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.

Importance of adult survival, local recruitment and immigration in a declining boreal forest passerine, the willow tit Parus montanus

“…Previous information on the survival of breeding adults between successive breeding seasons (Orell et al 1994a, 1994b, Orell and Belda 2002 and on adults and yearlings during the winter months were available from the willow tit population in this study (Lahti et al 1998) and from a population near Gothenburg, Sweden (e.g., Ekman 1984). All of these papers consistently show that adult willow tits have a rather constant and high survival rate between years, and that adult birds are more likely than yearlings to survive the winter.…”

“…We used a 12-year (1991-2002) capture-recapture data set from a population monitored year-round since 1986 (Orell et al 1994a(Orell et al , 1994b(Orell et al , 1999aKoivula et al 1996;Lahti et al 1998;Orell and Belda 2002). Based on line-transect censuses and winter bird censuses in Finland, the willow tit has declined in numbers by about 60% from the 1940s to 1950s until the 1980s and about 15% from 1983 to 2004 (Va¨isa¨nen et al 1998;Va¨isa¨nen 2005).…”

Home range size of willow tits: a response to winter habitat loss

“…Every time that we used the feeder to trap willow tits, we noted the identity of flock members. After marking, willow tits were trained to come when species-specific vocalizations were played back using a tape recorder (Koivula et al 1995;Lahti et al 1998). We removed the food from focal flocks at least 2 weeks before we started home range observation.…”

“…Huhta and Jokimäki 2001), postponed dispersal (Lens and Dhondt 1994), and increased movement costs of individuals (BØlisle et al 2001) associated with habitat fragmentation. However, for resident species inhabiting northern latitudes, such as many species of the genus Parus, population dynamics appear to be strongly determined by events occurring during the non-breeding season (Matthysen 1990;Lahti et al 1998;Doherty and Grubb 2002). Recent studies, dealing with the behavioral responses of individuals to relevant landscape elements such as forest edges, focused on wintertime (Rodríguez et al 2001;Tellería et al 2001).…”

Survival of male Tengmalm's owls under temporally varying food conditions

“…Annual survival rates of barn owls Tyto alba in Scotland were 80-85% in winters of vole abundance, but decreased to 45-55% in winters of vole scarcity (Taylor 1994). Most experimental studies have also revealed that food-supplemented birds survive winter better than unfed control birds (Krebs 1971;Jansson et al 1981;Brittingham and Temple 1988;Lahti et al 1998), underlying the importance of sufficient food resources under adverse conditions. Along with these studies our results suggest that variation in annual survival rates may be large, and 86 Table 2 Results for different survival (φ) and recapture (R) models in 1981 to 1996.…”