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Data from: Effects of ambient temperatures on evolutionary potential of reproductive timing in boreal passerines

Cite this dataset

Vatka, Emma; Orell, Markku; Rytkönen, Seppo; Merilä, Juha (2020). Data from: Effects of ambient temperatures on evolutionary potential of reproductive timing in boreal passerines [Dataset]. Dryad.


1. Many populations need to adapt to changing environmental conditions, such as warming climate. Changing conditions generate directional selection for traits critical for fitness. For evolutionary responses to occur, these traits need to be heritable. However, changes in environmental conditions can alter the amount of heritable variation a population expresses, making predictions about expected responses difficult.

2. The aim of this study was to evaluate the effects of ambient temperatures on evolutionary potential and strength of natural selection on the timing of reproduction in two passerine birds breeding in boreal forests.

3. Long-term data on individually marked Willow Tits (Poecile montanus, 1975–2018) and Great Tits (Parus major, 1969–2018) were analysed with random regression animal models to assess if spring temperatures affect the expressed amount of additive genetic variation (VA) and heritability (h2) in the timing of breeding. We assessed if ambient temperatures of different seasons influenced the direction and strength of selection on breeding time. We also evaluated if the strength of selection co-varied with evolutionary potential.

4. Levels of VA or h2 expressed in laying date were unaffected by spring temperatures in both study species. Selection for earlier breeding was found in the Willow Tit, but not in the Great Tit. In the Willow Tit, selection for earlier breeding was more intense when the temperatures of following autumns and winters were low. Different measures of evolutionary potential did not co-vary strongly with the strength of selection in either species.

5. We conclude that there is no or little evidence that climate warming would either constrain or promote evolutionary potential in timing of breeding through changes in amount of genetic variance expressed in boreal Willow and Great Tits. However, selection on the timing of breeding, a life-history event taking place in springtime, is regulated by temperatures of autumns and winters. Rapid warming of these periods have thus potential to reduce the rate of expected evolutionary response in reproductive timing.


Long-term monitoring of nesting attempts of Willow Tits (1975–2018) and Great Tits (1969–2018) in the Oulu study area (ca. 65°05’N, 25°33′E) followed routine procedures. Nests were visited at least weekly and their contents were recorded. The laying date of the first egg was as a rule calculated from the observed number of eggs in an incomplete clutch (i.e., incubation had not yet started) under the assumption that one egg is laid per day.

Both the parents and young were marked with individually coded aluminium leg rings and parents with unique combinations of plastic colour rings. Parental birds were aged as yearlings or older either by calculating from the ringing date of recruits, or based on the tail feather shape in Willow Tits or on plumage coloration in Great Tits.

Data of mean ambient temperatures for 1969–2018 were retrieved from the Finnish Meteorological Institute, Oulunsalo observatory.

Usage notes

Please read the "readme.txt" for specifications of variables.