I have been shooting pix in the Arsenal for many years; so much so that I kinda stopped going a few years ago. The last straw was the dropping water levels in Lower Derby Lake (due to Colorado's drought) which essentially shut the arsenal down as a winter fishing ground for bald eagles. There were times when I saw a dozen eagles in the trees surrounding Lower Derby Lake, and there was a roost tree where I saw dozens of eagles in that one tree on a few occasions in the dead of winter. I also have some (lousy) pictures of a bald eagle standing on the edge of the lake ice one day and grabbing a fish out of the water. I treasure the memory, even though I didn't get any good shots. The eagles have disappeared as the fish disappeared from the lake.
The 'original' wildlife drive is now a service road (no public access), but the 'new' wildlife drive goes deeper into the arsenal and exposes visitors to more habitat (a good thing fro photography). When I was a regular on the old wildlife drive, the herd would cross that road at a particular spot in early to mid-afternoon almost every day. You could pull onto the shoulder at the crossing area and when the herd came through, there would be bison within feet (sometimes inches) of your car. They are huge animals, and I feel sorry for the occasional tourist I read about getting gored up at Yellowstone (or elsewhere). I swear by the signs someone else mentioned in this thread - I stay in my car on the wildlife loop. With the new loop (and my declining attendance), I have not timed the bison crossings any more. But, I still did get lucky from time to time with the herd crossing the new loop at or near my car.
I decided (for old times' sake) to go back just a few weeks ago on a snowy weekend afternoon. Lower Derby Lake is now a sandy depression in the prairie, with no significant water in it. No signs of eagles anywhere, but I did see a few hawks around (quite possibly one of the red tails the OP shared). Deer and prairie dogs were around (as usual), and I had the good fortune to get caught in a bison crossing. Here's an image:
If you are in the Denver area, I would certainly recommend the trip. Depending on how fast you drive and the number of photo stops you make, the loop can be a 60-90 minute trip, and there's also a trail network (outside of the bison area) with some decent hikes. The east end of the Lake Ladora trail is a common area for deer sightings. The cattails and trees in this area provide good cover while the deer come off the prairie to get a drink from the lake. The pontoon bridge also gets you out onto the water for some interesting compositions.
It's busier, but the Lake Mary trail can offer some really good water fowl shots with the small island in the middle of the lake, and that trail also has a pontoon bridge (although it can be clogged with fishermen). There used to be a huge beehive in a hollow tree on the northwest side of Lake Mary. I'm not into bee photography (I have no interest in getting stung), but there were always lots of bees around it in the summer. The tree was still there when I visited recently, but there was no bee activity (maybe just due to the weather).
If you are a history buff, the new visitor's center has a decent exhibit describing the history of the arsenal, which can help you understand the overall layout of the place. They also have a full-size mounted bison in the lobby, along with other wildlife displays.
