Climate in Africa sequentially shapes spring passage of Willow WarblerPhylloscopus trochilusacross the Baltic coast

Abstract

Background Many migrant birds have been returning to Europe earlier in spring since the 1980s. This has been attributed mostly to an earlier onset of spring in Europe, but we found the timing of Willow Warblers’ passage to be influenced by climate indices for Africa as much as those for Europe. Willow Warblers’ spring passage through northern Europe involves populations from different wintering quarters in Africa. We therefore expected that migration timing in the early, middle and late periods of spring would be influenced sequentially by climate indices operating in different parts of the winter range. Methods Using data from daily mistnetting in 1 April–15 May over 1982–2017 at Bukowo (Poland, Baltic Sea coast), we derived an Annual Anomaly (AA, in days) of Willow Warbler spring migration. We decomposed this anomaly into three main periods (1–26 April, 27 April–5 May, 6–15 May); one-third of migrants in each period. We modelled three sequential time series of spring passage using calendar year and 15 large-scale climate indices averaged over the months of Willow Warblers’ life stages in the year preceding spring migration as explanatory variables in multiple regression models. Nine climate variables were selected in the best models. We used these nine explanatory variables and calculated their partial correlations in models for nine overlapping sub-periods of AA. The pattern of relationships between AA in these nine sub-periods of spring and the nine climate variables indicated how spring passage had responded to the climate. We recommend this method for the study of birds’ phenological responses to climate change. Results The Southern Oscillation Index and Indian Ocean Dipole in Aug–Oct showed large partial correlations early in the passage, then faded in importance. For the Sahel Precipitation Index (PSAH) and Sahel Temperature Anomaly (TSAH) in Aug–Oct partial correlations occurred early then peaked in mid-passage; for PSAH (Nov–March) correlations peaked at the end of passage. NAO and local temperatures (April–May) showed low correlations till late April, which then increased. For the Scandinavian Index (Jun–Jul) partial correlations peaked in mid-passage. Year was not selected in any of the best models, indicating that the climate variables alone accounted for Willow Warblers’ multiyear trend towards an earlier spring passage. Discussion Climate indices for southern and eastern Africa dominated relationships in early spring, but western African indices dominated in mid- and late spring. We thus concluded that Willow Warblers wintering in southern and eastern Africa dominated early arrivals, but those from western Africa dominated later. We suggest that drivers of phenological shifts in avian migration are related to changes in climate at remote wintering grounds and at stopovers, operating with climate change in the north, especially for species with complex and long-distance migration patterns.

Large-Scale Climatic Patterns Have Stronger Carry-Over Effects than Local Temperatures on Spring Phenology of Long-Distance Passerine Migrants between Europe and Africa

“…Negative and positive values of AA indicated, respectively, earlier and later passage of spring than the long-term average curve for the species in focus. We then used the annual anomalies (AA) for each species' spring migration as the response variable in multiple regression models, with 15 climate indices and calendar year as explanatory variables; we used the same variables as in [23]. These were based on groups of months pertinent to the birds' life-stages during the year preceding the spring migration (Figure 3); see [23] and Sections 2.4 and 2.5 for details.…”

“…We have shown that, in one long-distance migrant, the Willow Warbler, the combined effect of multiple indices of climate at both its breeding grounds in Europe and its nonbreeding grounds in western, eastern, and southern Africa explained not only the long-term trend to earlier arrival, but also nearly 60% of the year-to-year variation in timing of this species' spring migration in 1982-2017 at the Polish coast of the Baltic Sea [22,23]. Thus, we also expect that, in other long-distance migrants, a combination of several large-scale and local climate indices should help explain their spring arrival timing in Europe.…”

“…Many migrant birds have been arriving earlier in spring in Europe and in North America since about the 1980s; this has mostly been attributed to increased spring temperatures at their northern stopovers and breeding grounds-an influence of climate change [1][2][3][4][5][6][7][8][9][10][11][12][13]. However, long-distance migrants to Europe begin their spring journey to the breeding grounds several weeks earlier, at wintering grounds in Africa; thus, climatic variation at these remote non-breeding grounds also influences the spring timing of these species [2,[13][14][15][16][17][18][19][20][21][22][23][24][25][26][27].…”

“…Fewer studies have related the timing of spring's arrival in Europe of long-distance migrants to climate and habitat variability in eastern and south-eastern Africa [2,[19][20][21]26,43]. Even fewer studies have shown that climatic variation in different parts of Africa [2,22,23,26], or in both hemispheres [2,24,[44][45][46], might jointly influence spring arrivals of migrants to Europe for breeding.…”

“…The precipitation which migrants experience at their wintering grounds and stopover areas has been shown to influence their spring arrival in Europe [2,14,16,19,[24][25][26][27]40,48,49,51,52]. Climate variability in Africa and Europe, including rainfall and temperatures, is largely shaped by ocean-atmospheric interactions; thus, large-scale climate indices that reflect these interactions, such as the Northern Atlantic Oscillation Index (NAOI), the Southern Oscillation Index (SOI/ENSO), and the Indian Ocean Dipole (IOD), might be used as convenient proxies for ecological conditions that the migrant birds experience over wide non-breeding grounds at different stages of their life [2,15,[21][22][23]26,[35][36][37]43,53]. The Northern Atlantic Oscillation (NAO) and the Scandinavian Pattern (SCAND) are proxies for conditions on the breeding grounds in north-western, central, and north-eastern Europe for many long-distance migrants [22,23,45,54].…”