Ecological conditions during winter predict arrival date at the breeding quarters in a trans‐Saharan migratory bird

Abstract

We studied variation in arrival date to the breeding colonies in Italy of a trans‐Saharan migratory bird, the barn swallow Hirundo rustica, in relation to variation in ecological conditions, as reflected by the normalized difference vegetation index (NDVI), in the winter quarters. Arrival date of old but not young individuals captured during consecutive breeding seasons was earlier after winters with favourable conditions. Change in arrival date in relation to change in NDVI was similar in the two sexes. Change in arrival date significantly and positively predicted change in breeding date. As a result of increased frequency of second broods determined by earlier arrival, the number of fledged offspring per season was larger after African winters with good in comparison to poor ecological conditions for barn swallows. This is the first study demonstrating phenotypic plasticity in migration phenology of a long‐distance migratory bird in relation to ecological conditions during wintering.

Climatic effects on breeding grounds are more important drivers of breeding phenology in migrant birds than carry-over effects from wintering grounds

“…Although Sahelian rainfall affects the timing of migrant arrival in Europe [6,7], our result suggests that this is not the main determinant of breeding phenology. Migrants nest earlier in warmer springs, although it remains unclear whether this advance is sufficient to avoid negative population consequences of warming.…”

Manipulating individual state during migration provides evidence for carry-over effects modulated by environmental conditions

“…Until now, evidence for such COEs solely came from correlative studies [3,4,11,12]. Our results also suggest that conditions experienced on the breeding grounds can enhance or buffer COEs.…”

Rainfall-induced changes in food availability modify the spring departure programme of a migratory bird

“…Although it is difficult to use spatial patterns to predict temporal responses, it is possible that annual variation in rain and food resources could induce variation in reproductive success for the same birds over multiple years. This idea is supported by past research with both Palaeotropical and Neotropical migrants, where arrival date on breeding grounds and reproductive success depended in part on rainfall or allied climate cycles in the non-breeding period [11,19,29].…”

“…Conditions outside of the breeding period such as habitat quality and resource availability, and other factors such as evolutionary canalization of traits could either facilitate or limit adaptive responses to selection for earlier reproduction [7][8][9]. The outcome of these factors on reproductive phenology later in the year may also vary annually in relation to aspects of climate that are indirectly related to but distinct from temperature, including snowpack and rainfall [10,11].…”