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.

中文翻译：

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极端温度下降会改变大山雀的孵化延迟、繁殖成功和生理状况

有人认为，极端天气事件可以被视为自然实验，可以解开鸟类根据环境变化调整物候和繁殖参数的机制。2017 年，几天的气温突然大幅下降影响了许多欧洲鸟类。该事件发生在波兰中部大山雀（Parus major）种群的产蛋早期孵化期，导致孵化时间大幅延迟并产生持续的繁殖后果。2017 年繁殖活动开始后出现的寒流，紧随其后的是 2018 年温暖而稳定的繁殖季节。这一自然实验对大山雀繁殖参数产生了重要影响。我们发现孵化日期、雏鸟数量、孵化成功率、2017 年至 2018 年间的雏鸟成功率。2017 年每窝雏鸟数量比 2018 年减少约 2 只。雏鸟成功与 2017 年孵化延迟呈正相关，而 2018 年则呈负相关。两年间孵化成功率差异显着， 2018 年更高。2017 年雏大山雀血液中的平均血红蛋白水平（用作身体、生理状况的指标）高于 2018 年。我们认为孵化的时刻可能（至少在某些年份）与传统上使用的产蛋日期相比，它与山雀雏鸟的最大食物需求时刻相关。同样在极端年份，孵化延迟的表型可塑性可能不足以根据即将到来的极端天气事件调整繁殖时间。