I'm a proud member of the cropping club. I crop 98% of my bird images. Often fairly deep. Sometimes down to only 3-4MP left starting from 51MP
I'm totally happy with cropping and mostly just post online so even my heavy crops of say 3-12MP remaining still hold up on forums and Flickr and of course on cellphone dominant IG.
Unless you aim for a more "small in the frame" or "birdscape" type of portfolio (Ray Hennessy type work) I think one has to crop if shooting songbirds or smaller. There just aren't lenses with enough maximum magnification values (even with TCs) that can really fill the frame with smaller birds except for lenses that require you to get within 3ft (like a 100-400 type lens with TCs).
I don't think you will have much fun with bird photography if you force yourself to never crop.
And think of this scenario...two photographers standing next to each other shooting the same subject....one has D500/500PF and the other D850/500PF. Both are after a similar final composition. D500 owner manages to frame up his composition perfectly in the VF...no cropping done in post. D850 owner crops his image to the same composition in post. Because those two cameras have almost identical pixel density the final images are the same resolution. Should the D850 owner be upset that he had to crop? Is the D500 owner doing anything "better" by not having to crop? I'd answer no to both. Is the D850 owner wasting his MPs? Well only if he has to crop to DX every shot....then he probably wasted $$ over a D500. But I bet that D850 owner finds times he doesn't have to crop all the way to DX and then maybe gets to shoot landscapes with wider lenses or human portraits etc where his FF sensor gets him some benefits.