Biologists sampled fur from 1,665 migratory bats like a travel diary, and the two species most often killed at wind turbines were moving north and south each autumn, not just south

A bat crosses the sweep of a turbine blade somewhere over North America on an autumn night.

For years the assumption was simple: heading south for winter, crossing a rotor by bad luck.

The losses are not small. An estimated hundreds of thousands of bats die each year in turbine collisions, and three migratory tree-roosting species make up nearly 80% of those deaths.

Tracking hardware is impractical on animals this small, so researchers reached for an instrument already built into every bat: its fur.

What could a single strand of hair actually record about where a bat had been?

What a strand of fur records that a tracker cannot

Bat fur grows in summer, and as it grows it takes on a chemical signature from the food and water the animal takes in. Stable hydrogen isotope compositions vary across broadly latitudinal bands, and because tree-roosting bats molt in early to mid summer, their fur reflects the summering grounds where it formed even when the animal is sampled somewhere else entirely.

That makes a single strand a location record that needs no battery and no transmitter. Some of the fur analyzed came from bats recovered at wind energy facilities. Other samples came from bats caught away from those facilities, giving the team a direct comparison.

By setting the isotope signatures of turbine-site bats against those from elsewhere, researchers could ask a question tags had never answered: where had these animals spent the summer, and which way were they moving afterward?

The route that turned out to run more than one way

The answer did not look like a simple seasonal retreat. Lead author Caitlin J. Campbell said the bats “began moving in several directions, with many individuals traveling to the north and many to the south” each autumn.

Hoary and eastern red bat movements were consistent with a pell-mell migratory strategy, containing population-level movements to both higher and lower latitudes during autumn. Silver-haired bats, by contrast, matched the metrics of a conventional back-and-forth migration. That is not the tidy southward wave the field had assumed.

Silver-haired bats also make up the lowest share of wind turbine fatalities among the three species. Movement from summer habitat toward higher latitudes was linked to a greater chance of being found at a wind energy facility for hoary and eastern red bats, but not for silver-haired bats. That gap matters enormously for anyone designing a mitigation window.

How the chemistry inside the fur was read

The team analyzed fur collected from between the shoulder blades of 1,665 bats across North America: 309 silver-haired, 719 hoary and 637 eastern red. Most of those samples, 84.7%, came from salvaged carcasses. Those measurements were then combined with existing datasets in a framework built to scale from individual movements to continental migration patterns.

No tracker had to be fixed to an animal. No battery ran down. The record was already written into the fur itself, waiting to be read.

That simplicity is the method’s real power: a carcass picked up beneath a turbine carries its whole summer history locked in a few milligrams of keratin.

Where the finding changes the math on curtailment

Most efforts to reduce bat losses at turbines rely on curtailment strategies, the temporary shutdown of turbines during low wind speeds, which can substantially cut fatalities. Those windows are concentrated in late summer and autumn, when most tree-roosting bat deaths at wind energy facilities occur.

The new analysis, published in Ecology Letters, reports that the timing of pell-mell migration strongly overlaps with fatalities at the continental level. The authors say this supports the hypothesis that migration distance and strategy drive increased interactions with turbines, though the reason some bats head north at all remains unresolved.

Work on painted turbine blades has tested whether visual cues on a rotor can cut collisions for birds, a reminder that fixes designed around one animal’s behavior rarely transfer cleanly to the next species.

What the wandering migration means for three species already in trouble

The hoary, the silver-haired and the eastern red bat are all experiencing dramatic fatality numbers at wind energy facilities. All three roost in trees rather than gathering in countable cave colonies, so their population sizes remain poorly understood. In Canada, a national status assessment listed all three as endangered.

The fur chemistry method opens a way to map population-level movement without attaching a single device. A Wyoming survey of insects near turbines points at a parallel gap in what the field has been measuring around these machines.

The fur method cannot say how many individual bats cross a given turbine corridor in a given week. But it has done something no tracker array could: it has replaced the assumption that autumn means south with a chemical map of movement that runs in more than one direction at once.