How homesick are migratory European nightjars?

One of the contributors to the study, an adult male European nightjar. 

Photo: Gabriel Norevik

A study on the spatial consistency and repeatability of European nightjars Caprimulgus europaeus tracked across multiple years

By Gabriel Norevik

• Do European nightjars return to site they visit previous years?

• Birds with GPS-trackers reveal their site-fidelity across the annual cycle

• Nightjars regularly revisit the location where they have bred and wintered

• Migration routes are more flexible but still reveal individuality

• Ecological barriers influence route variability and subsequent paths

Background

Although we do not fully understand the mechanisms in motion it is well established that migrants demonstrate an inherent preference regarding where to go and what routes to take. Likewise, animals have been shown (to a varying degree) to incorporate experience when making decisions and to gradually modify their behaviour as they gain knowledge. Combined, innate and acquired behavioural response defines the individual from others. For migratory animals this individuality may for example be reflected in when and where they move. However, these movement variables are usually also influenced by external factors such as weather and wind, and by knock-on effects of previous events. To begin picking apart the different effects on the migration of birds one need repeated measurement from individual birds, meaning tracks from multiple journeys. By doing so, we can start answering question about if and to what degree the nightjars return to the same site every year, and how the similarity in route choice within an individual compare to the variation we see among individuals. While the former ask how faithful vs flexible individuals are in their space-use, the latter refers to its repeatability. That is, is there a degree of inheritable individuality in space-use separating individuals that selection can act upon or are the migration routes merely a result of a random movement towards a distant wintering range? Presumably it is more complicated than that, and the migrants are likely trading off a multitude of factors that are relevant to successfully return to the breeding ground in the seasons to come.

We have tracked ‘our’ nightjars for about a decade now and know from previous work that they undertake some of the longest migrations between the breeding range across Eurasia and the sub-equatorial wintering grounds. The seasonal migrations follow different routes forming a clockwise loop and during the flights the birds alternate flight altitudes frequently, which have led us to suspect that these birds are influenced by wind conditions enroute. It is well-established that migratory birds generally have a preference to fly in supporting winds. The rationale behind this is simple: imagine a bird flying at approximately 10 m/s (likely including most nightjars) even a moderate airflow will have a substantial effect on the realised movement towards the distant goal. If we assume that the time and energy spent on acquire the energy to fuel these flights correlates with the fitness of these birds, it makes sense that they response accordingly to the wind conditions enroute. So here the birds need to decide to either prioritise the gain in local experience to revisit known sites but spending additional energy on compensating unfavourable winds or just go with the flow and losing out on the benefit of local experience. Presumably, the realised flight route is a result of a compromise between the two extremes (complete wind compensation or full drift) depending on local landscape configuration and habitat, the birds' body condition, population specific goals, and other to us currently unknow factors.

What did we look for?

As the birds spend most of their time on the breeding and main wintering locations devoted to reproduction and the annual flight feather moult, we hypothesize that they would benefit from local knowledge about reliable foraging sites and where to locate safe places to rest and nest. During migration on the other hand, each stationary site is used rather briefly and, compared to for example shorebirds performing their epic migrations between reliable foraging hotspots, we interpret aerial insects that nightjars feed on to be more evenly distributed in the landscape. Hence, we expect individuals to become flexible in their space-use during migration and not limit themselves to specific routes or stopover sites. To make it a bit more complicated, if the birds allow themselves to be flexible, there are still decisions to make about the extent of flexibility to allow, especially if the endpoints of the migrations are fixed. In theory, for long enough migrations the effects of crosswinds from either left or right may cancel themselves out meaning that a nightjar may only need to perform minor compensations at the very end of the migration. In addition, the benefit of applying wind-drift increases when traversing landscapes associated with higher flight costs and/or risk of death, such as deserts and seas for a terrestrial migrant. In other words, it is safer and more economical to just focus on making it across such barriers and to devote energy- and time-consuming efforts of flight-route adjustments towards more benign regions where resting and fuelling is possible. Given that the biomes associated with the European-African migration system is organized like a birthday cake with thick layers of barriers that the birds need to cross, we expected to find distinct episodes of increased within-individual route variation, indicative for a use of wind-drift during these passages.

How did we go about?

We went to our long-term study population in Sweden, northern Europe, to capture breeding birds and equip some of them with tiny GPS tags that were programmed to collect data about the birds’ whereabouts throughout the annual cycle. With a positioning error of a few meters, the tags could even pinpoint the favourite spots of the birds within their territories, and how those may differ between years. To keep the weight of the tags (and their influence on the birds) at a minimum, the tags lacked a transmitter to send the data, meaning that we had to return to our site the following breeding to relocate and recapture the birds before we got our hands on the most sought-after data. As we wanted to get repeated tracks from the same individuals we had to repeat this procedure for year after year. One of the more faithful reencounters was a male that we trapped and ringed back in 2018 and later recaptured within an hour on the same location every season up to 2022.

What did we find?

We found that the nightjars were faithful to their breeding and wintering locations where all individuals returned to previously used sites, but that they only occasionally returned to the same stopover sites during migration. Still, individuals did not mix randomly during the migrations but appeared to follow individual-specific movement corridors. We also found that ecological barriers such as vast deserts, seas, and mountain areas were associated with the degree to which the birds stuck to the individual movement corridors, or in other words, how much the realised flight routes varied between years. For example, route variation increased when birds crossed the Sahara Desert and the Mediterranean Sea but decreased when passing the mountainous regions of central Europe.

Maps of repeatedly used wintering sites of two European nightjars from doi.org/10.1186/s40462-025-00537-6 licensed under CC BY 4.0. Different colours and point shapes refer to different years, lines show estimated utilization distributions.

What does this mean?

The high degree of site-faithfulness demonstrated by the nightjars suggest that experience gained from previous years are valuable enough for the birds to return the following years. The main wintering sites of these birds are distributed across the tropical savanna and shrubland in southern Africa associated with rather unpredictable distribution of seasonal rains and productivity. Regardless of the merits of local knowledge, to what extent nightjars are subject to such environmental variation and how they cope with it are interesting future research subjects.

Maps of migration routes of European nightjars from doi.org/10.1186/s40462-025-00537-6 licensed under CC BY 4.0. Different colours refer to a subset of repeatedly tracked individuals to facilitate comparison of routes between years. Dashed horizontal lines delimit different ecological barriers referred to in the paper.

Given the repeated use of the breeding and wintering sites it is not a surprise that also the realised routes followed similar, but not identical, paths each year. We noted that passages associated with the major barriers influenced the distribution of flight routes, and the consistency of individual birds. For example, an increase in route variation during the crossing of deserts and seas indicates that the birds allow themselves to be more flexible and, presumably, drift with the current airflows instead of spending valuable energy on route compensation. Given that both Sahara Desert and the Mediterranean Sea have large east to west distributions and rather uniform north to south width, it is reasonable that a better solution is to await more benign areas before spending energy on route adjustments. The opposite barrier effect was found when the nightjars were to circumvent the Alps in central Europe resulting in a funnelling effect on the flight routes. In a way, this is a terrestrial equivalent of migratory bottleneck of soaring migrants when approaching open waters, and when they are concentrating towards locations with the shortest passage. Interestingly we found route concentrations on an individual level prior to the Sahara crossing in spring, indicating that birds are orienting towards intermediate goal areas before proceeding across the desert. We can only speculate about the reason for this behaviour, but it indicates that it is positively associated with the birds’ fitness and that it is heritable.

We could not find conclusive evidence for a repeated use of important stopover sites where the birds fuel up prior on of the most demanding passages of the migration, although a few birds revisited the same sites between years. Rather, birds seemed to depart from similar longitudes each year indicating a consistent use of initiation point of the desert crossing. As with several of the findings in this study, it would be interesting to investigate to what degree, wind and habitat conditions along the route are associated with the behaviours documented here.

We hope that you have enjoyed reading this post about nightjars. It is our aim to help spreading the word of these enigmatic birds and to encourage communication and collaboration within the global community of nightjar enthusiasts. We are aiming to post regularly about nightjars, please contact us if there is a story you want us to write about.