I'd try a little lower ISO sometimes that does the trick for me t Roy 1600 then 800 see if that works .
I usually shoot these birds on Manual (not sure what Nikon is )
Sometimes manual at 250-1000 speed depending if they're moving slow or flying off . if you have low light of course you'd have to have a f4-5 or even 2.8and I hate to say it sometimes it can be the lens . And if all else fails put it i auto and see what it does for you I use the sigma 150-500 hand held and just love it my bet is it's the lens .
I hope some of these suggestions will help fix your problem .

well i like the dear in the snow allot if you put a vignette around them that would be really nice.
I know it s getting to be summer now so id say that the Mallard is the next choice just possiblycrop a little that a very nice sharp photo.

good job your on the right track love the photos .

such beautiful animals love your photos.

oh wow , really cool. Glad you have the one pic maybe you'll get some more in the future.

wow what fun to watch all those critters and get those shots.

here's my .2 cents worth , i shoot raw also and if i make a mistake with the exposure I have plenty of wiggle room to correct the image that's the beauty of RAW but also we should be able to correct in the camera first not rely on an editing program as we all know.

Id like to see a tighter crop on the 1st three even when photo graphing them .
the 4th on is just a "record shot" cant do anything with it way to busy and to much to clean up .
5th one tighter crop when shooting .(zoom in more if you can )
6th one this is nice i think it tells a story , it looks like he's thinking, so how about a vertical crop clone out the scraggly branch coming out of the tree .

If there's shadow you may have to move around your area .
Just some friendly advise thats all .
keep shooting .

lovely photo

How about sending in some of your Owl photos .

Here's a few more from the Waco zoo .
I just love the look on this Inca Tern Bird and the way he tilted his head i only wish id had backed up a little more but he caught me off guard as he came up to the window faster than i could get ready .

Oh and the King Buzzards were so fascinating to watch , on top of being somewhat different looking i couldn't help but enjoy there colors .

Inca Tern


King Buzzards

Awe that's so cute .Nice shot .

oh yes much better crop , Love the Rose id like to see it more vertical (that's just my preference)
and wow the Bee inside is so sharp and interesting , awesome .

thanks i just love these .

Here's a couple shots i did , mostly using a 300mm lens on manual ISO was 100 don't remember the rest but keep at it .
There's a full moon coming up Aug. 1st and 2nd so try it again.

Just moon & Clouds


Twilight Moon


Full Moon

Oh my goodness i just love the chimp on the steps that's hilarious and the cougar is so beautiful id love to hug him or her but we all know what would happen . lol.

Lets do some zoo animal photos .

Eating


Lemur family


Caged Tiger