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What do Owls Eat for Dinner?

2016/5/4 10:28:36

I have been a keen ornithologist for many years and owls have always fascinated me, especially the local barn owl which I watched hunting over nearby farmland on summer evenings. But I always wondered what they were catching for dinner.

Eventually I tracked the owl to its daytime roost - a deserted cow shed in the middle of a field. As I approached the derelict building, the barn owl flew silently away. I pushed open the rusty metal door and looked around. Lying on the floor beneath the roof beams were large, shiny black bundles of fur, about the size of my thumb. I picked one up and broke it open to find an incredible array of small bones, teeth, and two tiny animal skulls. This was my first glimpse into what gradually became a major interest, and I have collected and examined hundreds of owl pellets from many species of owls over the years.

So what exactly is an owl pellet? It's the undigested remains of the bird's previous meals. An owl does not rip the meat off its victims, as a buzzard or eagle does. Nor does it chew its food. Instead, it swallows its victims - mainly small mammals - whole. Once the food is swallowed, it passes into the gizzard, where the food is broken up, and then to the stomach for digestion. Some of the harder parts of the prey are not (or only partly) digested. These solid remains are prevented from passing any further down the gut. Instead, they are squeezed in the gizzard into a compact mass. This is then coughed up, or regurgitated as a pellet.

As pellets do not pass through the intestine, they are quite different from droppings. They do not smell, and are not unpleasant to work with. They consist of bones, teeth, claws, beaks, insect wing cases, frog skins, seed husks etc. These are enclosed by softer material like fur and feathers.

Studying owl pellets is extremely worthwhile because, with a bit of detective work, you can find valuable information about the diet of the owl. The solid remains can easily be extracted and identified. Most species of small mammals are easy to identify by their skulls and jawbones. It is therefore possible to identify precisely what the bird has been feeding on, which provides key information about the numbers of prey in that area, the food chains and the precise part the owl plays in them. Without studying owl pellets, this information would otherwise be very difficult to discover.

Dissecting an owl pellet is an easy process. It is advisable to use surgical gloves at all times. These can be obtained from your local pharmacy. Pellets can be teased apart when they are dry, but it is best to soak them first. Half fill a bowl with water and a drop of liquid disinfectant, and leave the pellets to soak for half an hour. Then take each one out and tease the pellet apart carefully using tweezers and cocktail sticks. As you come across bones, pick them out and lay them on kitchen paper. They will soon dry, after which they can be identified.

The most distinctive bone of a small mammal, and hence the easiest to identify, is the skull, followed by the lower jaws. There are four ways in which a skull or jaw can be identified: the actual size of the skull; the dental formula; teeth cusp patterns; and root patterns.

There are many books available showing clear illustrations of these four elements of a small animal's skull. You can also email me for a simple guide to the identification of skulls and jaws found in owl pellets.

The first study I conducted on my local barn owls threw up some interesting data. From the 138 pellets I analyzed over that summer, I discovered that the owl had eaten 481 prey items. These were 277 field voles, 119 common shrews, 41 wood mice, 22 pygmy shrews, 14 bank voles, 3 brown rats and 3 birds. In other words, in percentage terms, the owl's preferences were for field voles (58%) and common shrews (25%).

Contrast this data with an analysis I conducted of pellets of the tawny owl, which seems to eat a greater variety of prey items: bank voles (31%), wood mice (29%), common shrews (21%) and beetles (10%) forming the bulk of its diet. At the other extreme is the short-eared owl. A study I completed on this owl revealed that over 92% of its diet was composed of field voles.

These statistics reflect the different habitats of these three species of owl. The barn owl generally confines itself to open fields - the domain of field voles - but also hunts alongside hedgerows and canal banks, where shrews can be found. The tawny owl's hunting area is generally woodland, a rich habitat for all kinds of small animals, but also extends into parks and gardens, hence its very diverse diet. The short-eared owl is very much a bird of the upland moors; a harsh environment where only the field vole can breed successfully.

Analyzing owl pellets is a fun, harmless occupation for young and old, and it leads to simple but fascinating insights into the diet of these creatures of the night.

Denzil Walton runs a technical copywriting service from his home near Brussels, but also writes on nature and the environment, and has published four books entitled Nature Walks Near Brussels.