Over 300 bird species from a number of orders produce pellets including many raptors and all owls. Owls have a comparatively inefficient digestive system, lacking the crop and having weaker stomach acid (pH 2.2-2.5) than other bird groups (pH 1.3-1.8). This means some of their prey items remain undigested including bones, fur, and feathers. The opening from the foregut into the rest of the digestive tract is small and prevents any undigested material from passing through. Instead, the material remains behind where it is compacted into an oval pellet and is then actively regurgitated back up through the oesophagus.

Owl pellets are relatively easy to find, as owls tend to produce them on a favourite feeding post or roost. Regurgitating a pellet is a voluntary act on the owl's part and in the wild most birds will produce one prior to leaving the roost for hunting. Often, another smaller pellet will be produced during the night before the second main period of hunting around dawn. Pellets are highly informative about what the owl has been eating and the size, shape and appearance of the pellet is normally characteristic of the owl species.

Barn owl: Pellets can be quite large (30-70mm long), smooth and rounded. They are characteristically black in appearance (regardless of the colour of the prey it has eaten) often with a varnish-like gloss when fresh. They are very solid pellets, with the material highly compressed.

Tawny owl: Tawny owl pellets are usually greyer and more obviously furry than those of the barn owl. They are medium sized pellets (20-50 mm long) with a bumpy surface. The shape is long and narrow, but irregular and they tend to taper at one end. They are far less solid than barn owl pellets, and often break into fragments as they hit the ground. Since they are in the open they weather quickly, and the bones may be very prominent. They often contain sand, which may indicate earthworms in their diet.

Little owl: The pellets are small (15-40 mm long), elongated and pointed at one end. They are very light, and if pressed, may either crumble or feel spongy. Colour and content vary with diet. They often show insect remains such as wing cases. Again, because they are out in the open, they will quickly become weathered. They can be confused with kestrel pellets.

Short-eared owl: Pellets are medium to large (30-60 mm long) in size. They are elongated, rounded at one end and tapered at the other. They are grey with a slight sheen and very lightweight, but strong and dont break up as easily as many of the other pellets.

Long-eared owl: The pellets are slightly smaller than those of the short-eared owl. They are long, irregular and very slender.

Dissection of pellets is relatively easy: all you need is forceps, small pot for soaking pellets, and hand lens (x8 or x10) or dissecting microscope. Pellets are easier to pull apart if soaked in water for about half an hour. Then just carefully tweeze apart, removing bones and other fragments as you go for cleaning and identification.

The matrix may be made up of fur, indicating a mammal-based diet. With some knowledge, small mammals can be identified by better-preserved hairs. Feathers often break down into a loose powder, but you can still usually find small quills, the central stem of the feathers. Sand or soil usually indicates earthworms. You will need to use a microscope to find the minute bristles, called chaetae, which confirm this. Skull and jaw bones are the most useful when trying to identify prey species. Some insect remains are fairly easily recognisable, for example the wing cases of beetles. Other hard parts which survive include legs, jaws and even complete heads.

The Field Studies Council (FSC) chart British Owls and Owl Pellets is a great way to get started with identifying owls from pellets and their prey species. The Analysis of Owl Pellets is a great little book with guides to dissecting pellets and identifying remains. Hair of West European Mammals is an in depth guide to identifying mammal hair and ideal for those looking for a more comprehensive text on hair remains found in pellets.