SummaryNestling survival and nestling weights in P.domesticusandP. montanuswere studied in 1961 and 1963–64 at Oxford. This paper concludes a study of factors influencing the reproductive rate.Taking all losses into account,P. domesticusreared an average of 1.6 nestlings per brood (45%) andP. montanus2.7 nestlings per brood (59%). About a third of all broods of both species failed completely to survive the nestling period. InP. domesticusthese failures were most numerous in the middle part of the breeding season and are attributed to nutritional deficiencies derived from unsuitable food provided as a consequence of a seasonal food shortage, but inP. montanuscomplete brood failures occurred mostly in the second half of the nestling period and are attributed to predation.43 broods ofP. domesticusand one brood ofP. montanuswere weighed daily. Those of P.domesticuswere classified as (1) successful broods—in these some nestlings died in the larger brood‐sizes, apparently through starvation; (2) long‐lived unsuccessful broods—in these the nestlings died at intervals and failure was attributed to nutritional deficiencies; and (3) short‐lived unsuccessful broods. A slight decrease in the weights of nestlings in successful broods at the end of the nestling period is attributed to the utilization of insulating fat facilitated by the completion of the feather covering. Nestlings of both species left the nest at 88–89% of the adult weight.Taking all “successful” broods together, the percentage survival rates on nestling day 131/2(day of hatching = day1/2) inP. domesticuswere 81–82% in b/2–3, 70% in b/4 and 56% in b/5 (a situation paralleled in this respect by P.hispaniolensis), but in P.montanusthey werec. 82% in all brood‐sizes. Hence, in P.domesticusb/4 probably gave rise to the largest number of nestlings reared per brood, while inP. montanusmost nestlings were produced by the largest brood‐size. Weighings of many broods on day 131/2showed two trends in the weight of the nestlings: (1) in both species the weight of the nestling decreased as the number of survivors from each initial brood‐size decreased; (2) between successive initial brood‐sizes the weight of the nestling of P.domesticusdecreased with increasing brood‐size but in P.montanusthere was no change.The losses in the larger broods of P.domesticusoccurred mostly in the first half of the nestling period—apparently in association with the asynchronous hatching of the eggs and as a consequence of the limitation on the feeding frequency of the adults. Nestling survival was lowest in the larger broods in the middle of the breeding season and contrasted with the mid‐season increase in mean clutch‐size. It is suggested that in the study area there was a (possibly unnatural) shortage of food suitable for nestlings in the middle of the season.It is suggested that inP. domesticusthe unexpectedly low feeding frequencies of the adults with large broods, apparently causing their low survival rates, may be an adaptation evolved to obtain the maximum amount of food in the presence of other adults which would be attracted to a food source by higher rates of activity.The breeding success calculated from data derived from the whole of this study was 35% for P.domesticusand 49% for P.montanus(2.9 and 3.9 nestlings per breeding pair per year respectively).It is suggested that the population of P.domesticuswas much closer to a critical limiting factor, e.g. food supply, than that of P.montanus. This may account for the striking differences between the two species in their nestling survival rates and their nestling weights in relation to brood‐size; in particular, the success of the larger broods of P.monta