Guide to Small Mammal Bones in Barn Owl Pellets

Learn more about the tireless work that went behind creating 'A Photographic Guide to Small Mammal Bones in Barn Owl Pellets' By Susanna Ramsey.

I have been fascinated by barn owl pellets for many years now, so I was happy when Derek Crawley asked me to write a guide to the small mammal bones which you find in these pellets. I had just written an online Introduction to dissecting pellets for children and beginners, but this was to be an advanced version.

I had often struggled myself to identify the skulls and jawbones down to species level, so I knew this would be a challenge, but one worth doing. The previous guides contain great black and white drawings of the different small mammal skulls and jawbones but I intended to use large colour photos so that the identifying features could be made very clear. I also hoped to make the guide attractive and accessible to all, to draw new audiences into the thrill of owl pellet dissection.

Photographing the Tiny Bones

I have a stereo microscope to which I can attach my Nikon camera. This enables me to take photos of objects at up to x80 magnification, though the clarity is generally best at x40 or below. The small mammal skulls which I needed to photograph are just 10-15mm long and the jawbones 5-18mm long and 2mm wide. Individual teeth are about 1mm wide.

As well as photographing tiny bones, I often use the equipment for taking detailed pictures of insects, flowers and feathers. My favourite subject is the beautiful, overlapping scales on a butterfly’s wing.

I have a large box containing thousands of small mammal bones, so my first task was to find good examples of each of the skulls and jawbones, with and without the teeth, where the root holes were visible. The bones are often partially damaged in the barn owl’s digestive system, so finding complete skulls took a while!

The next job was to line up the tiny bones, one at a time, under the microscope at exactly the right angle to show the required features and to stabilise them for the photos, without the props being visible. Getting the right exposure was also difficult as white bones reflect the light.

Rarer species

I had most of the bones I needed from years of dissecting barn owl pellets with children in local schools, and I had access to the great Canva photo library for photos of the living animals, but I still did not have all the photos I needed for the guide. Some small mammals are only present on certain islands such as the lesser white-toothed shrew and the common vole; harvest mouse bones are rarely found in pellets and are incredibly fragile.

I therefore approached the mammal department of the Natural History Museum to ask if they could provide some of the missing skull and jawbone pictures, which they kindly agreed to do. They helped with photos of the bones for the harvest mouse, yellow-necked mouse, lesser white-toothed shrew, house mouse and weasel. I also approached the Barn Owl Trust for photos of the predator’s skull! 

The Isles of Scilly Wildlife Trust supplied photos of a living lesser white-toothed shrew and David Tosh and colleagues helped with pictures of the greater white-toothed shrew. The Jersey Barn Owl Conservation Network gave me some pellets so I could look for bones from the lesser white-toothed shrew. Jersey is the only island where there are barn owls and lesser white-toothed shrews! There are no barn owls on the Isles of Scilly although there are lesser white-toothed shrews!

Researching the Facts

To identify which mammal species a skull or jawbone is from, you need to look closely at the shape of the teeth and the holes which are left behind by the roots, when the tooth is removed. For shrews you need to see whether the teeth have red tips and, if so, scrutinise the first incisor, counting the number and arrangement of cusps which it has. The size is also significant; for example, water voles and rats have larger skulls and jawbones whereas pygmy shrews and harvest mice have smaller.

The identifying features of the small mammal skulls and jawbones are described in a few, very useful booklets. The most comprehensive is “The Analysis of Owl Pellets’ by DW Yalden, published by the Mammal Society in 2009. There are excellent drawings of the tooth root holes for all of the small mammals in “Mammals of Britain, Their Tracks, Trails & Signs” by MJ Lawrence and DW Brown published by Blandford Press. I also consulted books by GB Corbet and HN Southern for details such as the size of each skull.

Writing the Guide

Having assembled the photos and facts, the final challenge was to compose and design the guide, making it look appealing and accessible to people with varying levels of expertise. Derek Crawley was a great help, checking I had got the facts right and had selected the correct photos for each species. I have to admit wondering why I had undertaken the task, when I had only completed the descriptions of the vole species and still had mice, rats and shrews to go!!

The booklet is currently available to everyone to view and download for free on the Mammal Society website. 

As an integral component of our 'Searching for Shrews' project, the Photographic Guide to Small Mammal Bones in Barn Owl Pellets plays a pivotal role in guiding people through our owl pellet dissection activities. Through this, we aim to uncover vital insights into the presence of the invasive non-native Greater White-toothed shrew in mainland Britain, contributing to our ongoing efforts in ecological research and conservation.