DOI: 10.5061/dryad.s1rn8pk5w
title: Data from: Effects of ambient temperatures on evolutionary potential of reproductive timing in boreal passerines

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