Building on the earlier suggestion about balance and weight... I've been super cautious picking my first long lens because honestly, the thought of a $2k setup toppling over gives me major anxiety lol. Here is what I am looking at for a budget-friendly but safe start:

• Fujifilm XF 150-600mm f/5.6-8 R LM OIS WR: The reach is incredible for small birds. Since the barrel doesnt move when you zoom, it stays balanced on your tripod. Just be careful with the slow aperture in low light tho, it might get grainy.
• Fujifilm XF 70-300mm f/4-5.6 R LM OIS WR combined with Fujifilm XF 1.4x TC WR Teleconverter: This is way lighter and easier on the wallet. It gives you decent reach without the bulk. I would suggest this if you are worried about carrying heavy gear through the woods or straining your back. Whichever you pick, maybe get a really sturdy strap too? I am always worried about the mount snapping if I just let a heavy lens dangle from the camera body!

So basically, the thread has already covered the main zoom options and the reality of weight vs. tracking. I totally agree with the point that nailing the focus is way more important than just having the longest reach. Just a few technical bits to add to the mix that I've noticed over the years: * Internal zooming (where the barrel doesn't extend) is a huge advantage for keeping your setup balanced on a gimbal or tripod.
* Look for lenses with Linear Motors (LM); they handle those tiny focus micro-adjustments during high-speed bursts much better than older motor types.
* Remember that using a teleconverter doesn't just cost you light—it can also noticeably slow down your autofocus acquisition speed in tricky conditions. But yeah, like others said, focus on the tracking. Also, if you buy used, check the firmware version immediately. I’ve seen so many people complain about 'soft' glass when they’re actually just running outdated AF algorithms.

Yep, this is the way

I used the Fuji 100-400mm for a while. It's decent, especially for the price used (~$1400). Sharp enough for birds, though reach can be limiting. Depends on how close you get!

Consider adapted vintage lenses! Cheaper, but check sharpness; might need professional help with adapters!

Used XF 100-400mm. Image quality is great, and you'll save big versus new. No complaints!

Bird photography's thrilling! But safety first: consider weight distribution! A heavy telephoto like the Fuji XF150-600mm (amazing reach!) plus a lightweight tripod can be a topple hazard, especially on uneven terrain. Always use a secure tripod head and anchor it properly. A fall could damage your gear AND you! Be aware of your surroundings!

Beyond lens choice, consider Fuji's service intervals. Birding often means dusty/damp environments. Factor professional sensor cleaning (around $150) annually to maintain image sharpness and avoid costly repairs. Be careful about DIY cleaning! I'd suggest budgeting for this *before* pushing your lens budget to the limit. Regular maintenance is key!

Honestly? For birds, consider a used Sigma. More bang for your buck, especially in that price range.

Hey! Birding in Arizona heat/dust is tough on lenses. Consider a used Fujinon XF100-400mm. It's decent, around $1200 used, leaving room for a good weather-sealed bag. Do you get much humidity where you are? That can affect lens choice too, I think. Good luck!

DIY lens cleaning's risky! Fuji's 100-400mm's great, but maybe get a pro to service it? I've seen DIY go wrong.

Owned the 100-400 for 5 years; great reach, but had issues with sharpness. Consider the used market for better value.

Forget pure reach; consider the Fuji XF100-400mm. It's all about tracking speed. Pair it with a fast body. Expect around 1.4 kg; see if you can handle the weight. Sharpness is decent, but nailing focus is key.