E. B. Ford supported the bird-predation hypothesis. To experimentally investigate the issue he recruited Bernard Kettlewell in 1952 under a grant from Nuffield Foundation. Biology of the peppered moth By the time of Kettlewell, it was known in England that there were three varieties of peppered moth. The normal, typica, is whitish-grey in colour with dark speckles on the wings. The colour was a perfect camouflage on light-coloured trees covered with lichens. The new form, carbonaria, was completely black. There was an intermediate form, called insularia, which was light-coloured with speckled wings, but distinct from typica in that it was not whitish. The moths were active at night, and rested on tree trunks and boughs during the day. The experiment The main experiment, called mark-release-recapture, started in the summer of 1953 and lasted for three years. It consists of two continuous phases. Preparation Kettlewell first devised a standard procedure for scoring the moths. It was necessary to determine how far apart the moths should be placed so they were indistinguishable from their backgrounds. Correct separation would result in effective and selective predation by birds, because if the moths were all too close then birds would be able to differentiate even well camouflaged individuals. He tested his scoring method in the woodlands near Birmingham by releasing 651 peppered moths (consisting of typica, carbonaria and insularia), and then at an aviary at the Research Station in Madingley in Cambridge. In the aviary he released 69 moths, which he allowed two great tits (Parus major) to prey upon. He found that the initial procedure failed, as the birds actively looked for any moth, regardless of their colour or background. He succeeded only by using freshly captured moths consisting of 9 black and 8 white types, which he released separately. He found that the birds preferentially caught the moths according to the background colour on which the moths were present. First phase For his first experimental site, Kettlewell chose Christopher Cadbury Bird Reserve, near Rubery, Birmingham, because it was heavily polluted, but still inhabited by a number of bird species. He caught all three types of peppered moth and marked them underneath their wings with cellulose paint, so that he would be able to identify them later from non-experimental individuals after recapture. He started capturing the moths on the night of 26 June 1953, and lasted till 5 July. Out of his total capture,