To fledge or not to fledge: factors influencing the number of eggs and the eggs-to-fledglings rate in White Storks Ciconia ciconia in an agricultural environment

摘要

Numerous studies have explored the relationship between environmental factors and White Stork Ciconia ciconia reproduction, mainly expressing breeding success as the number of fledglings. Nonetheless, one of the most critical life-history stages in birds falls between egg-laying and fledging, and identifying the factors causing offspring mortality during this period provides valuable knowledge. We quantified the number of laid White Stork eggs and the proportion of eggs that turned into fledglings inan agriculture-dominated region in Eastern Germany. Moreover, we identified the factors among land cover, weather and arrival dates, which influenced these two reproductive measures the most, and analysed the monitored mortality causes. On average, foureggs were laid per nest, and 57.8 % of the eggs turned into fledglings. The number of eggs laid was best explained by the negative effect of the arrival date of the second stork, while the percentage of eggs that turned into fledglings was more dependent an weather: most important parameters were mean temperature in the fifth and seventh weeks after the assumed breeding start (i.e. around the assumed hatching date), and the number of consecutive days with precipitation when nestlings are assumed tobe approximately 3 weeks old. In an agricultural environment, weather effects that potentially disturb food availability might be more important than effects directly affecting the survival of White Stork offspring. The most frequent observed mortalitycause, nest fights, furthermore revealed the relevance of intraspecific competition in the study population.