Population dynamics of an expanding passerine at the distribution margin

Abstract

Individuals may be maladapted to novel environments at the species' distribution margin. We investigated population dynamics in a marginal habitat where reproduction has been proven poor. Survival, population growth rate (¿) and its components, breeding and natal dispersal were studied in great tits Parus major breeding at the northern margin of its distribution in northern Finland. We used long term capture-mark-recapture data sets. Study area size and population density were used to explain adult survival rates. The average annual estimates of adult survival rose from 0.371 to 0.388 between the periods of 1971-1984 and 1999-2009. The estimates are slightly lower than estimates of small passerines in Europe. Low local survival rate of fledglings (0.050-0.055) probably reflects intensified emigration from this low quality area. Temporal variation in ¿ was large (0.498-1.856). Despite of low adult survival and recruitment rates, the mean estimates of ¿ (1.008 and 1.033) indicate an overall stability in the population size. Indeed, our results suggest that the immigration has an important role in the population dynamics of northern great tits. Thus the population is demographically and genetically dependent on core habitats which may cause adaptive problems due to intensive gene flow. Given those limitations, options for evolution of local adaptations in northern distribution margins are discussed.