Flight altitude dynamics of migrating European nightjars across regions and seasons

Abstract

Avian migrants may fly at a range of altitudes, but usually concentrate near strata where a combination of flight conditions is favourable. The aerial environment can have a large impact on the performance of the migrant and is usually highly dynamic, making it beneficial for a bird to regularly check the flight conditions at alternative altitudes. We recorded the migrations between northern Europe and sub-Saharan Africa of European nightjars Caprimulgus europaeus to explore their altitudinal space use during spring and autumn flights and to test whether their climbs and descents were performed according to predictions from flight mechanical theory. Spring migration across all regions was associated with more exploratory vertical flights involving major climbs, a higher degree of vertical displacement within flights, and less time spent in level flight, although flight altitude per se was only higher during the Sahara crossing. The nightjars commonly operated at ascent rates below the theoretical maximum, and periods of descent were commonly undertaken by active flight, and rarely by gliding flight, which has been assumed to be a cheaper locomotion mode during descents. The surprisingly frequent flight-altitude shifts further suggest that nightjars can perform vertical displacements at a relatively low cost, which is expected if the birds can allocate potential energy gained during climbs to thrust forward movement during descents. The results should inspire future studies on the potential costs associated with frequent altitude changes and their trade-offs against anticipated flight condition improvements for aerial migrants.

Foraging on the wing for fish while migrating over changing landscapes: traveling behaviors vary with available aquatic habitat for Caspian terns

“…Previous research has demonstrated that the selection of flight altitudes may be key in achieving optimal wind support and play an important role in successfully surpassing ecological barriers [ 22 ]. Also, a recent study on European nightjars has shown that altitude shifts are frequent during migration [ 68 ], supporting the high within-season variation in flight heights we found for Caspian terns.…”

Migration routes and timing of European Nightjars (Caprimulgus europaeus) breeding in eastern Mongolia

“…Of the 16 deployments in 2019, we were only able to recover one multi-sensor logger due to the COVID-19 pandemic preventing the organization of extensive fieldwork in 2020 and 2021. The recovery rate of 15%, although low, lies within the variation in recovery rates in Western European populations where trapping efforts have been constant across years (Norevik et al 2021), and can most likely be attributed to bad weather conditions during a 2-week trapping session in Mongolia (July 2019). The two loggers recovered in 2019 recorded partial migration cycles, stopping 17 days and 58 days after departure from the wintering site.…”

“…First, we used the activity data to determine stationary periods, defined as periods of minimum 12 h during which a bird stays within the same location. As nightjars use flapping flight for migration (Norevik et al 2021), we used an automated k-means classification algorithm to classify periods of flapping flight as migratory flight. Since the pressure analysis relies on high precision of this classification, we manually edit the activity and the pressure timeseries following the recommendation from Nussbaumer et al (2022b).…”

The spatial consistency of migratory route and stopover choice in European nightjars quantified by nearest neighbour analysis on multi-annual GPS tracks

“…To maximise the recapture probability of birds with tags also birds in proximity to the territory of the deployment site were targeted for trapping during the following year(s). For more information regarding recapture rates of tagged nightjars within the study population see Norevik et al (2021).…”