Delayed egg-laying in Red-backed Shrike Lanius collurio in relation to increased rainfall in east-central Poland

Climate change has affected the breeding parameters of many animal species. In birds, most studies have focused on the effects of temperature on clutch phenology and clutch size. The long-term influence of other weather factors, including rainfall, on breeding parameters have been analysed much less often. Based on a 23-year dataset and 308 broods, we documented shifts in the timing of breeding, clutch size and mean egg volume in a long-distance migrant, the Red-backed Shrike Lanius collurio, from a central European population. We found a 5-day shift towards delayed breeding, but no differences in brood size or egg volume during those 23 years. The GLM analysis showed that the mean May temperature had a positive influence on the clutch initiation date, whereas the number of days with rain delayed laying. During the period 1999-2021, there was no change in the mean May temperature, but total precipitation and the number of days with rain in May increased. Thus, delayed nesting in this population was probably due to the increase in rainfall during this period. Our results provide a rare example of delayed nesting in birds in recent years. Predicted changes in the climate make it difficult to assess the long-term impact of global warming on the viability of Red-backed Shrike populations in east-central Poland.