Dovecots [0] have been used for (likely) millenia. And, because of migration, the birds can ultimately bring nutrients in from disparate locations - as opposed to purely localally grazing livestock.
Per leathers: one, we don't have to completely eliminate cattle. Two, synthetics exist. Three, alternatives are being made/investigated [1].
Per leathers: one, we don't have to completely eliminate cattle. Two, synthetics exist. Three, alternatives are being made/investigated [1].
[0] https://en.wikipedia.org/wiki/Dovecote [1] https://www.felsie.co.uk/ (and others, if you google around)