We fitted male willow warblers with Intigeo–W30 geolocators (Migrate Technology LTD, 0.3 g, c. seven months capacity) in East Denmark (55.61°N, 12.57°E; catching range 500 m) from May to mid-June (n = 17 in 2014, n = 20 in 2015) using leg-loop harnesses [39 (link)] made of 1 mm braided nylon cord. 17 birds were recaptured the year after tagging (n = 11 in 2015, n = 6 in 2016). Two loggers from 2014 contained no data.
Positions were estimated using the GeoLight package [40 (link)] in R [41 ]. A threshold of 3 lx was used and sun elevation angles between −3° and 0° provided the best fit using Hill-Ekström calibration [38 (link)] (breeding area calibration produced similar spatiotemporal patterns, Additional file1 : Appendix S2-S4).
Periods of no overall change in longitude for ≥5 days were considered staging. We excluded latitude from positions within ten days of equinox. Position outliers >10° from median longitude/latitude at each staging site were excluded (Additional file1 : Appendix S1).
Normalized Difference Vegetation Index (NDVI) was used to estimate vegetation conditions [42 (link)]. NDVI was obtained from the MODIS satellite product MOD13C1 [43 ]. Mean NDVI within a radius of 50 km for each wintering site were extracted with the adehabitat R package [44 (link)].
Data were pooled for all analyses because t-tests revealed no differences between the two years in average latitude (p = 0.44), longitude (p = 0.79) or NDVI (p = 0.23).
The western ‘detour’ between the staging sites before and after the Sahara coincided with Equinox. We estimated average westernmost latitude projecting from the mean position of the last European staging sites assuming a speed of 300 km/day (daily migration speed of willow warblers ringed in Denmark [45 ]).
Longitudinal spread of birds in winter was estimated using the loxodromic distance between longitude of the centre of mass for all individuals and the latitude and longitude of the centre of mass for each individual in five-day intervals in R using SDMTools [46 ] and geosphere [47 ].
We correlated arrival date, body mass, wing length and NDVI with longitude to evaluate causes of winter spread using Pearson’s r (Note that a weak relationship with arrival date is expected because of extra travel time). We tested for consistent north-south or east-west directional changes and direction of change in NDVI between consecutive winter sites using Sign tests. Lastly, we investigated trends over time in NDVI within sites using Pearson’s r. Potential effects of variation in longitudinal distribution of NDVI in earlier years on termination of migration were investigated by correlating site-specific NDVI among the last three winters before capture.
Positions were estimated using the GeoLight package [40 (link)] in R [41 ]. A threshold of 3 lx was used and sun elevation angles between −3° and 0° provided the best fit using Hill-Ekström calibration [38 (link)] (breeding area calibration produced similar spatiotemporal patterns, Additional file
Periods of no overall change in longitude for ≥5 days were considered staging. We excluded latitude from positions within ten days of equinox. Position outliers >10° from median longitude/latitude at each staging site were excluded (Additional file
Normalized Difference Vegetation Index (NDVI) was used to estimate vegetation conditions [42 (link)]. NDVI was obtained from the MODIS satellite product MOD13C1 [43 ]. Mean NDVI within a radius of 50 km for each wintering site were extracted with the adehabitat R package [44 (link)].
Data were pooled for all analyses because t-tests revealed no differences between the two years in average latitude (p = 0.44), longitude (p = 0.79) or NDVI (p = 0.23).
The western ‘detour’ between the staging sites before and after the Sahara coincided with Equinox. We estimated average westernmost latitude projecting from the mean position of the last European staging sites assuming a speed of 300 km/day (daily migration speed of willow warblers ringed in Denmark [45 ]).
Longitudinal spread of birds in winter was estimated using the loxodromic distance between longitude of the centre of mass for all individuals and the latitude and longitude of the centre of mass for each individual in five-day intervals in R using SDMTools [46 ] and geosphere [47 ].
We correlated arrival date, body mass, wing length and NDVI with longitude to evaluate causes of winter spread using Pearson’s r (Note that a weak relationship with arrival date is expected because of extra travel time). We tested for consistent north-south or east-west directional changes and direction of change in NDVI between consecutive winter sites using Sign tests. Lastly, we investigated trends over time in NDVI within sites using Pearson’s r. Potential effects of variation in longitudinal distribution of NDVI in earlier years on termination of migration were investigated by correlating site-specific NDVI among the last three winters before capture.
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