The breeding season of P. domesticus in 1961 and from 1963 to 1964. at Oxford began in April and ended in early August; that of P. montunus began slightly later and ended slightly earlier and this species also laid fewer clutches. In P. domesticus laying birds fell into two groups, namely “early-” and “late-starters”, which, from a comparison with data obtained from birds of known age, appeared to be mostly females two or more years old, and females one year old, respectively. In P. montanus, however, it appeared that birds of all ages were beginning breeding at about the same time in the season. The laying of first clutches of the early-starting birds of P. domesticus and of the earlier starting members of the one group of P. montanus was correlated with the prevailing air temperature; it is estimated that P. domesticus began its laying at slightly lower air temperatures than P. montunus. It is suggested that laying is adapted to begin at a particular air temperature because at lower temperatures the adult would have insufficient energy available to produce and incubate a clutch.