Thanks for he feedback.

Can I ask you what model of Birdy you have and what is the size of the crankset chainring ?

I have now a 11s 9-34t cassette with a 50t chainring on my Birdy III and there is some space left between the chain and the right rear arm, so I am pretty sure that 36t will be OK, but 40t seems to me too much with my 50t chainring I think that the chain will rub against the rear arm on the 40t cog, a smaller one will probably help.