Testing traditional assumptions about regional migration in bats

Testing traditional assumptions about regional migration in bats


In earlier posts I shared behind-the-scenes stories and background about my postdoc project. Now here are the results.

The majority of migratory bats have movements that cover much shorter distances than most migratory birds, and are commonly referred to as “regional” migrants. A few bat species do move long distances, and those get a lot of the attention, but the diversity of species making regional movements is relatively poorly studied. As a result, bat biologists have been forced to make assumptions based on what we know about a few better-studied species. Our recent paper on regional bat migration looked at a relatively well-studied species, the endangered Indiana bat (Myotis sodalis), in a specific area of its range where longer-than-usual movements had been documented in the past, to try to address this problem.

Telemetry stations and bat movement in spring. The central state is Indiana. Circles represent tower locations and short thin lines represent the direction of antennas (with 5 km distances). Thick lines connect bat movements from the Wyandotte Cave tagging site to other towers where they were detected. Each line represents a bat movement; some lines represent similar movements by multiple bats. Inset shows detailed movement around Wyandotte Cave.

The traditional story about regionally migrating bats is that they move relatively short distances (less than 100 km) between winter hibernacula (often caves) and summer maternity roosts. Unlike long-distance migrants and birds, this movement is not primarily in a north-south direction; rather, bats move in a star-like pattern across the landscape in many directions. This movement is thought to happen along linear landscape elements, for example rivers or coastlines. Finally, in autumn, for many species migration is complicated by pre-hibernation mating behavior known as swarming. However, we generally assume that swarming behavior happens at the hibernacula and thus the movement is characterized as a simple return trip reversing the spring movement. We make these assumptions in part because bats are very small, but new technology has allowed us to track bats individually during migration across the landscape.

Autumn swarming station deployments and bat movement. Thick black lines indicate bat movement northward from the two tagging sites, Coon Cave to the north and Wyandotte Cave to the south. Inset shows detailed movement around Wyandotte cave.

These assumptions have big implications for conservation. Bats are exposed to many threats during their movements, but it’s risky to spend conservation dollars to protect them when and where they travel based on assumptions. Should we protect flyways near rivers? During what months is it crucial to protect the area around hibernacula or maternity colonies? When and where are bats passing through areas where wind turbines might be constructed? Given that these bats are often struggling with threats from both White Nose Syndrome and wind turbine mortality, these questions urgently need answers.

We tagged 108 Indiana bats in southern and central Indiana and used a regional network of radiotelemetry receivers to study movement patterns. In spring, bats left the hibernaculum immediately and primarily moved north despite available maternity roosts in all directions. We found no evidence that bats follow rivers, the predominant linear element in the landscape. Despite earlier studies documenting movements of > 500 km in the region, only six tagged bats traveled > 100 km, which suggests that longer distance movements may be outliers, and the distance assumption may be valid. In autumn, only two bats visited multiple hibernacula after swarming. Surprisingly, after swarming some females moved > 100 km to areas without known hibernacula, thus the simple to-and-fro seasonal movement may not be a valid assumption in autumn. Common generalizations about regional migrant movements may not be representative of population behavior and care should be taken with respect to management decisions based on those assumptions.

Krauel JJ, McGuire LP, Boyles JG (2017) Testing traditional assumptions about regional migration in bats. Mammal Research:1-9. doi:10.1007/s13364-017-0346-9

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