Desert crossing strategies of migrant songbirds vary between and within species

Abstract

Each year, billions of songbirds cross large ecological barriers during their migration. Understanding how they perform this incredible task is crucial to predict how global change may threaten the safety of such journeys. Earlier studies based on radar suggested that most songbirds cross deserts in intermittent flights at high altitude, stopping in the desert during the day, while recent tracking with light loggers suggested diurnal prolongation of nocturnal flights and common non-stop flights for some species. We analyzed light intensity and temperature data obtained from geolocation loggers deployed on 130 individuals of ten migratory songbird species, and show that a large variety of strategies for crossing deserts exists between, but also sometimes within species. Diurnal stopover in the desert is a common strategy in autumn, while most species prolonged some nocturnal flights into the day. Non-stop flights over the desert occurred more frequently in spring than in autumn, and more frequently in foliage gleaners. Temperature recordings suggest that songbirds crossed deserts with flight bouts performed at various altitudes according to species and season, along a gradient ranging from low above ground in autumn to probably >2000 m above ground level, and possibly at higher altitude in spring. High-altitude flights are therefore not the general rule for crossing deserts in migrant songbirds. We conclude that a diversity of migration strategies exists for desert crossing among songbirds, with variations between but also within species.

Figure 1

Light intensity (log-transformed), minimal and maximal temperatures (if available) recorded by geolocators during autumn desert crossing for six individuals of different species. Green rectangles represent possible flight bouts. (a) ortolan bunting R328: FLP during three consecutive days; (b) whinchat BG251: long FLP on second day and short FLP on fourth day, so not on consecutive days; (c) spotted flycatcher BN276: FLP in the third day, with lower temperatures in the morning of that day. (d) wood warbler BM370: non-stop flights during three nights and two days, revealed by complete FLPs and low temperatures; (e) tree pipit BL909: third and fourth days with obvious FLPs; no temperature data available. (f) nightingale BE506: two nights with large drops in minimal and maximal temperatures, the second followed by a short FLP, corresponding to a low minimal temperature saved at 9:39, so recorded during the four previous hours (5:39–9:39), while sunrise occurred at 6:00.

Figure 2

Light intensity (log-transformed) and minimum and maximum temperature (if available) recorded by geolocators during spring desert crossing, for six individuals of different species. Green rectangles represent possible flight bouts. (a) ortolan bunting R313: FLPs over two consecutive days, and temperature drops in the third night revealing a third night of active migration at a relatively high altitude; (b) whinchat BG416: two full-day FLPs during two consecutive days; species foraging in the open, like whinchat and flycatchers, where a full-day FLP is harder to distinguish from usual patterns than in species foraging in bushes and trees, like wood warbler and nightingale; (c) spotted flycatcher BN275: non-stop flight during two nights and a complete day, followed by a prolongation during most of the second day, for a flight bout that lasted potentially up to 45 hours, while the low temperatures during the following night reveal another nocturnal flight bout; (d) Mediterranean flycatcher BD454 spring: full-day FLP on the second day, and FLP in the morning of the third day, for a non-stop flight bout that lasted potentially up to 40 hours; (e) wood warbler BM424: a non-stop flight that lasted potentially up to 48 hours; (f) tree pipit BL919: a non-stop flight that lasted potentially up to 40 hours.

Figure 3

Detailed autumn overview of daytime light pattern anomalies (FLP) recorded by geolocators while birds were crossing a desert. Each category is accompanied by a representative figure of recorded light intensities, description of the anomaly and the most plausible interpretation, numbers of individuals and species-specific % of occurrence, in line with previous publications. OB: ortolan bunting, WH: whinchat, SF: spotted flycatcher, MF: Mediterranean flycatcher, WI: willow warbler, WO: wood warbler, RW: Eurasian reed warbler, TP: tree pipit, NG: nightingale, SR: rufous-tailed scrub-robin. The first four categories total 88.1% of tracked individuals (stopovers in the desert), the last three categories 11.9% (probable non-stop flight over the desert.

Figure 4

Detailed spring overview of daytime light pattern anomalies (FLP) recorded by geolocators while birds were crossing a desert. Each category is accompanied by a representative figure of recorded light intensities, numbers of individuals and species-specific % of occurrence, in line with previous publications. OB: ortolan bunting, WH: whinchat, SF: spotted flycatcher, MF: Mediterranean flycatcher, WO: wood warbler, RW: Eurasian reed warbler, TP: tree pipit, NG: nightingale, SR: rufous-tailed scrub-robin. The first four categories totalize 55% of tracked individuals (stopovers in the desert), the last three categories 45% (probable non-stop flight over the desert) – see Fig. 3 for the description and interpretation of light patterns.

Figure 5

Boxplot of the time of landing after sunrise for days with FLPs not lasting a complete day, during the autumn (7 species) and spring (6 species) migrations. Average, box of 25^th and 75^th quartiles, and 5%-95% confidence intervals, with dots showing potential outliers. Labels of the x-axis are codes for species names: OB: ortolan bunting, WH: whinchat, SF: spotted flycatcher, MF: Mediterranean flycatcher, WI: willow warbler, RW: Eurasian reed warbler, TP: tree pipit, NG: nightingale.

Figure 6

Boxplot showing the differences (average, box of 25^th and 75^th quartiles, and 5%-95% confidence intervals) in maximum temperature recorded on a day of prolonged nocturnal flight compared to the maximum temperature recorded at the same hour the previous day, or on the next day that occurred without a prolonged flight during daylight. Labels of the x-axis are codes for species names and seasons: OB: ortolan bunting, WI: willow warbler, WO: wood warbler, SF: spotted flycatcher, MF: Mediterranean flycatcher, SR: rufous-tailed scrub-robin, NG: nightingale.

Figure 7

Stationary sites of individual birds (median ±25th/75th or 20th/80th percentiles of location estimates) just prior and after the occurrence of the full light pattern(s) detected during autumn (a–d) and spring (e–h) migration; red = ortolan bunting (a,e); light blue = spotted flycatcher, dark blue = Mediterranean flycatcher (b,f); light green = willow warbler, dark green = wood warbler (c,g); orange = Eurasian reed warbler, dark red = rufous-tailed scrub-robin (d,h).