For bird photography, I typically just use one camera and one lens, namely the Canon EOS R10 and the RF 100-400 mm f/5.6-8 lens. In the near future, I will also be buying a Sigma 150-600 mm lens to complement the RF 100-400. The main appeal of this setup is its unbelievably light weight; I can easily and very comfortably carry this camera and lens combination all day without any problems.
My previous setup (a Canon 70D and a 150-600 mm Tamron lens) did make my arms tired after being out with it all day. Additionally, my Tamron, unfortunately, had a couple of issues with its autofocus and especially its stabilisation so I had to send it back for a full refund after a few months of using it.