Little is known about the process or causes of fledging or nest-leaving in passerine birds because researchers can rarely predict when fledging will occur in a given nest. We used continuous videotaping of nests to both document the process of fledging in the house wren, Troglodytes aedon, a small, cavity-nesting songbird, and test hypotheses as to what might cause fledging to begin. Fledging began any time from 14 to 19 d after hatching commenced. Slower-developing broods fledged later than faster-developing broods. Fledging typically began within 5 h of sunrise and over 80% of all nestlings fledged before noon. All nestlings fledged on the same day at 65% of nests and over two consecutive days in most other nests. We found no evidence that fledging was triggered by changes in parental behaviour. Parental rate of food delivery to nestlings did not decline during a 3-h period leading up to the first fledging, nor was the rate of feeding just prior to the first fledging lower than the rate at the same time the day before. Moreover, parents did not slow the rate of food delivery to nests after part of the brood had fledged. Hatching is asynchronous in our study population which creates a marked age/size hierarchy within broods. At most nests, the first nestling to fledge was the most well-developed nestling in the brood or nearly so (as measured by feather length). This suggests that fledging typically begins when the most welldeveloped nestlings in the brood reach some threshold size. However, at about one-fifth of nests, the first nestling to fledge was only moderate in size. At these nests, severe competition for food may have caused smaller, less competitive nestlings to fledge first to increase their access to food. We found no strong support for the suggestion that the oldest nestlings delay fledging until their leastdeveloped nestmate reaches some minimum size, although further experimental work on this question is warranted.