Demography and dynamics of a partial migrant close to the northern range margin

Thesis event information

Date and time of the thesis defence

Place of the thesis defence

Linnanmaa, auditorium TA105

Topic of the dissertation

Demography and dynamics of a partial migrant close to the northern range margin

Doctoral candidate

Master of Science Juhani Karvonen

Faculty and unit

University of Oulu Graduate School, Faculty of Science, Ecology and Genetics Research Unit

Subject of study

Animal Ecology

Opponent

Docent Aleksi Lehikoinen, University of Helsinki

Custos

Doctor Veli-Matti Pakanen, University of Oulu

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The northern great tit population is dependent on immigration

The dissertation study found that the northern great tit population had large temporal variation in the population growth rate, but indicated an overall increasing population size. However, without immigrants entering from outside the study area, the population would have declined due to consistently low adult survival and low local recruitment. Immigration formed 39–43% of the annual population growth rate indicating that the population is demographically dependent on immigration.

The study population’s demography differs most from other great tit populations in terms of adult survival which is lower than estimates from more southern Europe. This difference may reflect the impact of more difficult winters weather conditions in the north. Results from the wintering population support this line of reasoning.

Within-winter movement was lower during mid-winter (January to February) and decreased during cold periods. This pattern is probably linked to energy saving and predator escaping strategies during these demanding periods when energy expenditure is high and birds have limited daylight hours to forage. Site fidelity was lower for juveniles than adults.

Survival showed strong links to winter weather. Survival of wintering great tits was linked to temperature variation with juveniles showing a stronger response to cold temperatures. When mean daily minimum temperatures declined below –15 C° degrees, survival started to decline. Low winter temperatures thus provide one explanation for the lower annual adult survival of this population. The results suggest that great tits suffer from the cold conditions of the north: higher mortality increases turnover allowing for strong immigration.

Northward expanding southern species, such as great tits should benefit from warming winters caused by climate change. Climate change causes range expansions, but neither the population parameters nor the ecological mechanisms behind range expansions are well known.

I studied population dynamics and demography of the great tit in northern Finland, close to the northern range margin of the species’ distribution. I used long-term capture-recapture data collected from breeding populations in Oulu to examine demography and the importance of demographic variables to population growth, and from a separate wintering population in Oulu, Hietasaari to examine winter conditions, such as temperature, as limiting factors on winter site fidelity and survival of fed great tits.
Last updated: 4.12.2019