TIME AND FOOD DEPENDENCE IN WILLOW TIT WINTER SURVIVAL

Abstract

Timing of mortality was studied in Willow Tits (Parus montanus) during three winters, 1991–1993 and 1995–1996, near Oulu, northern Finland. The aim of this study was, by investigating monthly winter survival rates, to examine (1) whether late winter is the most difficult time for birds, and (2) whether temperature, especially in late winter, correlates with survival, as previously proposed. We also sought information on sex and age differences in timing of mortality, rarely studied among passerines. After color-marking, individuals were resighted monthly from September to May. Capture–recapture models were applied to separately estimate survival and resighting probabilities. Survival estimates given by three methods of varying complexity (simple counts, Jolly-Seber, Cormack-Jolly-Seber) were compared. The three different methods yielded similar survival estimates. Because simple counting does not account for emigration, consistent results from different analyses support the earlier view that Willow Tits are sedentary. Adults survived better than yearlings in all winters and exhibited more constant survival rates. Two-year data also suggest that winter and summer survival rates were similar in adults, but not in yearlings, for which mortality was greatest in winter. Males survived better than females in one winter. Temperature per se did not correlate with survival, but deviations to colder than long-term average temperature seemed to be associated with low survival probability, which could reflect occasional food limitation. A feeding experiment was conducted to test the winter food limitation hypothesis. Fed birds survived winter better than did control birds, significantly so in yearlings and males, but not in adults or females. Previously, late winter has been suggested as being a difficult time for birds. However, no support was found for this hypothesis. Small passerines wintering at high latitudes seem to be able to cope with high energy demand and the limited foraging time of winter.