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Abstract
Migratory birds usually respond to climate change by modifying breeding and/or wintering areas, as well as by reproducing earlier. In addition, changes in winter habitat use or breeding phenology could have important carry-over effects on subsequent breeding success. Here, we studied age- and sex-dependent carry-over effects from wintering to the breeding stage of a small aerial insectivorous long-distance migratory bird, the barn swallows (Hirundo rustica) breeding in Denmark during 1984–2013. First, we used stable isotope analyses combined with ringing recoveries to identify wintering areas. Second, we found that environmental conditions as inferred by Normalized Difference Vegetation Index (NDVI) have improved at the wintering grounds. Third, we used confirmatory path analysis to quantify the indirect effect of winter conditions on subsequent breeding success. Males delayed onset of breeding and raised fewer fledglings in the first brood when ecological conditions during the previous winter improved. This response was age dependent, since yearlings did not respond to this environmental cue but the response was increasingly stronger as males aged. Females showed a similar response to winter conditions, although not statistically significant. These results highlight the importance of studying carry-over effects within the context of climate change, especially in relation to age of individuals.
Highlights
Migratory birds usually respond to climate change by modifying breeding and/or wintering areas, as well as by reproducing earlier
Wintering areas were largely spread through Cameroon, Central African Republic, Gabon, Congo, Democratic Republic of Congo, Angola, Zambia, Zimbabwe and some patches across Botswana and South Africa (Fig. 1H)
We determined with reasonable precision the wintering areas of barn swallows breeding in Denmark, being distributed from Central to South Africa
Summary
Migratory birds usually respond to climate change by modifying breeding and/or wintering areas, as well as by reproducing earlier. Females showed a similar response to winter conditions, not statistically significant These results highlight the importance of studying carry-over effects within the context of climate change, especially in relation to age of individuals. Arrival time to breeding areas usually determines the start of breeding, and this in turn influences the number of offspring produced[14,15,16] Because all these variables flow sequentially in a time series fashion, Structural Equation Models (hereafter: SEM) provide a powerful and elegant approach to investigate carry-over effects in migratory birds[17,18,19,20,21]. Lower temperature and higher rainfall may result in greener vegetation, higher abundance of insects[31,33,35,36], and favorable conditions for aerial insectivores wintering in the tropics, whereas higher temperatures and lower precipitation may result in the opposite[5,7,37,38,39]
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