Do White Storks Ciconia ciconia always profit from an early return to their breeding grounds?

Abstract

Capsule Arrival date strongly influenced date of breeding and breeding success. Aim To check our hypothesis that in years with low April temperatures, i.e. when storks started to breed, the relationship between timing of breeding and success would be absent, meaning that migrants would have no advantage by returning early to their breeding area. Methods We collected data in Poznań province, western Poland during the period 1983–2002. Based on local climatological data we selected eight cold spring years and compared them with the 12 ‘normal’ years. We analysed how weather affected the timing and arrival pattern of White Stork. Results The two groups of years did not differ significantly in population size, but in normal years the arrival date of both parents was earlier. Arrival date strongly influenced (was positively correlated with) date of breeding and (negatively correlated with) breeding success. Conclusion The slopes describing the above relationships did not differ significantly between the two groups of years. Therefore, we speculate that natural selection strongly favours birds that return early from the wintering grounds.