How to spot a ‘batty’ tree!

article written by Caitlin Coombs, Berkshire Biodiversity Officer

Summer, is over, ecology survey season has come to an end, right? WRONG! There is still plenty to explore over the winter months.

Around three quarters of British bat species are known to roost in trees. The remaining species tend to favour human-made structures; but this is only because of a lack of suitable and available tree habitats.

But not just any old tree! Bats are picky and have requirements. They need a tree which harbours specific features which provide shelter for roosting during the day, hibernating during the winter, and to form maternity roosts. In other words, bats need a tree with ‘potential roosting features’ (PRFs).

Since bats are not able to bore holes or make nests, they use whatever gaps are available, including cavities and crevices made by other animals, the natural decay of the wood or even by arboricultural methods.

A phase 1 bat survey (also known as a bat roost potential survey, or a preliminary bat scoping survey), is an ecological survey that can be done all throughout the year and involves assessing the potential of a tree (or building) to support roosting bats.

An ecologist would estimate them‘potential’ of  tree if it supports roosting bats by looking for ‘potential roosting features’.

Roosting features can include a range of things:

  • Woodpecker holes
  • Rot holes
  • Splits or cracks in branches
  • Fissures
  • Sheltered areas created by thick flaking or peeling bark
  • Rot holes
  • Cracks and splits in stems or branches
  • Partially detached thick bark
  • Knot holes arising from naturally shed branches, or branches previously pruned back to the branch collar
  • Man-made holes
  • Cavities created by branches tearing out from parent stems
  • Other hollows or cavities, including buttress-rots
  • Hazard beams
  • Double-leaders forming compression forks with included bark and potential cavities
  • Gaps between overlapping stems or branches
  • Dense ivy with thick stems
  • Bat, bird or dormouse boxes

Apart from roosting features, other signs of bat activity include bat droppings, odour emanating from a PRF, audible squeaking at dusk or in warm weather, and staining below the PRF.

Winter is the best time of year for this kind of exploring. As leaves fall from trees, you can get a much better view of the crown and upper branches.

So, what are you waiting for?

Get searching! A good pair of binoculars will help you see branches and areas of trunk higher up the crown. ‘Batty’ trees don’t have to be in the woods; even a garden tree can host a range of roosting features. Oak and ash seem to be particular favourites, and as trees mature, they generally tend to host a wider range of features suitable for roosting.

Apart from the physical features of a tree, you can also ask those who may know about bat activity or roosts in the area. Contact us or your local bat group to find out more! See below for information on how you can learn more about bats, bat surveys, and getting involved.

Useful links and resources:


Posted: November 10, 2020