Overlap between Breeding and Moulting in the Great Tit Parus major and Willow Tit P. montanus in Northern Finland

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Abstract
The timing of the molt was studied in 386 great tits and 46 willow tits in Oulu, northern Finland, in 1969-1977 and 1970, 1975-1977, respectively. The birds were also weighed during and after breeding. The willow tits started molting about 3 wk earlier than the great tits. The molt was retarded by 2-3 wk in the latter species when compared with populations further south. Male great tits started molting before females, and completed the primary molt in approximately 74 d [days]. The corresponding value for females was 70 d. The molting period of the great tit population covered approximately 105 d. The willow tits started molting more frequently during breeding than did the great tits. In the latter species this overlap was more pronounced than further south. The early start and termination of molt in the willow tits are due to the fact that the tits must have time to collect their winter food storages in autumn. The weights of the tits tended to increase during molt.