Researchers designed an experiment to test the hypothesis that the evolution of coloration in lice is driven by birds. researchers transferred 25 lice from a random sample of lice obtained from wild‑caught rock pigeons to each individual in groups of black pigeons and white pigeons. half of the pigeons from each color group were fitted with a harmless mouthpiece that stopped the beaks from entirely closing. the mouthpiece prevented pigeons from removing lice but did not affect the pigeons’ ability to feed. each pigeon was isolated and kept in identical environmental conditions for temperature, humidity, and amount of light. the lice have a generation time of 24 days. the researchers observed the phenotypes of the lice over a period of four years.