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Bats and Bridges

Each summer the organised chaos of bat survey season descends on us at HED Ltd., and every season brings with it something unexpected. For those wondering why that is, given we’ve been conducting surveys for several years, the answer is simple, even to the most experienced bat worker, scientist, or ecologist; bats are unpredictable. The most knowledgeable of individuals will readily inform you that the more we learn about bats, the less we know.

Their unpredictability extends to their choice of roost sites. These hideaways can be found in crevices in old or decaying trees, buildings (e.g., beneath flashings, tiles or tucked in between old wooden beams), caves (I don’t think that’s news), or perhaps most surprisingly, bridges.

As structures, bridges might not seem the most accommodating of places, but to a bat that couldn’t be further from the truth. They are often positioned over ideal foraging habitat, such as rivers with bordering woodland and other vegetation (referred to as the riparian corridor), which is a great place to find their invertebrate prey.

Species regularly recorded roosting beneath bridges include pipistrelle species (Pipistrellus sp.), Natterer’s (Myotis nattereri), brown long-eared (Plecotus auritus), and for us the most frequent suspects; Daubenton’s bats (Myotis daubentonii).

Daubenton’s bats are a personal favourite of mine. Their echolocation call is akin to a machine gun, with a rapid and regular pulse rate in contrast to that of pipistrelle species, which sound like wobbling a sheet of metal. They hunt 5-25cm from the surface of the water, skimming freshwater invertebrates using their hairy feet and tail membrane in synchronicity.

This species delivered one of our unexpected surprises this year. Within the literature Daubenton’s bats are freshwater associated, feeding over calm water on freshwater invertebrate species. So, you could imagine my surprise when observing them hunting over a saltwater inlet beneath a bridge on the west coast of Scotland. Amusingly, I’d just advised our bat surveyors over walkie talkie that we were unlikely to find Daubenton’s bats due to the lack of freshwater.

The bats in this case were probably reminding me never to be as bold as to try and predict their behaviour. I’ve discussed this finding with others and the only explanation that has arisen is that nearby freshwater might have been feeding into the inlet, making the water more of a mixture of fresh and salty (brackish).

Whatever the cause, remember, never try to guess what a bat might do. They are one of nature’s greatest mysteries and will continue to firmly defend that title.


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