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Titre :	Influence of weather on the autumn migration of waders 'Charadrii' at Blavandshuk, western Denmark 1963-2017
Auteurs :	Hans Meltofte, Auteur ; Jan Durinck, Auteur ; BENT Jakobsen, Auteur ; Claus Nordstrom, Auteur ; FRANK F. Riget, Auteur
Type de document :	Article
Dans :	Dansk Ornitologisk Forenings Tidsskrift (112(2) 2018)
Article en page(s) :	40-52
Résumé :	Meltofte & Rabol (1977) analysed the weather dependence of the visible autumn migration of 12 wader species at Blavandshuk on the west coast of Denmark using the first eight years of data from Blavand Bird Observatory. Eight years is not long when it comes to such a variable factor as weather, so here we update the earlier analysis based on 50 years of data for 19 species. The much bigger dataset has the further advantage that we could focus the analyses more specifically on the periods with the most concentrated migrations of adults (Tab. 1), where most birds head for the Wadden Sea of Denmark, Germany and The Netherlands.Observations were made from the beach or the outer dunes from around sunrise using binoculars and telescopes, and only days with at least three hours of observation were accepted for analysis. Furthermore, only species with more than 400 recorded individuals and only birds heading south were included. Weather data was taken at 6 a. m. from Thyboron c. 125 km north of Blavandshuk, i.e. from the general area from where the migrating birds at Blavandshuk appear. We excluded weak winds, i.e. below 1.5 m/s, from the wind direction analyses.All tests for correlation (Pearson's product-moment correlation coefficient) between the migration intensity of a species and a meteorological parameter were carried out as two-sided t-tests. Correlation between migration intensity and wind direction (VR) is not straightforward, because wind direction is a circular variable. Therefore, our determination of the correlation coefficient follows the method used by Meltofte & Rabol (1977). The analysis was performed stepwise on the transformation cos (n x 20 degrees - VR), where n denotes the numbers 0, 1, 2,...17. The largest positive correlation coefficient of the 18 values thus found indicates the specific wind direction, which has the best correlation with the migration intensity. An average vector for each of the 19 species was determined based on the wind direction and migration intensity in 12 direction categories (0 degrees, 30 degrees,. 330 degrees). As a measure of the spread of the migration intensity in relation to compass direction, a descriptor termed concentration was calculated for each species as the size of the average vector divided by the sum of the mean migration intensity per day for all 12 direction intervals. Concentration can attain values in the range 0-1, where values close to 1 denote migration intensities closely centred around a particular wind direction. Correlation analysis between each of the 19 individual species was performed to detect possible similarities in the migration of the species (Fig. 3).The results of our extended analysis to a high degree sharpen the picture that emerged in the first analysis (Fig. 1 and Tab. 2). The migration of the majority of the species involved is most intensive in winds from easterly directions. This is most pronounced for Ruff Calidris pugnax, Curlew Sandpiper Calidris ferruginea, Dunlin Calidris alpina, Little Stint Calidris minuta, Common Snipe Gallinago gallinago, Common Sandpiper Actitis hypoleucos, Spotted Redshank Tringa erythropus, Common Greenshank Tringa nebularia and Common Redshank Tringa totanus. However, intensive migration of all of these species includes a more or less pronounced component of southeasterly winds. This is even more so for Grey Plover Pluvialis squatarola, Eurasian Golden Plover Pluvialis apricaria, Common Ringed Plover Charadrius hiaticula, Eurasian Curlew Numenius arquata and Bar-tailed Godwit Limosa lapponica, while Ruddy Turnstone Arenaria interpres, Red Knot Calidris canutus and Sanderling Calidris alba has an even stronger southly wind component, and Whimbrels Numenius phaeopus migrate most intensively in winds in the entire southern sector.Eurasian Oystercatcher Haematopus ostralegus differs markedly from this pattern of preference for eastern to southern winds. For this species, the wind preference has two peaks, with the highest migration intensity in westerly to northwesterly winds and a secondary peak in easterly and southeasterly winds (Fig. 1).A number of other weather parameters also show highly significant correlation with individual species (Tab. 2). For several species preferentially migrating in easterly winds, temperature and temperature increase correlate with high migration intensity. Likewise, air pressure and especially air pressure decreases (i.e. in relation to the days before) correlate with the migration of a number of these species. This pertain to Common Sandpiper, Spotted Redshank, Greenshank and Redshank in particular, which also show significant negative correlation with cloud cover.For Ruddy Turnstone, Red Knot and Sanderling (plus Dunlin), which have a pronounced element of intensive migration in southerly winds, it is striking that there is strong positive correlation with air humidity (= haze) and to a varying degree also with air temperature (Tab. 2). For Red Knot and Sanderling, as almost the only species, there is also a strong negative correlation with wind speed, while the Ruddy Turnstone differs in a number of weather elements.Oystercatchers are also different in this respect, since they often migrate with high intensity in overcast, cool, windy and rainy weather with 'long term' air pressure decrease (from two days before; Tab. 2). Even the migration of Whimbrels is correlated with precipitation, cloud cover and low air pressure.We also found strong positive correlation and never any negative correlation between high migration intensity in the 19 species (Fig. 3).Based on these results, we can delineate the general weather patterns associated with intensive migration at Blavandshuk. For the many migrants occurring in easterly winds, the high degree of correlation with warm weather is a consequence of easterly winds in late summer. However, the correlation with short-term air pressure decrease may be the result of a low pressure system that has just begun to appear from the west. For the three 'south wind species' - Turnstone, Knot and Sanderling - together with Dunlin and Common Snipe that also migrated at high intensity in southerly winds, the pronounced correlation with high humidity (haze), and to some extent also with high temperature and temperature increase (Tab. 2), point to a weather situation just after a warm front has passed. And lastly, the 'preference' of 'west wind Oystercatchers' for overcast, cool, windy and rainy weather with low and 'long term' falling air pressure is typically related to the tail-end of a cold front of a low pressure system moving eastwards.An important part of the motivation for Meltofte & Rabol's (1977) analyses was to use the wind directions with the most intensive migration together with the numerical dominance of the individual species at Revtangen in southwestern Norway and at Falsterbo and Ottenby in southern Sweden in relation to Blavand to unravel the geographical origin of the migrants. We can confirm most of these assessments. We find it most likely that migrants in easterly and southeasterly winds mainly originate from northern Russia and have reached the west coast of Denmark from the east after passage over southern Scandinavia (see Meltofte 2008). By contrast, the 'west wind Oystercatchers' must originate from Norway and have reached the west coast of Denmark after passage over the North Sea. The 'south wind' migrants are harder to elucidate. According to the numbers at the three passage points mentioned above leading to Blavandshuk, most Turnstones, Knots and Sanderlings appear to have passed over the North Sea and reached the coast from the west. But unlike the Oystercatchers, they may have come directly from the Nearctic breeding grounds without staging in Norway. An important proportion of Whimbrels, Curlews and Common Snipes seems to originate from the east but after having initiated the migration much closer to Blavandshuk than most other species.We already know that many Greenland Sanderlings pass southwestern Scandinavia in autumn (Ferdinand 1953, Grimeland 1967, Meltofte 1993, Reneerkens et al. 2009). It is unclear, however, how many Siberian Sanderlings join the East Atlantic Flyway in autumn (Reneerkens et al. 2009). While roughly the same numbers pass Revtangen and Blavandshuk, only a few percent of these numbers occur at Falsterbo and Ottenby (Tab. 4 in Meltofte & Rabol 1977). Sanderling migration is also very modest at Cape Poosaspea in Estonia, where their numbers are counted in the hundreds (Ellermaa et al. 2010). Considering that most of the migration takes place at great height and thus out of sight (cf. Meltofte 2008), these observations are an indication that modest but appreciable numbers of Siberian Sanderlings reach West European coasts in autumn just like they do in spring (Fischer & Meltofte 2015).It has been known for a long time that the majority of Oystercatchers passing Blavandshuk are Norwegian breeding birds (Preuss 1961, Thelle 1970, Meltofte & Rabol 1977, Meltofte 1993). However, the large numbers that pass Blavandshuk in easterly and southeasterly winds support the conclusion by Meltofte (1993) that many of the Oystercatchers in the northern part of the Wadden Sea are from the inner part of the Baltic and the White Sea. If we separate the total numbers of Oystercatchers passing Blavandshuk in winds between 195 degrees and 29 degrees from birds passing in winds between 30 degrees and 194 degrees, and leave out birds passing in wind speeds below 1.5 m/s, about one fourth of the migrants are likely to be from the Baltic and the White Sea. In rounded figures this gives 40 000-50 000 'Norwegian' and 15 000 Baltic and the White Sea birds out of an estimated maximum of possibly 70 000-75 000 Oystercatchers that passed Blavand each year in the beginning of the study period -dependent on the dominating wind patterns during the peak migration period in the individual years. Both populations hav

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