Why Do Owls Cough Up Bones? Sorting What Cannot Be Digested

Many birds swallow prey whole or in large pieces rather than processing it, having no teeth and in most cases no means of separating flesh from bone. Digestion takes place in two chambers, with the first producing acid and enzymes and the second, a muscular gizzard, grinding and sorting. Material that cannot be digested, including bone, fur, feathers, insect cuticle and scales, accumulates in the gizzard and is compacted into a mass, which is then passed back up and ejected through the mouth rather than continuing through the gut, where it would cause damage. The bird produces one roughly every day or so, generally at a regular roosting place, and cannot feed properly until it has done so.

The habit is far more widespread than the association with owls suggests:

• •Owls, whose pellets contain unusually complete bones because their digestion is less acidic
• •Hawks, falcons and eagles, whose stronger acid dissolves much of the bone
• •Gulls, crows, herons, kingfishers and grebes
• •Many insectivorous songbirds, producing tiny pellets of insect cuticle
• •Swifts, shrikes and nightjars
• •The difference in bone preservation between groups matters enormously for anyone studying the contents

Pellets provide a diet record that is difficult to obtain any other way. Prey can be identified to species from skulls, jaws and teeth, since small mammals are readily distinguished by dentition, and a single roost can yield hundreds of individuals over a season. That makes them a survey method for the small mammals of an area as much as for the bird's diet, and owl pellets have been used to document the presence of species that trapping had missed and to detect the arrival of introduced species. Long series from the same site record change over decades. The method is cheap, requires no licence in most jurisdictions, disturbs nothing, and produces quantitative data, which is a rare combination in field ecology.

The practical procedure is simple and is used in schools and in laboratories alike. A pellet is dried and, if it came from the wild rather than from a supplier, sterilised by heating, since the material can carry organisms including salmonella. It is then teased apart dry with fine tools, or soaked in water and separated, with the second method faster and harder on delicate bones. Contents are sorted into skulls, jaws, limb bones and vertebrae, and the jaws and skulls are matched against identification keys that work from tooth shape and count. Counting the number of individuals means counting whichever element is most numerous rather than adding bones up, since one animal contributes many. The whole exercise takes an hour or two and produces a genuine dataset, which is why it has become a standard school activity.

The record is not a straightforward sample of what is out there and the distortions are known. Different predators take different prey, so a pellet series describes what that species hunts rather than what lives nearby, and owls take more of what is active at night and at the size they prefer. Digestion destroys small and fragile bones more than robust ones, so the smallest prey is underrepresented, and species with stronger digestion leave less identifiable material. Prey taken far from the roost is mixed with prey taken nearby, so the catchment is uncertain. Seasonal change in diet means a single collection misrepresents the year. And soft-bodied prey including earthworms and slugs leaves almost nothing, so a diet including them appears to consist of something else entirely.

Birds swallowing prey whole compact the indigestible parts in the gizzard and eject them through the mouth, usually about once a day at a roost. Owls preserve bones better than hawks because their digestion is less acidic. The contents identify prey to species and serve as a small mammal survey, with known biases towards robust bones and nocturnal prey.