Bird Pellets: A Complete Photographic Guide

By Ed Drewitt

This book is the first comprehensive guide to bird pellets, the undigested remains of food that form together into a ball or sausage-like shape and are regurgitated. It showcases the range of pellets that different bird species produce, including owls, hawks, falcons, corvids such as ravens and magpies, as well as waders – and even garden birds! The common items found in them, such as small mammal skulls and bones, are analysed in detail, with the discussion accompanied by numerous colour illustrations.  

The book progresses methodically from an introduction to pellets, covering what they are and how they are formed, to instructions on dissection and analysis and how this can be used in research, followed by a closer look at the pellets of each bird species in turn – from the golden eagle to the dipper. We learn how to identify the remains of small mammals including bats, as well as reptiles, amphibians, fish, invertebrates and of course other birds.

Dissecting bird pellets and identifying what is inside can be an important tool for discovering what birds are feeding on as part of more detailed diet studies. It is also an activity often delivered at family-friendly events or in schools by wildlife organisations. Extracting information from pellets also has sound scientific value: while it does not capture everything a bird has been eating, it still goes a long way in revealing the diet of birds and how this may change over time, in different habitats and different parts of the world.

• Language: English
• Publisher: Pelagic Publishing
• Release date: Jun 20, 2024
• ISBN: 9781784274726

Ed Drewitt

Ed Drewitt is a professional naturalist, wildlife detective, and broadcaster for the BBC. He has been studying urban Peregrines for over 15 years, and specialises in colour ringing their chicks, and identifying what they have been eating.

Book preview

1 INTRODUCTION

Dissecting bird pellets and seeing what is inside is the ultimate fun way of discovering what a particular group of birds has been eating. This is often an activity offered at family events or in schools by wildlife organisations – it has all the fascinating and disgusting elements of play that children love. Extracting information from pellets also has sound scientific value: while it does not capture everything a bird has been eating, this method still goes a long way in revealing the diets of birds and how this may change over time in different habitats or parts of the world. Pellet analyses may also help indicate the presence of species not otherwise detected in a habitat, or provide early indications of the decline or increase in populations of species, such as marine fish. They can also be used to look at the role birds have in the dispersal of seeds and fruits and to study what bird predators have eaten thousands or even millions of years ago.

Most birds in Britain and Ireland probably produce pellets, especially if they are eating anything that contains indigestible material such as hair, bone or the hard, external body parts from invertebrates. While birds of prey, including owls, eagles, hawks and falcons, are well known for producing pellets, even garden birds, such as robins and blackbirds will produce them. Other more surprising species include reed warbler, sedge warbler, spotted flycatcher, nightjar, woodpigeon and capercaillie (Tucker 1944).

In this guide I showcase pellets from a wide range of different bird species that are likely to be encountered and reveal how to identify common items found in them, such as small mammal skulls and bones. While the book focuses on pellets from bird species in Britain and Ireland, it also features some images that have come from the same species found in other parts of the world. Any images without a person’s name in the caption were found or taken by myself. The scientific names of most species are summarised in a list at the back of the book.

As a boy I loved finding and collecting anything that came from birds – feathers, skulls and pellets. They gave me a connection and insights into different species that I was unable to get from a book or photos. Some of my first pellets were those from garden birds: a robin that was nesting in the garden and a sparrowhawk that had a favourite plucking perch in the woods near where I lived. Those original pellets are still safely laid out in small boxes on a layer of sawdust. In my late teenage years I started studying the diet of urban peregrines in Bristol and other cities and would regularly find their powdery, crumbly pellets. In the mid-2010s I had the pleasure of working in the teaching laboratory in the School of Biological Sciences at the University of Bristol. One of the practical sessions each spring was ‘Mammals as prey’. Over 200 barn owl pellets would be dissected each year, revealing a brilliant dataset of what barn owls had been eating at Avonmouth Sewage Works, Bristol, Chew Valley Lake or Salisbury Plain. It was these practical sessions that particularly inspired me to look closer at bird pellets and discover what lies within them. I hope this book inspires you too to do the same.

2 WHAT IS A PELLET?

A bird pellet is the undigested remains of food that has been collected in the stomach, rather than passed through the alimentary canal and been excreted as poo. Instead, it is packed up into a pellet and regurgitated back up out of the mouth. Contrary to common belief, it is not just birds of prey such as owls, hawks, falcons and eagles that produce pellets. Most birds that eat substantially hard and indigestible parts of animals or plants in fact regurgitate pellets, including woodland and garden bird species such as blackbirds, song thrushes and robins, corvids like magpies and rooks, and seabirds such as gulls, terns, skuas and cormorants.

Depending on what a bird has been eating, its pellet may appear soft and hairy, or rough and textured where it is made up of the hard parts of seeds, insects or reptiles. Hairy or feathery examples may feel solid and hard to the touch thanks to an outer coating of mucus that seals the pellets, creating a more robust outer surface that deteriorates over time. In the case of seabirds such as cormorants, this mucus is present in abundance and solidifies like glue, turning pellets rock-hard.

The quantity and quality of identifiable material found in pellets depends on how birds feed – whether they pick off food from prey items or eat them whole – and their ability to partially or fully digest prey. For instance, peregrines largely feed around the bones of a bird and often tear off wings, heads and legs without eating them. Therefore, their pellets comprise undigested feathers packed together with the occasional bird foot, bone fragment or bird identification ring. A barn owl on the other hand will eat a small mammal whole and regurgitate most of its bones and hair, although other owl species will digest bones more easily. Sometimes traces of flesh will still be apparent on skulls, feet and tails where they have not been fully digested. The hard parts of invertebrates, such as beetle wing cases (elytra), remain unscathed and just as shiny and bright as when they were swallowed.

Distinguishing a pellet from poo

Pellets are often confused with faeces or poo from other animals, in particular mammals that may consume similar prey. Generally, though, pellets contain parts of invertebrates, hair, feathers, bones, teeth and other hard body parts that are more or less intact – unlike in the case of mammal excrement. They haven’t been crushed or mushed up by the hard, crunching teeth of mammal predators or scavengers. These pellets can be pulled apart with hands or tweezers and do not contain the soft, squidgy parts of poo that we see from mammals. They also tend to have a cleaner or musty smell.

Some mammals do produce pellet-like poo, depending on what they have been eating. When foxes have been eating birds or mammals, the feathers and hair are included in their faeces. These are long, thin and often twisted, tapering at their ends. They may smell musty and appear more like a dog poo rather than a bird pellet. The remains of any bones or parts of bodies such as feet are more likely to be chewed and fragmented, or indistinguishable. Squirrel poo may resemble small garden bird pellets. They look fibrous with partly digested plant material – a little like the texture of a rabbit or hare poo, although shaped like a small sausage. However, they lack the whole seeds or pips of fruit and invertebrate remains such as spiders or woodlice that are found in the pellets of songbirds.

The poo of birds is generally watery, white splashes with a darker solid mass. In pigeons these are somewhat more wholesome, soft and brown-green with white deposits of uric acid. Those of waterbirds, such as geese and swans, are soft, green and fibrous, while those of grouse are fibrous and dry. Birds of prey produce copious white splashes that whitewash favourite cliff ledges and vegetation below nests. The green woodpecker produces long, cylindrical poos packed full of the exoskeletons of ants encased on the outside with a thin layer of white uric acid.

Pellet sizes

Pellets come in all sorts of shapes and sizes. Each species of bird will produce pellets of varying size, and this can make identification more challenging as they do not always relate directly to the size of the bird they came from. For example, ravens produce both large pellets (80mm long) and small pellets (50mm long or less). Pellets from the same species vary in size depending on what they have been eating and on their age: young birds will produce small pellets relative to those produced by older individuals. Pellets that have been around for a while may also appear larger than when they were first regurgitated. For instance, barn owl pellets begin to expand and appear larger as they dry out and the outer mucus coating slowly breaks down. This process may be even faster if detritivores – invertebrates that love eating feathers and hair – get to work (see ‘What else might you find in a pellet?’).

How does a pellet form?

A pellet forms in the stomach of a bird. How much bone is regurgitated is determined by how much has been swallowed and by the digestive power of a given species’ stomach juices. A bird’s stomach is made up of two main parts: the proventriculus and the ventriculus or muscular stomach, which is known as the gizzard. The undigestible remains from a meal – seeds, hair, feathers, bones, teeth, horny parts, such as claws and hard parts of invertebrates – are formed together in the gizzard and stored in the smaller, weaker proventriculus prior to regurgitation.

In medium- to large-sized owls, pellets are formed within a six- to eight-hour period (Grimm & Whitehouse 1963; Smith & Richmond 1972). Pellets are regurgitated in response to how much food the bird has been eating, when it last fed and by the stimulus of hunting new prey (Smith & Richmond 1972). Large birds such as owls and cormorants produce one pellet per day, sometimes two, while skuas and gulls produce one per meal (Marti 1973; Barrett et al. 2007). By contrast, green sandpipers produce pellets every 20 minutes when feeding (Holt & Warrington 1996)!

Variable colour

Pellets vary in colour depending on the diet of the bird that produced them. Those of crows and rooks are very light and yellow in appearance if they have been feeding on grain, while an abundance of insects may result in black or brown pellets. For little owls and jackdaws, these may be black, glossy and iridescent due to the shiny beetle wing cases. Birds of prey produce grey, dark brown or almost black pellets when feeding on small mammals. The remains of voles, mice and rabbits tend to form grey-brown pellets – although they still tend to look black or dark brown in barn owl pellets – while moles and shrews (if eaten as the