Birds eat the moths Biston betularia and Phigalia pilosaria as they rest exposed on tree trunks. At Eastham Ferry, Merseyside, and Hawarden, Clwyd, predation of the two species, which fly in summer and winter respectively, is similar. Migrant species of bird contribute little to the overall size of the population of predators. Immigration of local ground-feeding species increased in density during winter. More insectivorous birds were noted at Eastham Ferry than at Hawarden; this was related to a higher rate of predation of P. pilosaria at the former locality. In the two species of moth the effects of predation appear to be similar and so other phenomena need to be invoked to explain the existence of the polymorphisms for melanism.