1. A migratory breeding population of Eurasian wigeon Anas penelope L., was studied over the period 1975-95 on 54 km(2) of freshwater habitat at Lake Myvatn, northern Iceland. Spring numbers of males averaged 1076 (range 526-1887). The sex ratio was 0.97 females per male and the proportion of yearling males averaged 0.21. The mean brood was 3.93 and a mean of 2.83 young per female (range 0.09-5.14) was produced. Production of young was positively related to chironomid abundance and negatively to cold and wet weather. 2. Both sexes seem to be faithful to their winter range, but males are much less faithful to the summer range than are females. The apparent return rate of males is presumably an expression of the return rate of females, which had previously bred locally or been produced at Myvatn. 3. The apparent return rate of old males (2 years or older), mean 0.84 (range 0.58-1.22), was positively correlated with chironomid abundance in the previous summer, which varied greatly. It is suggested that abundant chironomids in late summer may provide a source of highly nutritious food which would increase the probability of wigeon returning to Myvatn or favour the survival of Myvatn wigeon on winter grounds shared with birds originating from other breeding areas. Chironomid numbers showed serial correlation, hence it may be advantageous for the birds to assess the food supply before departure and return accordingly. 4. The apparent return rate of yearling males, mean 0.22 (range 0.06-0.56), was negatively related to the number of young produced in the year before. 5. The density of breeding wigeon at Lake Myvatn was mainly determined by food conditions in the breeding area, as experienced by the adults 1 year previously, and events in the winter range appeared to have less influence. Flyway populations may be limited by a complex set of local and global fluctuations in resources in (i) breeding localities, determining density and production in each, coupled with (ii) staging and wintering localities in each of which local resources would determine density.
