The biscuits are compressed and swell when wet by the wood glue. I see no reason why more than one species would not work. Biscuit joiners were invented by Lemmelo who is Swiss, I think. Beech is popular in Europe, so the biscuits that first appeared were beech. I guess somebody found birch more available or cheaper. There are few things that can only be made from one species. Just me rambling. If the birch biscuits are no good, please get back to us.
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