Supplemental food, in the form of millet seed, was provided to half of an island Song Sparrow population during the 1978-1979 winter to test if winter food influenced: (1) overwinter survival; (2) winter wights; (3) breeding density in 1979 and (4) 1979 breeding performance. Territorial males were most dominant at feeders and may have restricted access of young to feeders. Young females were most subordinate at feeders. Adult survival was not affected by supplementary food, but young survival was higher than in 6 previous years and young seen to visit feeders may have survived better than young not seen at feeders. Young females were more variable in weight on the unfed half of the island than on the fed end. The breeding population increased by 38% from 1978 to 1979, but it is not known how much of this increase resulted from food addition. Pairs of birds with feeders on their territories began to lay 25 days earlier in 1979 than control pairs, but delayed longer than controls before a second breeding attempt. One-year old females began to lay significantly later than adults on the control area, but not on the fed area. Other measures of breeding performance were not affected by supplemental food. Winter food may be more important and male territorial behaviour less important than previously supposed in limiting numbers in the Mandarte Island Song Sparrow population.