or the self timer) so that you aren't touching the camera when the shutter trips. Live View also may allow you to "zoom in" on the preview to see if you are focused accurately. Try this at different apertures. It's quite possible the lens is sharper at some f-stops than at other. (All your test shots appear to be done at f/8, if the image EXIF is to be believed.)

If you have a filter on the lens, remove it and try without. Depending upon the quality of the filter, some of them can have a pretty nasty effect on image quality.

Basically, with tests like this you try to eliminate everything else that might effect image sharpness, so you're truly only testing the lens sharpness when focused. Obviously you won't be able to use these "ideal" focusing methods with active subjects, especially fast moving, difficult ones like birds in flight! Those are about the toughest type of subject to focus manually.

If you see a lot of variability in focus accuracy in your test shots, it may be wear in the lens focusing mechanism. I don't see any variation in sharpness across the image frame in your samples. So I wouldn't suspect a problem like a de-centered element. That usually shows up as part of the image soft while other parts are sharp. Of course, these particular images might simply not show where there's a problem. Test shots like above will confirm whether or not there is any problem like this.

I do see some significant chromatic aberration in the three pelican image (didn't look that closely to the other two images, it may be in them, too). It's mostly off-center... most prominent in the bird's wings nearest the edges of the images. This may simply be the nature of the lens and isn't uncommon in telephotos (modern ones often use super low dispersion and fluorite elements to reduce chromatic aberration.... I would guess a lens that age uses neither).

I hope you don't mind.... I tried sharpening the three pelican image in Photoshop, as well as some work to reduce the chromatic aberration in it... To me it looks pretty good up to 66% or maybe a little more. Maybe more could be done if working from the original image, rather than this JPEG. See what you think...

Have you tried using the focus confirmation that your D750 has built in?

It may work differently on your camera, but I still shoot film on my Nikon F100 (sweet camera) and it has it. On the F100 there are two arrows in the lower left corner of the viewfinder, one points left and the other points right. There is a dot between the arrows. If one of the arrows is lit then that indicates the direction you must turn the focus ring to achieve focus. When focus is achieved, the dot between the arrows will light. On your D750 I believe there will also be a "beep" when you're in focus.

Check your manual and give it a try.

I think it should work with AI lenses, but all mine are AI-S so I'm not positive.

You're probably heard all the hype about mirrorless cameras? One of the reasons is how they enable / enhance manual focus lenses. Take your coot for an example. We can download the image and click to the 1:1 details and see the missed focus.

Now imagine if that 1:1 detail had been available in your view finder for you to 'see' and accurately focus the manual lens on the bird's head in passing. Imagine too if you could confidently shoot at a slower shutter knowing the in-body stabilization will handle any camera shake.

Imagine if you could zoom to the details in the EVF (Electronic View Finder) of those far buildings at Point Richmond and focus exactly on those houses and capture a sharply focused image? If you had set-up a tripod and used the zoom display on the DSLR's rear LiveView, you'd have a similar ability to see and focus on those small-in-frame details.

Finally, imagine if the EVF also highlighted with a yellow (or white or red) shimmering color telling you where the camera was focused. That shimmer is most useful when you can't zoom to the 10x details in the EVF for a moving subject. As these pelicans flew by, you could use that shimmering color to help judge the plane of focus and help to adjust to a bird as it passed. Although, BIF is a challenge even with an AF lens.