Migrating shorebird killed by raptor at 3000 m above ground as revealed by high-resolution tracking

Recent tracking technology has revealed that some birds fly at extreme altitudes up to and above 7000 m above sea level (asl) during nonstop migration flights (Lindström et al., 2021; Sjöberg et al., 2021). An untested hypothesis put forward is that birds migrate this high above the ground to avoid encountering predators. However, information about predation danger during the migratory flight is scant, and the altitudes where migrants are at risk are unknown.

While migration enables animals to exploit favorable habitat conditions in various locations throughout the year to optimize overall fitness (Lack, 1968), it can also be costly and is often associated with higher mortality rates (Klaassen et al., 2014). A significant factor contributing to these high mortality rates is the increased risk of predation during migration (Lindström, 1989; Sillett & Holmes, 2002; Walter, 1979). Consequently, safety is considered an important evolutionary driver of avian migration strategies (Alerstam et al., 2003; Alerstam & Lindström, 1990; Lank et al., 2003; Ydenberg et al., 2004).

In birds, predation has mostly been studied at migratory stopover sites (Dierschke, 2003; Lindström, 1989), but information about predation risk during the migratory flight is largely lacking, especially concerning at what altitudes migrants are at risk. It was recently revealed that during long nonstop flights, some migrants ascend at dawn to diurnal flight altitudes up to and above 7000 m asl (Lindström et al., 2021; Sjöberg et al., 2021). These studies propose that the birds try to avoid being predated by flying very high above the ground. Eleonora's Falcons Falco eleonorae, which specialize in hunting migratory passerines, have been recorded to make flights up to 3500 m (Xirouchakis & Panuccio, 2019), but the knowledge on how high avian predators ascend to hunt is very limited.

Using GPS trackers with on-board accelerometry, we tracked Arctic-breeding Grey Plovers Pluvialis squatarola (Figure 1a,b) migrating across Northern Europe. In January 2023, we tagged eight adult Grey Plovers with Druid MINI 2G transmitters on the island of Griend (53°15′ N, 5°15′ E) in the Dutch Wadden Sea. Tags were attached using leg-loop harnesses constructed from flattened spectra ribbon (2.5 mm width) and aluminum crimps (combined mass of transmitter and harness <8 g, equaling <5% of body mass). The solar-powered transmitters record GPS positions (including altitude), and ODBA (overall dynamic body acceleration, a measure for activity (Wilson et al., 2006), hereafter “body acceleration” for short). Data were transferred via the 2G GSM network. Interval settings varied from 2 to 24 h for GPS positions and 1–30 min for body acceleration measurements. The onboard tri-axial accelerometer sampled data continuously at 25 Hz which were summarized into one body acceleration value every 10 s following Wilson et al. (2006). These values were then used to calculate average body acceleration values based on the set interval (e.g., mean of 6 values for 1-min body acceleration setting).

At 21:58 local summer time on May 27 (25 min after sunset), one of the tagged birds suddenly stopped its migratory flight (Figure 2a,b). At the same moment, the body acceleration signal (1-min average) of the tracking device on this plover made a steep drop from 1.45 to 0.27 g (Figure 2c). Two minutes later (i.e., 22:00), the logger recorded a GPS fix at an altitude of 2882 m above ground (about 2900 m asl). Additional information from this fix was in strong contrast with the previous fixes: a change in course from 64° (NE) to 194° (SW) and a decrease in speed of flight from 14.4 ± 4.1 m/s to 6.4 m/s (Figure 2c). The subsequent GPS fix at 00:00 h on May 28 and all subsequent fixes came from a single position 8 km away.

Here, the tag and remains of the bird (Figure 1c) were found within 200 m of a peregrine nest, undoubtingly suggesting predation by a peregrine falcon (Falco peregrinus) as the cause of death. Together this suggests that the plover was captured by the falcon at 21:58. Considering the 2-min delay of GPS fix after the predation, the predation altitude was at least 2882 m above ground level (Figure 2c) but could have been higher. In the 15 min right before the predation event (21:43–21:58), the plover displayed increased body acceleration (1.44 ± 0.07 g, Figure 1c), which was significantly higher than the previous 15-min period body acceleration (1.26 ± 0.04 g, t(22) = −8.6, p < 0.0001). Increased body acceleration is associated with increased flight intensity (Krishnan et al., 2022), which could indicate that the plover spotted the predator peregrine falcon at around 21:43 and made an attempt to escape.

Peregrines use various hunting techniques to capture birds in flight (Zoratto et al., 2010), including a characteristic surprise stoop dive from above, which requires the peregrine to ascent to higher altitudes than its prey to hunt. In the case of the observed predation event, the potential attempt to escape (based on the observed 15-min increase in dynamic body acceleration) of the Grey Plover suggests that the peregrine would have used a pursuit tactic, which is also commonly observed (Dekker, 2009; Zoratto et al., 2010).

We also compared migration timing and flight altitude of the predated individual with three other tracked individuals from which we received data for the period from departure from the Wadden Sea and up to passage into the Russian Arctic (37° E). The route taken by the predated plover was similar to that of the other tracked individual plovers (Figure 2a). The predated plover started its migratory flight on May 26, which was also comparable with the other three tracked individuals (May 26–28). All four plovers flew higher above land (mean flight altitude 1726 ± 966 m, Figure 2b) than above sea (302 ± 717 m, Figure 2b). The altitude at which the predated plover was captured (2882 m) was at the higher end of the altitudinal range of flight over land (0–3216 m) in our dataset.

Although in-flight predation of birds in active migration is indirectly documented through the large numbers of migratory prey species in the diet of falcons (Xirouchakis et al., 2019; Zuberogoitia et al., 2013), direct evidence is limited to ground-based observations, by necessity limiting the altitudes at which predation events can be recorded (Rudebeck, 1950, 1951). This limitation is lifted by the recent application of ever lighter high-resolution tracking devices on small-bodied birds, enabling us to remotely record in-flight predation (e.g., Bom et al., 2023). The observed predation event constitutes an exceptional record of high-altitude hunting by Peregrine Falcons. These falcons are specialized in hunting birds and capturing them in flight (Xirouchakis et al., 2019; Zuberogoitia et al., 2013). During migratory flight, Peregrine Falcons can reach altitudes up to 5600 m (Dixon et al., 2017), but up to which altitude they hunt is unclear. GPS-tracking of Greater Noctule Bats Nyctalus lasiopterus, known to prey on migratory birds (Ibáñez et al., 2001), has unveiled they forage up to 1659 m above ground level (Naďo et al., 2019). In areas and periods with sufficient prey availability at high altitude (e.g., migration corridors during shorebird migration), hunting at high altitudes may be a profitable strategy.

While predation risk is considered an important evolutionary driver of migration strategies (Alerstam & Lindström, 1990; Lank et al., 2003; Ydenberg et al., 2004), its role in shaping flight altitudes remains only speculative. The application of high-resolution tracking now makes it possible to address questions on where, when, and up to which altitude birds risk predation during migratory flight, and for example whether flying at extreme altitudes during the day may be a successful adaptive strategy to avoid predators. Our observation suggests that this may not always be the case, as increasing flight altitude up to 3000 m above ground is clearly not enough for migrants to avoid predation risk.

The authors declare no conflicts of interest.

Permits to catch, handle, and tag Grey Plovers were granted to the NIOZ under protocol number AVD80200202215943.