It's not necessarily saying wonderful results can't be obtained with many different types of cameras, from superzoom small sensors to high-end bridge cameras to mirrorless interchangeable lens cameras to APS-C DSLRs to full-frame pro body DSLRs.
But if you had to decide overall based purely on the maximum diversity and availability of lenses, reach, focus speed and performance, tracking ability, low-light IQ, high-ISO performance, low-light focus ability, control over DOF, and so on, that the top system would be a full-frame DSLR with high res and advanced focus, followed by crop body DSLRs and APS-C sensor mirrorless cameras, then by full-frame mirrorless (only ranking below APS-C mirrorless because of less lens adaptability and focus only a match or exceeding on 1 current body), followed by Micro4:3, followed by 1" sensor cameras, followed by smaller sensor bridge cameras. Taking all into consideration.
In perfect conditions, perfect light, the right shooting distance, there's precious little difference in overall IQ between a wide range of cameras. And the photographer's skill and experience can make a massive difference - ie: an unskilled newbie with a D850 and top of the line 500mm lens can still get worse results than a highly-experienced birder with a 1" sensor camera.
I've settled on an APS-C sensor, mirrorless interchangeable lens camera as my absolute favorite birding and wildlife rig, having shot through various smaller sensor superzooms to larger sensor bridge to DSLRs over the years. I still have a DSLR system with lenses from 300mm primes to 150-600mm zooms. But my mirrorless system is the right size, right design, right sensor, right focusing ability and speed, right tracking, and right lenses to work for me 99% of the time. While I know I might get better results with a full-frame DSLR and top of the line lens in some conditions or situations, I can get good enough in those situations with the system I have now, excellent in all other areas where I most commonly shoot, and have a system that's relatively affordable and decently light and portable (relative of course to what you're used to), so for me, it's the best choice. For someone else, a bridge might be the best mix of compromise vs desires...others, only a full-frame DSLR with $10,000 lens will make them happy. It's all OK.