Challenging a 15‐year‐old claim: The North Atlantic Oscillation index as a predictor of spring migration phenology of birds

Abstract

Many migrant bird species that breed in the Northern Hemisphere show advancement in spring arrival dates. The North Atlantic Oscillation (NAO) index is one of the climatic variables that have been most often investigated and shown to be correlated with these changes in spring arrival. Although the NAO is often claimed to be a good predictor or even to have a marked effect on interannual changes in spring migration phenology of Northern Hemisphere breeding birds, the results on relations between spring migration phenology and NAO show a large variety, ranging from no, over weak, to a strong association. Several factors, such as geographic location, migration phase, and the NAO index time window, have been suggested to partly explain these observed differences in association. A combination of a literature meta-analysis, and a meta-analysis and sliding time window analysis of a dataset of 23 short- and long-distance migrants from the constant-effort trapping garden at Helgoland, Germany, however, paints a completely different picture. We found a statistically significant overall effect size of the NAO on spring migration phenology (coefficient = -0.14, SE = 0.054), but this on average only explains 0%-6% of the variance in spring migration phenology across all species. As such, the value and biological meaning of the NAO as a general predictor or explanatory variable for climate change effects on migration phenology of birds, seems highly questionable. We found little to no definite support for previously suggested factors, such as geographic location, migration phenology phase, or the NAO time window, to explain the heterogeneity in correlation differences. We, however, did find compelling evidence that the lack of accounting for trends in both time series has led to strongly inflated (spurious) correlations in many studies (coefficient = -0.13, SE = 0.019).

Peer Review #2 of "Climatic variation in Africa and Europe has combined effects on timing of spring migration in a long-distance migrant Willow Warbler Phylloscopus trochilus (v0.2)"

“…Forchhammer, Post & Stenseth, 2002;Hüppop & Hüppop, 2003;Ahola et al, 2004;Vähätalo et al, 2004;Rainio et al, 2006;Gordo, 2007;Jonzén et al, 2007;Saino & Ambrosini, 2008;. At Helgoland, several passerines begun arriving early after a positive December-March NAOI (Hüppop & Hüppop, 2003), though NAOI explained only a small part of the variation in spring migration phenology in short-and long-distance migrants (Haest, Hüppop & Bairlein, 2018). Similarly, in our study, November-March NAOI explained only 13% of the variation in the timing of Willow Warblers' spring migration (Table 2).…”

“…Advances in spring arrival in Europe have been related to the winter and annual North Atlantic Oscillation Index (NAOI), which reflect weather patterns in western Europe and northern Africa (Forchhammer, Post & Stenseth, 2002;Møller, 2002;Cotton, 2003;Stenseth et al, 2003;Ahola et al, 2004;Gordo, Barriocanal & Robson, 2011;Grimm et al, 2015). NAOI explained only 0%-6% of the variance in the phenology of spring migration in 23 species of migrants in Helgoland, Germany (Haest, Hüppop & Bairlein, 2018), probably because this index does not reflect the conditions the migrants experience farther south.…”

Different trends of neighboring populations of Lesser Kestrel: Effects of climate and other environmental conditions

“…Finally, the North Atlantic Oscillation (NAO) index between December and March (“winter NAO”) was used to assess general ecological conditions experienced by birds during spring migration (stage 4, Figure ). Winter NAO is one of the most used predictors in studies of the effects of climate variability on the migration ecology of European birds (e.g., Gordo, Barriocanal, & Robson, ; Guéry et al, ; Sæther et al, ; Saino et al, , but see Haest, Hüppop, & Bairlein, for criticism on its use). Although the spring migration of the Lesser Kestrel spans only few days (stage 4, Figure ), the winter NAO index calculated during this period is a good proxy of the general winter and early spring meteo‐climatic conditions over large areas of the Northern Hemisphere (see e.g., Guéry et al, ; Wang, Ting, & Kushner, ).…”

Predictable shorebird departure patterns from a staging site can inform collision risks and mitigation of wind energy developments

“…weather at the time of arrival), due to dissociated internal and external cues experienced at the previous departure site (Alerstam & Lindström , Haest et al . ). In contrast, we predicted that departures would be related to both local time of day and weather, with birds departing around sunset and with winds blowing towards the north (assisting winds) (Ma et al .…”

“…Although weather systems can predict migratory bird arrivals at larger temporal and spatial scales, we predicted that arrivals would show weak or no relationships with short-term, local environmental conditions (i.e. weather at the time of arrival), due to dissociated internal and external cues experienced at the previous departure site (Alerstam & Lindstr€ om 1990, Haest et al 2018. In contrast, we predicted that departures would be related to both local time of day and weather, with birds departing around sunset and with winds blowing towards the north (assisting winds) (Ma et al 2011, Gr€ onroos et al 2012.…”