Ongoing studies in the region of Zurich, Switzerland, on the Dipper date back to 1987. The Dipper population (about 25 breeding pairs) is located on two streams flowing into Lake Zurich. All the birds are individually marked. Over the years moult records on 247 individuals trapped once or repeatedly per season were collected, however, due to re-trapping of individuals total number of records amounted to 358. Exact age of 238 individuals was known. Moult process was outlined on the basis of all primaries and the outer six secondaries as well as in tail feathers (rectrices). All birds recorded belong to the local population, thus data on individuals and their breeding specifics had been collected over the years and could thus be analysed as to their influence on the moult process.
Among passerines flight feather moult in Dippers is exceptional. Feather replacement occurs in accelerated and sporadic waves. This particular trait however does not shorten the overall time required for a moult: the waves occur in sequences and between these periods moult is slowed down. Moult in the secondaries, which, in general, is not initiated until the primaries have obtained their second wave, follows the same pattern. For the Dipper, moult starts on 23 June and ends on 15 September on average; 95% of all initiations lie between 1 June and 15 July. Moult lasts 80 to 88 days (84 days on average) and is thus within the range known for other similar songbird species. No support was found of the hypothesis that given its aquatic lifestyle at least two thirds of the wing feathers have to be intact. Observations on behaviour show that during moult the Dipper forages on the bankside or in shallow water. Analyses of stomach content have confirmed this behaviour. During the stage in which a massive loss of feathers occurs real diving is very rare and the Dipper becomes quite sedentary and even cryptic with otherwise little activity evident. Its wave-like moult seems to be a compromise between a short moulting period within which flight and diving capability is limited.
On the whole, male Dippers start to moult 5.2 days earlier than females. A comparison of age classes shows that 89 of secondyear birds initiate moult 5.6 days prior to the 149 older-than-second-year birds. The earlier moult in second-year birds can be explained by the fact that they have retained their plumage for a longer time (hatching date to average moult date in their second year versus average moult date to average moult date in older-than-second-year birds). Among the older-than-secondyear birds the median starting date remains stable. A striking difference becomes evident when comparing 176 early breeders (brood or young reaching independence prior to 22 June) with 71 late breeders (breeding activites after 22 June): the advance group begins moulting 12.5 days earlier than those birds with prolonged breeding acitvities. Within the delayed group males moult 6.1 days earlier than females. In conclusion, moult is dependent on sex, breeding date and breeding activity as well as age.