Fig 1.
Locations of stationary sites and stopover durations of migrating European nightjars.
(A) Locations of breeding (square), wintering (triangles), and stopover (circles) sites. Colors correspond to panels B and C. (B) Individual stopover durations in the Temperate and Sahel zones, respectively. Vertical bars indicate medians, boxes indicate 25th to 75th percentiles, whiskers indicate ranges, and black dots indicate outliers. (C) Pooled distribution of stationary durations showing a peak at 22 days. The map was plotted using the R package rworldmap [20]. Underlying data are found in S1 Data.
Fig 2.
Individual activity pattern and presumed daily foraging time during the nonbreeding season of European nightjars.
(A) Actogram showing the hourly activity index of an individual with no (white) to continuous (black) flight activity throughout the nonbreeding season. Gray corresponds to intermittent flight activity representing foraging flights. The bird’s activity is confined to the night and only exceptionally is activity recorded during daylight hours, which demonstrates the species’ crepuscular and nocturnal lifestyle. Black vertical lines represent periods of high activity spanning over several hours (indicative of migratory flights), and aggregations of black lines represent consecutive nights of migratory flights. Intermittent flight activity, presumably related to foraging, is concentrated towards dusk and dawn (horizontal arrows to the right). Following the daily cycle of the moon, intermittent flight activity is also registered during nighttime, resulting in diagonal bands throughout the nonbreeding season (vertical arrows). Notice the shorter activity periods from mid-April to May and in August, which reflect the shorter nights at northern latitudes. (B) The number of hours with registrations of presumed foraging activity per day and individual of nightjars in 2016–2017 (small dots) and daily means (large dots). Superimposed is a sine curve with a 29.54-day period, approximating the lunar cycle. Underlying data are found in S2 Data and S3 Data. GMT, Greenwich Mean Time.
Fig 3.
Daily travel intensity of European nightjars.
(A) Daily fraction of individuals tracked during 2015–2018 undertaking a migratory flight, ranging from none (0) to all (1). Filled and open circles represent the timing of new and full moon, respectively. (B) Travel intensity data from the migration periods pooled relative to the lunar cycle. The daily fraction of traveling nightjars increases during the waning moon and peaks 11 days after the full moon. Underlying data are found in S4 Data.
Fig 4.
Realized departure events and relative benefit of optimal departure timing under a range of stopover durations.
Box plots at median stopover durations corresponding to the four annual stopovers (see Fig 1). Bars indicate median departures, boxes indicate 25th to 75th percentiles, whiskers indicate ranges, and black dots show outliers. Departures could be triggered by dwindling fuelling rates as a fixed proximate factor regardless of stopover duration (horizontal dashed line). Alternatively, the nightjars may strive to maximize the ratio between cumulative foraging time and total stopover duration by centering the stopover around full moon. This would lead to a positive correlation between stopover duration and departure timing relative to the lunar cycle, increasing from full moon to new moon as stopover duration increases towards one lunar month (solid line). Underlying data are found in S1 Data.