Picking Bones & Pulling Teeth

I’d never pulled a tooth out before this day but I got the knack of it fairly swiftly, being mindful not to squeeze too hard or pull too quickly. After all, identification hinged on this outcome.

This day was a Sunday afternoon in October and I was with sixteen other forensic investigators piecing together several scenes of our countryside. Or, to be more precise, I was with sixteen other interested folk who were finding out the contents of barn owl pellets from several sites across Worcestershire!

What is a barn owl pellet?

An owl pellet is a regurgitated compact package of indigestible parts of their prey – the fur and bone – which they usually swallow whole. What a gift to us mammalogists! Dissecting the pellets can tell us exactly what an owl has eaten. A lot of birds actually produce pellets but for our purposes barn owls offer the perfect subject as the pellets can easily be found below reliable roosts or nests – predominantly in barns or boxes.

And so we set about collecting pellets throughout the year – with licenced buddies at nest sites and from roost sites, where disturbance was minimised. All pellets were labelled with site location and collection date and were then stored in the freezer to keep grubs of clothes moths from developing and eating the pellet.

During the workshop each team of two had a bag of pellets from one of the five Worcestershire sites collected from and, fuelled by tea and biscuits, clad in surgical gloves and armed with scalpels and tweezers, we got to it.

The aroma in the room was a little different to the usual smell of your Sunday roasts cooking but as time went on nostrils quickly became immune as the teasing apart of grey matter started and bones extracted. With the support and guidance of our experts, Johnny & Jane, and of many different identification guides, we started to separate the voles from the mice and the common shrews from the pygmies. And that’s where the tooth extraction comes in. Different species have different formations of teeth and it is the roots – or lack of, number of holes and arrangements of holes left behind which indicate what species you have in your hand. ‘Another field vole!’ my team-mate would announce after another tooth had slid from its jaw. (There was something strangely satisfying about the whole thing – maybe that’s just me..?!) But it was the skulls or upper mandibles that were the treasure we were really after. We made sure to only count the skulls so as not to potentially count different parts of the same individual and over record.

In some ways, delving into a pellet is like uncovering pieces to a puzzle or clues to a mystery. We can’t always be out there seeing it with our own eyes – recording every time a vole runs between anthills or a barn owl lifts a shrew out of grassland – and so here in a lumpy, grey capsule is evidence of what is where, and a good indication of how much of it there is.

And so not only was it a fun, interesting and educational Sunday afternoon – and probably a very different one for many of us – it was also a way of collecting valuable data and contributing important records to help map the presence and distribution of small mammals in the county. All records made at the workshop have been sent to the Worcestershire Biological Records Centre.

What we found

Our results were entered onto our ‘live’ spreadsheet, broadcast on a big screen so we could all see the figures as they came in (our own election results of small mammals)! In total we managed to get through 60 pellets, with an even spread of pellets dissected from each site.

We discovered 8 mammal species, the most common by far being field vole with 74 individuals recorded across all sites. At one site, from 10 pellets 16 field voles were recorded with nothing else but a single common shrew. This may be unsurprising as field vole is often named as the main prey of the barn owl, but they are birds of varied habitat and will diversify if they get the chance. Consequently, we also recorded remains of wood mouse, house mouse, common shrew, pygmy shrew and bank vole. One team even found the jaw of a rat in one pellet – the biggest find of the day.

Most sites had pellets containing only two species – mainly field vole and wood mouse, with the occasional shrew species thrown in. But one of the headlines of the day was the diversity of results from one site. This happened to be a site managed by the Worcestershire Wildlife Trust, whereas all of the other sites were privately-owned farmland. The pellets from this site held 7 of the 8 species we recorded on the workshop (only missing rat) and was the only site to have pellets containing pygmy shrew. By far and away the most diverse but also the pellets contained the most individuals of any other site – 30 individual small mammals from 9 pellets. The results suggest that this site has far more better-quality habitat which is able to support larger populations of more species – ensuring the barn owls are getting well fed!

You really don’t have to be an expert to have a go at owl pellet dissection – I’m proof! – but you do need to follow the correct procedures for collecting pellets (the barn owl is a protected Schedule 1 species and obviously you’ll need to get access permission for private land), store the pellets appropriately and wash your hands thoroughly afterwards! And of course – let us know what you find!