The Use of Intrinsic Markers for Studying the Migratory Movements of Bats

Mortality of migratory bat species at wind energy facilities is a well-documented phenomenon, and mitigation and management are partially constrained by the current limited knowledge of bat migratory movements. Analyses of biochemical signatures in bat tissues (“intrinsic markers”) can provide information about the migratory origins of individual bats. Many tissue samples for intrinsic marker analysis may be collected from living and dead bats, including carcasses collected at wind energy facilities. In this paper, we review the full suite of available intrinsic marker analysis techniques that may be used to study bat migration, with the goal of summarizing the current literature and highlighting knowledge gaps and opportunities. We discuss applications of the stable isotopes of hydrogen, oxygen, nitrogen, carbon, sulfur; radiogenic strontium isotopes; trace elements and contaminants; and the combination of these markers with each other and with other extrinsic markers. We further discuss the tissue types that may be analyzed for each and provide a synthesis of the generalized workflow required to link bats to origins using intrinsic markers. While stable hydrogen isotope techniques have clearly been the leading approach to infer migratory bat movement patterns across the landscape, here we emphasize a variety of lesser used intrinsic markers (i.e., strontium, trace elements, contaminants) that may address new study areas or answer novel research questions.

Transcontinental 2200 km migration of a Nathusius’ pipistrelle (Pipistrellus nathusii) across Europe

A male Pipistrellus nathusii ringed in Pape Natural Park (S Latvia) in August 2015 was recovered recently dead in Pitillas’ Lagoon Natural Reserve (N Spain) in March 2017. At 2224 km in SSW direction, this is the first documented bat migration between these countries and worldwide the longest migration record of a bat. We also report other observations of this species in autumn in Northern Spain, suggesting that the Iberian Peninsula may be an important wintering area for Nathusius’ pipistrelles. Conservation measures should be agreed on by countries along the migration routes to improve the protection of this species.

Assessing the use of rivers as migratory corridors for temperate bats

Investigating landscape-level movement patterns of migratory animals can be challenging, but this is a major component of some animal’s life history and behavior. In particular, bat migration has been difficult to characterize, yet recent research on bat migratory ecology has made major advances. It has been largely accepted that rivers and other linear landscape features may be important migratory corridors for bats during both long- and short-distance migrations. We assessed the migratory behavior of multiple temperate bat species along the Missouri River, a major river corridor in North Dakota, during March through October of 2016–2017. Bat detectors with paired microphones were deployed and oriented parallel to the riverbank. This configuration permitted detection of directional passes of bats, approximately 10–20 m above the microphones and 40 m into the river, which were used as an estimate of migratory behavior. We found the effects of season and species explained less than 2% of the variation of directional passes, indicating an absence of season-specific movement patterns along the studied river corridor. Although our study only assessed a portion of a major river corridor, the results suggest that migratory movements of bats along rivers may not be as straightforward as once thought, highlighting the need for future studies investigating the fine-scaled movement patterns of bats during migration.

Variability and repeatability of noctule bat migration in Central Europe: evidence for partial and differential migration

Each year, large numbers of bats move across Europe between their summer and winter areas, yet even though many of them are endangered and legally protected, we are unaware about many aspects of their migratory behaviour. Here, taking Nyctalus noctula as a model species, we used stable hydrogen isotopic values in fur ( δ 2 H f ) as an endogenous marker to shed light on the migratory behaviour of more than 1000 bats from hibernacula across Central Europe. Specifically, we asked the following questions: how flexible is migration in temperate zone bats? Which general migration pattern do noctule bats follow? How repeatable and thus predictable is the migratory behaviour of individuals? Do morphological correlates of migration occur in bats? Our study confirmed that noctule bats engage in partial and female-biased migration across Europe, suggesting the strongest migration pressures for northern populations. Further, we revealed a combination of partial and differential migration patterns with highly variable migration distances which lead to a pronounced mixing of different source populations in hibernacula where mating occurs. Most individuals were consistent in their migration strategy over time, i.e. 86% could be repeatedly assigned to either long-distance or regional origin across years. This is consistent with our finding that the between-individual component explained 84% of the variation in δ 2 H f values, suggesting specialized individual migratory behaviours and a strong natal philopatry. We discovered a positive correlation between forearm length and migration distance and support for sex-specific effects of migration on body condition. Our study elucidated migration patterns over large geographical scales, demonstrating that considerable numbers of migratory bats originating from distant populations depend on hibernacula across Central Europe, calling for international conservation management.

Bat activity and bat migration at the elevation above 3,000 m at Hoher Sonnblick massif in the Central Alps, Austria (Chiroptera)

Recent studies on the presence and activity of bats at high elevations show, surprisingly, that large numbers of bats cross the Alps up to 2,500 m a. s. l. This study takes the next step: to investigate bat activity at an elevation above 3,000 m a. s. l. The main study site was located on the top of Hoher Sonnblick at 3,106 m a. s. l. (Salzburg, Austria). Bat activity was monitored during September and October 2014, and permanently from March to November 2015, with an automated recording device. To compare bat activity at a lower location, a study site at 2,273 m a. s. l. was also monitored from May to October 2015. Contrary to our expectations, we found bats present at 3,106 m from mid-April to mid-September. However, periods of bat activity at these high altitudes were shorter than at lower elevations and were interrupted by longer periods with no activity. Among the recorded species there were all the long-distance migrants in Europe: Nyctalus leisleri, N. noctula, Pipistrellus nathusii and Vespertilio murinus. Eptesicus nilssonii, a predominantly sedentary species, was also recorded on the mountain top as well as Pipistrellus pygmaeus. Bat activity was linked to milder weather conditions. However, we did record bats at wind speeds of up to 12.2 m/s and temperatures as low as –2.1 °C.

Determinants of spring migration departure decision in a bat

Migratory decisions in birds are closely tied to environmental cues and fat stores, but it remains unknown if the same variables trigger bat migration. To learn more about the rare phenomenon of bat migration, we studied departure decisions of female common noctules ( Nyctalus noctula ) in southern Germany. We did not find the fattening period that modulates departure decisions in birds. Female noctules departed after a regular evening foraging session, uniformly heading northeast. As the day of year increased, migratory decisions were based on the interactions among wind speed, wind direction and air pressure. As the migration season progressed, bats were likely to migrate on nights with higher air pressure and faster tail winds in the direction of travel, and also show high probability of migration on low-pressure nights with slow head winds. Common noctules thus monitor complex environmental conditions to find the optimal migration night.