Temperature and rainfall anomalies in Africa predict timing of spring migration in trans-Saharan migratory birds

Abstract

The long-term advance in the timing of bird spring migration in the Northern Hemisphere is associated with global climate change. The extent to which changes in bird phenology reflect responses to weather conditions in the wintering or breeding areas, or during migration, however, remains to be elucidated. We analyse the relationships between the timing of spring migration of 9 species of trans-Saharan migratory birds across the Mediterranean, and thermal and precipitation anomalies in the main wintering areas south of the Sahara Desert and in North African stopover areas. Median migration dates were collected on the island of Capri (southern Italy) by standardized mist-netting during 1981 to 2004. High temperatures in sub-Saharan Africa (Sahel and Gulf of Guinea) prior to northward migration (February and March) were associated with advanced migration. Moreover, birds migrated earlier when winter rainfall in North Africa was more abundant. The relationships between relevant meteorological variables and timing of migration were remarkably consistent among species, suggesting a coherent response to the same extrinsic stimuli. All these results were obtained while statistically controlling for the long-term trend towards the earlier timing of spring migration across the Mediterranean that has been documented in previous analyses of the same dataset, a trend that was confirmed by the present analyses. In conclusion, our results suggest that thermal conditions in the wintering quarters, as well as rainfall in North African stopover areas, can influence interannual variation in migration phenology of trans-Saharan migratory birds, although the ecological mechanisms that causally link meteorological conditions to the timing of migration remain a matter of speculation.

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.…”

Populations of migratory bird species that did not show a phenological response to climate change are declining

“…This fact prevented us from quantifying change in migration phenology in relation to local climate, because climate during any part of the migratory path could potentially affect change in phenology (44,45). Therefore, we analyzed change in phenology over time, assuming that such change was due to change in climate, as has been done in other studies (8,9,21,35,46,47). Changes in migration dates were the slope of the simple linear regression of migration date on year, expressed as days/year.…”

Climate change and timing of avian breeding and migration throughout Europe

“…Increasing migration speed under high temperatures (Cochran & Wikelski 2005, Marra et al 2005, Saino et al 2007, Sparks & Tryjanowksi 2007 may seem an important adaptation to track climatic variability, because it allows birds to arrive and breed earlier in warm years. However, if the temperatures en route do not have high predictive power for the circumstances at the breeding grounds, this strategy fails, and may even lead to individuals arriving too early if they do not halt their migration when encountering adverse circumstances (Brown & Brown 1998).…”

“…Furthermore, there may even be increased selection for later, instead of earlier, arrival at more northern sites. We know that migration speed is strongly related to climatic effects at the wintering grounds and en route (Cotton 2003, Ahola et al 2004, Cochran & Wikelski 2005, Marra et al 2005, Hüppop & Winkel 2006, Gordo 2007, and the advanced progress made at the midway point during the migratory route from the African wintering grounds to the European breeding grounds is most likely a phenotypic response to changed environmental conditions, although an advance was still found after including African climatic variables (Saino et al 2007). …”