Quoted from document (ec.europa.eu/food/animal/welfare/international/out67_en.pdf), pp. 53-54

All farmed mink live in wire mesh cages with a nest box. Most farmers use the same cage for: single housing of females and the much larger males, littering and nursing until an age of 6-8 weeks and housing of growing kits in pairs from 12 weeks until pelting. The bottom of the cage is always made out of wire mesh because other materials
can cause hygienic problems (de Jonge and Leipoldt, 1994b).

In the breeding period (from a few weeks prior to partition until the kits are six weeks old) a plastic covered ‘kit’ wire mesh (1 x 0.5 inch mesh size) is placed in the cage, covering the bottom. Usually newspaper and straw are placed between this and the cage’s normal wire mesh bottom. This protects the kits from draughts and gives them an almost solid floor for the first six to eight weeks of their lives.