Multi-scale habitat selection throughout the annual cycle of a long-distance avian migrant

GPS-tagged Nightjar being released, by Jody Lawrence / BTO

Author(s): Lathouwers, M., Dendoncker, N., Artois, T., Beenaerts, N., Conway, G., Henderson, I., Shewring, M., Cross, T., Ulenaers, E. & Evens, R.

Published: October 2023

Journal: Ecological Indicators Volume: 156

Article No.: 111099

Digital Identifier No. (DOI): 10.1016/j.ecolind.2023.111099

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Long distance migrants, such as those breeding in western Europe and wintering south of the Sahara in Africa, face perilous journeys during autumn and spring. During the annual cycle, the habitats they use vary considerably between breeding and wintering sites, as well as on essential migration stopovers in very different landscapes on different continents.

There are several geographical barriers to negotiate and large expanses of inhospitable habitat, such as sea and desert, to cross. In this collaborative study, high resolution GPS tracking data were used to investigate how the Nightjar achieves this journey, and the strategy these birds use to locate essential foraging habitat along the way.

Small to medium sized migratory birds cannot store enough fat reserves to complete the journey in a single movement. Instead, they must stop to refuel at various stages to successfully complete each subsequent leg of the journey using a series of intermediate locations as ‘stepping stones’. This can be problematic for these migrants, as they first need to find suitable foraging habitat and feed sufficiently to get over the next barrier. Aerial insectivores have the added advantage that they can visually select good looking habits and feed opportunistically en route, but Nightjars can only do this when active during the hours of darkness.

The results of this study showed, unsurprisingly, that Nightjars rapidly cross barriers and inhospitable habitats, including Mediterranean Sea, Sahara Desert and equatorial forests. Either side of these barriers, Nightjars generally slowed their migration and spent more time in mixed habits, where they could then more intensively search out locations providing foraging hotspots. In both spring and autumn, around nine separate stops were made en route, each ranging between one and 27 days in duration.

This study demonstrates how Nightjars optimise their migratory flights and search for prime stopover sites by selecting areas with very diverse habitat composition, which by their nature support prey hotspots. The birds can then home in upon these with limited effort. It is still not known how Nightjars assess the habitat, but it is likely that they can visually assess the structure in the limited night-time light, or during better illuminated twilight periods.

Abstract

Long-distance migrants are constrained by widely separated hospitable habitats in geographically isolated locations, making them vulnerable to environmental change, both through natural and anthropogenic causes. Knowledge about their resource selection decisions is imperative to understand the drivers of their declines. The distinct periods within an annual cycle, when individuals experience different environmental circumstances, are inextricably linked through carry-over effects which can have important consequences for the individual, and consequently the population. In this study, we employ precise archival GPS-tracking data of European Nightjars (Caprimulgus europaeus) and high-resolution global land cover data to examine habitat selection during the sedentary wintering and breeding periods, as well as during autumn and spring migration, using a correlational approach. We demonstrate how nightjars use general habitat characteristics, such as landscape diversity, for high-order habitat selection, while resource selection at a finer spatial scale is reliant on fine-scale variables related to a habitat’s suitability, such as surface area of grassland and shrubland. We show that nightjars favour spatially diverse landscapes, which allows them to minimize time spent searching for optimal habitats. The considerable variation in the drivers of habitat selection between and within seasons shows how anthropogenic land-use change can have an array of different impacts on migrants by influencing large- and fine-scale habitat selection. This study shows the advantages of an individual based GPS-tracking approach, combined with high spatial resolution remote sensing data, and highlights the need for full annual-cycle research on scale dependent habitat selection of long-distance avian migrants.