I got started with birding with a Nikon DX (i.e. APS-C) Kit-level 55-300 zoom, which maxes out at 450mm FF equiv. One capture at least with that one I shared here: this one.
Looking over my collection of images rated 3 stars with that lens, I note 2 things:
- Quite a few are of larger birds - pelicans, great egrets, swamp hens, swans
- But not a few are of small or even tiny birds; yes, you do struggle to fill the frame with those, and cropping down does impact the quality.
I could lust after long glass with price tags approaching $10,000 (AUD), but they were never realistically an option for me. I ended up waiting for a sale at a time I was expecting my tax refund and got a Sigma 150-600mm (FF, so 225-900mm on APS-C) in their (C)ontemporary series - which is the cheapest - and spent a little under AUD $1400 in 2019. More expensive are the (S)port and still dearer are the (A)rt. But the quality does not necessarily increase as the price does, according to some reviews, and the contemporary is already quite heavy enough, thank you.
Is this Sigma Contemporary as good as perfect for me? No: it has its best sharpness in the mid-range, and the fastest aperture decreases out to f/6.3 at the long end. It is, however, the best I have by a large margin. And at 300mm it leaves the Nikon DX kit-level for dead.
You can currently find this lens for Nikon-F mount for just over USD 1000 online