Shift of bird nesting time in Central Siberia due to climate warming: Phenotypic plasticity or genetic shift?

Numerous facts of birds breeding time advancement on the background of climate warming have been accumulated over the past decades. Most studies note a correlation between the population laying date and the temperature of a certain period, but the reasons and mechanism of this relationship remain uncertain. Individual-based genetic studies, despite significant success, are too complex and time-consuming, and their resolution is still insufficient to prove directed genetic changes associated with climate warming. We have implemented a population-based approach to the analysis of laying dates in 6054 nests of most abundant species in the poorly studied natural region for the period from 1974 to 2016. In search for the strongest climatic relationship, we iteratively sorted out three weather indicators, different dates of influence, and all successive fractions of the population entering into reproduction. In search for maximum climatic dependence, we checked the influence of three weather indicators, different exposure dates, and the response of each fraction of the population that successively enters breeding. We have found that the maximum response is associated with a phenological indicator based on the sum of the active temperatures. The greatest influence fell on the 35% quantile of the laying dates in the season, which coincided with the modal day in the population. The phenological peculiarity of the situation at the beginning of nesting was maintained for a long time and thus could serve as a reliable source of information on the timing of prospective food abundance. This should increase the efficiency of selection for an optimal laying date according to the results of reproduction, thereby explaining the mechanism of reaction to climate. We decomposed the observed laying dates and the dates of achievement of the necessary phenological conditions into components associated with a long-term trend and annual fluctuations. Comparing these data series, we obtained an independent assessment of the environmental influence on annual deviations and long-term displacement of breeding dates. The annual deviations of the conditions caused a proportional response: the short-distance migrants compensated for the environmental change by about half, the long-distance migrants by nearly 1/3, and the eastern ones did it poorer than the western ones. Under extreme phenological conditions, the response of some species was less than proportional, which indicated the limit of phenotypic plasticity. In years separated by a long interval, but similar in spring phenology, egg laying occurred significantly earlier. During the whole observation period, the spring phenology has shifted for more than 12 days and caused the same advance in the nesting time in shortdistance migrants, while long-distance migrants compensated for the shift by less than 2/3. The long-term shift in the laying dates has exceeded their standard deviation in populations by the average of 2.7 times. Thus, the described phenomena go well beyond phenotypic plasticity and suggest significant evolutionary change of the laying dates.