The effectiveness of rodent control is measured at intervals of around two months, from soon after baiting starts through to late summer. Tunnels with a peanut butter bait in the centre are placed in the localities shown above. Ink is applied to pads next to the bait, and as the rodent exits its footprints are picked up on blotting paper pieces that it has to cross.
A total of 190 tunnels are in place, in random positions, so that a variety of habitats are sampled. These are compared with 30 tunnels at three locations outside the Ark.
The protocol is to set the baits and papers on a day when there is little rain, and the papers are collected next day. Typically the incidence of rat footprints is around 2-5% in the Ark, and 70%+ outside.