Difference between 300mm and 500mm in crop terms

I have an all purpose Tamron 18-2701 which is great. I've just bought Sigma 150-500 and am still trying it but the results I am getting are very good.

I picture I took yesterday of a white throated kingfisher. I find the lensnot bad at all comparing to the much more expensive Nikon lenses. The picture was taken without tripod.
Focal length 500mm
F 6.3
1/640 sec
ISO220

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