What Makes a Bat Super Highway?

The landscape north of Welwyn Garden City in Hertfordshire comprises in its woodlands and treelined hedgerows some of the fragmented remains of Great Hitch Wood. During the Saxon period this stretched from Hitchin in the north, over the Mimram valley to Hatfield parish in the south. Figure 1 shows you a small part of this landscape, northward to Woolmer Green.


 Figure 1 –Fragmented woodlands north of Welwyn Garden City, possibly comprising part of the former Great Hitch Wood (Edina Digimap, 2019)


Look closely and you will find in that network of hedgerows, watercourses and woodland edges many potential bat commuting routes traversing the landscape. I don’t know which ones are most important, but bats are long lived creatures of habit, so the long-term stability of this landscape suggests they could have been using the same routes since their post Saxon creation. 


I have a great interest in landscape ecology. I also have a thing about maps and tracks and “where things lead”, which means bat commuting routes really fascinate me. Over the past year I’ve become increasingly interested in how central Hertfordshire functions as a “bat landscape”. This was partially triggered by a magical evening out in Welwyn Garden City, where on a trip to Black Fan Valley (Figure 2), I found a “bat super-highway” along a wooded escarpment leading from the Mimram to Black Fan Lagoon, shown below in Figure 2.



. Figure 2 - Black Fan Lagoon (Nigel Cox) 


I found a “pinch point” through which the bats streamed in multitudes past my head and legs at touching distance and I stood amidst them, feeling the rush of wingbeats past my face and marvelling at their ability to pass so closely without hitting me . When the rush died down, I followed them to the lagoon to hear them feed. My bat detector sounded continuously for the duration of my visit, with all three pipistrelles, at least one myotis species and a barbastelle noted. As a newcomer to bat work, the sheer scale of activity was overwhelming and the magic of it sent shivers up my spine. I think that was the day I caught “the bat bug”. 


Returning to my “bat super-highway", this brings us back to the topic of ancient landscapes. The tree lined escarpment I mentioned runs along the former eastern edge of the ancient Digswell manor and is probably another relic of Saxon woodland. You can still see traces of the former boundary baulk and it contains some fine veteran trees. Its topography as an escarpment makes it an imposing feature and my theory is that this is an ancient bat routeway that has been in use for centuries.   


That said, there is evidence that bats preferentially forage over waters with plentiful bankside vegetation. (e.g. Russ & Montgomery, 2002; Zeale et al. 2012) so the importance of this commuting route could be evidence for that rather than antiquity. I found scant evidence of bat activity associated with the non-vegetated bank and the lagoon is a recent feature, originating from a sewage works operational in the 1950’s.  


Nevertheless, historical evidence shows Black Fan, which once hosted a hamlet farmstead was always an insect rich wetland area, so I think my antiquity hypothesis still stands. The name “fan” arises from “fen” and the area had a well and several springs. It must have been wet as grazing was restricted to dry seasons. Figure 3, from the 1880’s shows the Digswell parish boundary running from the river valley in the north east, round to the southwest where it enters Sherrardspark Wood where the balk is still extant. 



Figure 3 - Digswell manor showing the parish boundary in the 1890's (Edina, 2019)


In Figure 4 - you can see how the parish boundary and its surrounding grassland today exists as a piece of encapsulated countryside. It is cut off from the woods but is still well connected to the Mimram valley. But where do the bats come from? From the north through Dawley Warren or down the train line? From east and west along the Mimram valley? 



Figure 4 -Digswell parish Boundary in Welwyn Garden City (Edina digimap, 2019). 

Another factor driving my interest in this topic is the presence of barbastelle Barbastella barbastellus in the area. I’m a member of the Hertfordshire & Middlesex bat group and earlier this year we discovered a nationally important, 90 strong maternity colony in a fragment of the former Great Hitch Wood. Those bats and any we find in other remnants will likely move around the landscape via an ancient network of commuting routes. Barbastelle travel considerable distances and I would love to know the routes they follow (that said, they are known to traverse open spaces when it's dark enough, which complicates things!)


I would like to find a way of testing my hypothesis that amongst commuting routes the “bat super-highways” are represented by ancient landscape features that originate from piece meal enclosure. Perhaps it could be a fun idea for a phD? 



References; 

Edina (2019). EDINA Historic Digimap Service, 10.12.2019. 


Hertfordshire Chamber of Commerce (2019) Rare bat found near Stevenage. Available at: https://www.hertschamber.com/news/chamber-news/rare-bat-population-found-near-stevenage


Macnair, A., Rowe, A., & Williamson, T. (2015). Dury and Andrews Map of Hertfordshire: Society and landscape in the eighteenth century. Windgather Press. 


Russ, J. M., & Montgomery, W. I. (2002). Habitat associations of bats in Northern Ireland: implications for conservation. Biological Conservation, 108(1), 49-58. 


Welwyn Garden City Urban, District Hatfield Rural District and Welwyn Rural District (1961). Annual report of the medical officer of health and public health inspectory of the districts for the year 1961. 


Ward, D. (1953). Diswell: From Domesday to Garden City. Welwyn Garden City: Welwyn and District Regional Survey Association. 


Zeale, M. R., Davidson-Watts, I., & Jones, G. (2012). Home range use and habitat selection by barbastelle bats (Barbastella barbastellus): implications for conservation. Journal of Mammalogy, 93(4), 1110-1118. 

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© 2020 by Fay Brotherhood Fine Art & Music, Welwyn Garden City, Hertfordshire, contactfaybrotherhood@gmail.com 

Background photo by Andrew Merritt Photography, 2020.

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