How weather conditions in non-breeding and breeding grounds affect the phenology and breeding abilities of white storks

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BackgroundIt has often been suggested that conditions in wintering grounds affect the breeding abilities of migratory birds. This is known as the carry-over effect. Heretofore, many studies have reported the relationship between conditions in wintering grounds, dates of departure from and arrival at breeding grounds, and breeding success. However, very few studies have shown how these conditions affect the capacity of females for egg production. AimsTo describe how conditions in the remote non-breeding areas in Africa affect reproductory abilities of migratory birds breeding in Europe. MethodsWe recorded 863 arrival dates for 191 nests (in 2005–16), clutch sizes of 412 clutches, and egg dimensions of 1725 eggs (in 2003–16) of white storks in western Poland. We used generalised climatic indicators such as the Sahel precipitation index (SPI) and Indian Ocean Dipole (IOD) to assess conditions in wintering grounds prior to breeding and precipitation and temperature in breeding grounds during the breeding season. ResultsThe white stork arrived significantly earlier in seasons preceded by a warmer and rainier March, while conditions in Africa did not affect arrival dates. Females laid more (and larger) eggs in seasons preceded by rainier winters in Africa. Breeding success was not affected by conditions in wintering grounds, but only by the weather during breeding season. ConclusionsConditions in wintering grounds may affect migratory birds, not only in terms of their arrival dates, but rather with respect to females' egg production capacities. Studies on stable isotopes may yield more detailed findings as to how this process takes place.