If you're really thinking of low light, canopy cover, dark areas, a monopod and a lens with IS will go a long way. Even with long focal-lengths. Monopod + IS just makes for a very forgiving platform so you can use virtually any lens as long as it has IS along with it to just add that extra stabilization on top of the monopod's stabilization. Great combo for low light without being super imposing to carry around (like a tripod). So you can use a 70-200 flavor with IS (F2.8 can be great, but if you're very close to the subject at 200mm and at F2.8, do you really want that super shallow DOF so that something on the subject is out of focus? Things to consider, fast focal-ratio has a compromise in this sense, while it's nice for exposure, it can be a problem sometimes for DOF on some subjects that you want entirely in focus if you're close to them). Your 100-400 could totally just be the lens you use, at F5.6, on a monopod. No purchase needed for a new lens. A monopod will stabilize it that well.
Example, under heavy canopy cover, here's a 150-600 on a monopod with IS enabled, and one with a 300mm F4L IS on monopod with IS enabled.
F6.3, 1/40s, 500mm
IMAGE LINK: https://flic.kr/p/JexMpC
IMG_3647
by
Martin Wise
, on Flickr
F6.3,
1/50s, 552mm
IMAGE LINK: https://flic.kr/p/FLVsnh
IMG_2574
by
Martin Wise
, on Flickr
F6.3,
1/60s, 600mm
IMAGE LINK: https://flic.kr/p/FgLyGX
IMG_2579
by
Martin Wise
, on Flickr
F4.0, 1/100s, 300mm (this is my wet setup, I like to drag the shutter for things like rain, etc).
IMAGE LINK: https://flic.kr/p/Kvg6cN
IMG_2082
by
Martin Wise
, on Flickr
Very best,