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

BACKGROUND

It 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.

AIMS

To describe how conditions in the remote non-breeding areas in Africa affect reproductory abilities of migratory birds breeding in Europe.

METHODS

We 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.

RESULTS

The 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.

CONCLUSIONS

Conditions 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.

Factors determining the occurrence of anthropogenic materials in nests of the white stork Ciconia ciconia

Birds have been using anthropogenic materials for nest construction for the past few decades. However, there is a trade-off between the use of new nesting material, which is often linked to greater breeding success, and the higher risk of nestling mortality due to entanglement or ingestion of debris. Here, we investigate the incorporation of anthropogenic materials into nests of the white stork Ciconia ciconia, based on a long-term study of a population in Western Poland. We recorded at least one item of debris in 50 and 42% of nests at the egg and nestling stages, respectively. More debris was found in nests located in territories with higher number of anthropogenic material in the surrounding environment. We found a relationship between the age of females, the number of debris in the area surrounding a nest, and the number of debris in the nest. We found no significant effect of the total number of debris in nests on clutch size, number of fledglings, or breeding success. Studies on the influence of the age and sex of individuals in understanding this behaviour and its drivers in bird populations should be continued.