Extreme temperature drop alters hatching delay, reproductive success, and physiological condition in great tits.

It has been suggested that extreme weather events may be treated as natural experiments that may unravel the mechanisms by which birds adjust their phenology and breeding parameters to environmental variability. In 2017, a sudden and heavy drop of temperatures for several days affected many European bird populations. This event occurred during the laying-early incubation period in the great tit (Parus major) population in central Poland, causing a large delay in hatching and had sustained reproductive consequences. This cold snap occurring once breeding activity had already started in 2017 was followed by the warm and invariable breeding season of 2018. This natural experiment had an essential influence on great tit reproductive parameters. We found a significant difference in hatching date, number of fledglings, hatching success, and fledging success between 2017 and 2018. In 2017, there were about two fledglings per nest fewer than in 2018. Fledging success was positively associated with hatching delay in 2017, while the relation was negative in 2018. Hatching success differed significantly between both years, being higher in 2018. Mean level of hemoglobin (used as index of body, physiological condition) in the blood of nestling great tits was higher in 2017 than in 2018. We argue that the moment of hatching may be (at least in some years) more tightly related to the moment of maximum food demand of tit nestlings than the traditionally used laying date. Also in extreme years, phenotypic plasticity of hatching delays may be insufficient to adjust the timing of breeding to the upcoming extreme weather events. Further examining its limits may be an important goal for future research.