Temperature can have a profound influence on the optimal laying date of insectivorous passerines by affecting the timing of peaks in spring food abundance. If individuals do not synchronise their reproduction with respect to this food peak, it can have detrimental impacts on their reproductive success relative to well-matched breeders. In a population of Blue Tits studied for over a decade, we find that large between-year variation in the onset of laying is tightly coupled to pre-breeding spring temperatures, and that this population-level advancement in laying date in warmer springs can be completely accounted for by individual plasticity. Notwithstanding substantial plasticity within individuals across years, laying date is highly repeatable, with certain individuals consistently early or late. The substantial repeatability appears underlain by heritable variation for relative laying date (h2 = 0.30 - 0.18 (SE), although this estimate has large standard error and is not significant. This makes assessment of the potential for evolutionary change in laying date problematic, despite observing strong directional selection favouring early-laying individuals. We found that part of the between individual variation in laying date was explained by the quality of the breeding territory and the age of the breeding female. Ornithologists have long sought to quantify variation in avian life histories, with particularly sophisticated experimental approaches and analyses applied to hole-nesting passerines. Nevertheless, there is still great value in presenting descriptions of long-term trends in life history traits, as has been done for laying date in this paper. Characterising such traits provide a useful baseline that is prescient in the face of a changing climate, which may decouple trophic interactions and reveal heterogeneous responses across populations. It also reinforces the value of replicating results and experiments in a field that is increasingly driven by novelty.
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Vol. 103 • No. 1