Nikon's own AF-S 80-400mm f/4.5-5.6G VR is excellent... "pro quality" or nearly so... It's also your "fastest" option. I.e., it maintains larger apertures to zoomed focal lengths well beyond what the 3rd party lenses do... Although considerably lighter than any of the 150-600s, it's about 1/2 lb. or more heavier than the 3rd party 100-400 lenses and rather pricey ($2300).

Sigma's 100-400mm f/5-6.3 OS HSM is the least expensive ($700), but there's no means of attaching a tripod mounting ring. It's slightly smaller and lighter than the Tammy, but also the slowest... It starts out at f/5 and the variable aperture steps down at shorter focal lengths than the others. For example, it drops from f/5 to f/5.6 at only 112mm. The Tamron starts at f/4.5 and doesn't stop down to f/5.6 until 181mm. Both these lenses end up at the same f/6.3... the Sigma at 234mm and the Tamron at 281mm. (In comparison, the Nikkor and Canon variants end up at f/5.6... the Nikkor doesn't stop down that far until 250mm and the Canon until 312mm).

Tamron's SP 100-400mm f/4.5-6.3 VC USD is $800, a little more expensive than the Sigma, but has the option to add a tripod mounting ring. There's add'l cost of $129 for the ring, but I wouldn't want to have a 400mm lens without one and Tamron's is a neat design that incorporates an Arca-Swiss compatible dovetail (I really wish all manufacturers would do that!) As noted, it's slightly faster than the Sigma through much of it's zoom range.

As a Nikon shooter it doesn't help you, but IMO the Canon 100-400mm f/4.5-5.6L IS USM II is the best of the bunch. It's the only one of them that uses fluorite, which helps it have superb image quality at all focal lengths, is pro quality build and performance, and while not cheap at $2000, costs a little less than the Nikkor.

All these lenses have image stabilization and high performance ring type ultrasonic focus drive. They weigh roughly 3 to 3.5 lb. (with hood and tripod ring, where avail.) and are pretty close to the same size. The Nikkor and Canon are fairly large diameter and use 77mm filters. The slower third party lenses are a little smaller diameter (which is why they have smaller apertures) and use 67mm filters.

Any of these lenses will outperform "do it all" zooms like 18-300mm and 18-400mm in most ways.... better image quality, faster auto focus, better build, better weather sealing, yada, yada.

For BIF specifically, I don't use any longer than 400mm... have often used 200 to 300mm, in fact. Some critters will be out of reach, but longer than 400mm on an APS-C camera is often just too difficult to manage for fast moving subjects.

None of these lenses are great candidates to use with a 1.4X teleconverter. It's possible, but especially with BIF where you need fairly fast shutter speeds, you'll need very good light since about the best you'll get with a TC added will be f/7.1, f/8, or f/9 apertures.

You can compare these lenses' specification, sample image quality test shots and much more at https://www.the-digital-picture.com/Reviews/Tamron-100-400mm-f-4.5-6.3-Di-VC-USD-Lens.aspx

Bryan doesn't fully review most Nikon gear (he's a Canon shooter). But he has informative reviews of both the 3rd party lenses and some info/image quality samples for the Nikkor as well, making possible close comparison.

Below also can be a useful means of confirming and comparing the specs of up to four different lenses:

https://www.bhphotovideo.com/c/compare/Nikon_AF-S_NIKKOR_80-400mm_f_4.5-5.6G_ED_VR_Lens_vs_Tamron_100-400mm_f_4.5-6.3_Di_VC_USD_Lens_for_Nikon_F_vs_Sigma_100-400mm_f_5-6.3_DG_OS_HSM_Contemporary_Lens_for_Nikon_F_vs_Canon_EF_100-400mm_f_4.5-5.6L_IS_II_USM_Lens/BHitems/936121-REG_1362803-REG_1321313-REG_1092632-REG

Grace, we have an equal mix of Nikon zoom (and single focus) Nikon and Tamron glass. If all I had was three days to rent and shoot, our V1/A011's (AP150-600mm) might not have ever had a chance to stay; it seems awesomely huge if you aren't able to have extended time to get used to it. We bought an open box/demo G1, and loved it so much, we bought a second new copy when the G2 came out, and the price dropped. We tried the G2 for two weeks; for the price and bit of extra weight, I couldn't figure what the buzz was about. The original A011 did and does everything for me as well as the G2 did in side by side tests, on every body we tested, DX and FX. If you have tender/delicate wrists, isometrics and other exercise can help; I pick up our Shitzu several times a day; he's much more of a handful than this lens. Arthritis made the G1/G2 a love it or leave it proposition, and after a week, it was here to stay. Never tried this Tammy with a monopod but tripods and gimbals are made for this one, and a tether is an indispensable insurance policy. I don't do extended outings over a couple of hours due to a shoulder replacement, so exercise keeps me working; wife says it too big for her to use but doesn't mind hanging onto it while I'm futzing with other gear. Packing the $10K Nikon equivalent was much more of a chore. I'll spend an hour with the 150-600 each day, handheld or mounted on the tripod/gimbal, tracking the local eagles and various other critters roaming our sky and property. We have woods and two connected valleys; can't beat country living. Recently saw Adorama had of couple of pre-owned G1s for $600, in extremely good condition; they vanished quickly. I wish these had been available around in decades when I was working/traveling cross-country in film/wooden-shutter days.

My 2nd Tammy favorite is the 44oz 175D, 200-400mm AF, used whenever the A011 isn't. It was panned by over the years, and they're rather hard to find. Skip the $35 ebay examples, but good ones bring upwards of $175; we found ours at Adorama and wouldn't ever with it. It's great handheld in the car, sharp/fast and good on a monopod or gimball. I like the Nikkor H300 Auto AiS as much the 175D. The Tammy 185D 28-300mm went out of production in 1999, it's short/light/fast and isn't an 18-400 VCII by any stretch, which is what those two would be replaced with. The little B018, 18-200 VCII is sweet and the shortest zoom we use. Normally don't wax on about the joys of our beloved older Tammys, but in context the 150-600mm is already classic and destined for greatness in the large lens hall of fame. Good luck whatever lens you choose and happy shooting. sv