I'm quite happy with my 100-400mm. I started with a 300/4 IS, but wanted more focal length, and I still wanted IS. In the <$2K range, that left me with the 100-400mm. I've considered the 400/5.6 as well, but I really wish it had IS - the IS is quite helpful for static subjects, and owlets in the nest qualify! The 400/5.6 is great for birds in flight, where you need a high shutter speed, but otherwise, I'd plan on using it on a tripod. FWIW, both the 100-400mm and the 400/5.6 have "moderate dust and moisture resistance for their switches, focusing rings and zoom rings where applicable." (quote from an email exchange I had with Chuck Westfall, Canon's technical guru).

The other thing I like about the 100-400mm is it's relatively compact length - when retracted, it's about the same length as a 70-200/2.8, and fits in a Lowepro Toploader Pro 75 AW case. The 400/5.6 doesn't retract, and it's a pretty long lens. OTOH, the 400/5.6 is a little sharper away from the center, and the AF is slightly faster than the 100-400mm. Admittedly, I was pretty tempted by a 400/5.6 that came up on my local Craigslist for $800, but I passed.

Since you mention owlets, here's a relevant shot with the 100-400mm:


EOS 7D, EF 100-400mm f/4.5-5.6L IS USM @ 400mm, 1/500 s, f/5.6, ISO 400