The development of the Rook's (Corvus frugilegus) colony in the face of shifting spring phenology

Abstract

Many birds species breed in colonies. One of the species in the Corvidae family nesting in groups is the Rook (Corvus frugilegus). The construction of the nest is costly for birds and needs high energy expenditure. Therefore, birds should optimize the time of nest building in relation to environmental, especially weather, conditions. Furthermore, birds should adapt their breeding phenology, including the date of starting the construction of nests for climate change. We observed the dynamics of increasing numbers of nests in a colony of Rooks in relation to air temperature, wind, rainfall, snowfall and day-length (indirect indicator of photoperiodism). Observations were carried out during three breeding seasons (2015–2017) in a large rookery in a big city in north-eastern Poland. The increase in the number of nests was correlated only with day length and no effect of climatic factors on the number of nests in the colony was observed. Number of nests varied significantly between successive years of research. In the case of the Rook (predictable breeders), the photoperiod seems to be the most important factor influencing breeding behavior, especially nest construction. In the last three decades, increasing temperature and time of spring phases have advanced and have influenced changes in the phenology of the availability of food resources. Simultaneously, a rapid decline in the numbers of breeding pairs of the Rook in Poland has been observed, especially in the last three decades. Conservative attachment to the length of the day, which is the main factor responsible for the development of the breeding colony, makes the Rook appear to be a species with low plasticity in the face of rapid climate changes. Thus, we suggest that may indirectly influence the decrease in the breeding population of the Rook.