it depends on your budget
the lens i use is a canon 300mm F4 L and a 1.4x teleconverter but get beyond the typical garden variety species that can be enticed closer with seed it's still a struggle
sigma have a 150 - 500mm that gets good things said about it and on the cheaper end a second hand sigma 170 - 500mm ( no longer in production ) can be had for not a lot of money and stays sharp up to around 400mm
i have owned the canon version of this lens and had some nice results with it but it did lose some sharpness above 420mm
it's all about the depth of your pocket
an example photo at 400mm with the sigma 170 - 500mm
an example photo at 500mm with sigma 170 - 500mm
if you click on the images you can navigate to full screen images
both acceptable images but as you can see the 400mm photo is noticeably sharper
good luck in your quest...