Summary.1Nestling RobinsErithacus rubeculawere weighed once each day from hatching until leaving the nest. Such regular visits largely eliminated the fear and premature flight reactions normally found in young Robins in the later stages in the nest.2Nestling weight increases extremely rapidly from hatching until about day 8 or 9, when the nestling may weigh ten times what it did at hatching. After about day 9 the weight stays approximately constant until the young leave the nest.3There are extremely marked individual differences (up to 80%) in the weights of young of the same age under natural conditions. Members of the same brood sometimes differ much, and sometimes only very little, in weight.4Round Oxford in 1946 and 1947 size of brood appeared to have no influence on the weight of the individual nestlings. However, it is possible that both years provided unusually favourable conditions of feeding for young Robins.5In 1947 the young in the two earliest broods to hatch died much below weight after a few days, apparently because caterpillars had not yet hatched in sufficient numbers to provide enough food for them.6The secondary coverts normally split open on day 6‐5, the primaries on day 7‐5. This happens irrespective of the body‐weight of the nestling. The nestling period also seems independent of body‐weight.7There were a few late developers, in which the wing feathers split open one or two days late. These individuals were at first low in weight, but continued to put on weight for one or two days after the age at which most nestlings had stopped.8The feeding visits of parent Robins to nestlings 7–14 days old varied from 3 to 33 visits per hour, averaging 13‐8 per hour. Broods of large size appeared to be visited rather more often than broods of small size, but the feeding rate did not increase proportionately to the greater number of young.9Reactions to predators near the nest are described.10The young can defaecate up to 7 % of the body‐weight. In the later stages in the nest, fsecal pellets were removed at an average rate of 0.8 per nest