How to dissect owl pellets

There’s no better way to spend a cold, dark winter’s day than staying indoors and analysing owl pellets.

This fascinating activity provides valuable information on the diets of our different owl species, helps us to collect records of rarer species, such as harvest mice, and is a valuable means of monitoring changes in numbers of small British mammals.

So, when you have finished analysing your owl pellets, make sure you send your records to The Mammal Society.

Owls swallow their prey whole – pellets are the regurgitated fur and bones. Besides small mammals, which are the most frequent prey, pellets can comprise the remains of birds, amphibians, insects and even the chaetae (small bristles in the skin) of earthworms, though these are only visible under a microscope

• To see what is in an owl pellet, soak it in water. When soft, gently tease it apart with forceps.
   
• Slowly pick out all of the bones and bits of insect and put to one side for identification.
   
• Count everything – you may only have one skull but three lower jaws, so check carefully to see how many animals are represented in the pellet.
   
• Owl pellets can be stored dry. Make sure you include details of the species of owl, collection site and date.
   
• If the owl has eaten a lot of earthworms (quite common in tawnies), the pellet may consist largely of soil and bits of grass or dead leaves. Most of the remains will be of shrews, voles and mice, which are easy to tell apart.

• Shrews have continuous rows of sharp, pointed teeth with red enamel tips that all look much the same.
• Pygmy (smallest), common and water shrews (largest) are easy to tell apart by size – after analysing a few pellets this will be obvious.
• Occasionally you may find a mole, which looks like a very large shrew but without the red tips to the teeth.
• Bats are similar and even rarer; they have shorter jaws, white teeth and prominent canines.

• Rodents have a conspicuous gap between the incisor teeth at the front of their skulls and the flat grinding teeth at the back. The cheek teeth of voles have zig-zag chewing surfaces, whereas rats and mice have three distinct rounded cheek teeth.
• Most voles found in owl pellets (especially barn owl pellets) are field voles. Bank voles are most common in tawny owl pellets; water voles are larger and quite common in wet areas.
• Most of the mice are likely to be wood mice; yellow-necked mice are very hard to tell from wood mice, and house mice are very rare. Harvest mice can be common in areas with rough grass; their skulls are very small and fragile. Young rats are easy to recognise by their large size.

• Bird bones are very fragile, and almost-transparent feathers in the matrix of the pellet are often the first indication that bird remains are present. Identifying species of birds is very difficult, unless some characteristic feathers remain.
• The skull is generally fragmented, but the beak can give an indication of the size and type of bird – seed eaters, such as sparrows, have short, heavy bills, starlings have slender bills with a bent jaw, while blackbirds are more robust and curved.

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