"What may seem simply to be discarded, unwanted food, in fact serve a key purpose in ensuring good health."
They often go unnoticed. A small bundle of valuable data that is both satisfying and fascinating to dissect.
Pellets are a mass of undigested food regurgitated by many species of birds.
Once you know what you’re looking for, finding pellets can be an exciting experience for all lovers of wildlife and the great outdoors. What’s more, their construction and purpose is profoundly intriguing.
Collecting and studying pellets is a great way for researchers and enthusiasts to determine the diets of the birds producing them, especially when trying to establish things such as seasonal variation in diet. The pellets themselves can include all sorts of matter, such as plants, bones, feathers and even insects. Dissecting the pellets is a much more ethical method than killing and dissecting the birds the birds themselves to gain such information. Establishing the diets of the birds is also a good indicator of the prey availability in given areas such as nesting sites.
Whilst it’s quite commonly known that owls produce pellets, there are actually several other species that likewise regurgitate indigestible matter. Species such as hawks and eagles produce some of the largest pellets amongst predatory birds. From kingfishers to swallows, herons to some corvids, all produce these curious lumps of evidence. What may seem simply to be discarded, unwanted food, in fact serve a key purpose in ensuring good health.
Regurgitating pellets removes indigestible food from the gizzard; a part of the digestive system found in birds. Once prey has been ingested, if it is not being stored it passes through into the proventriculus, otherwise referred to as the glandular stomach. The proventriculus is lined with mucus secreting, columnar epithelial cells which secrete hydrochloric acid and pepsinogen. The acid activates the conversion of pepsinogen to pepsin enzymes which aid in the digestion of proteins. This cocktail of acids and enzymes creates a pH level of 2.0 – 3.5 within the proventriculus, that successfully breaks down and separates the soft tissues from the indigestible matter.
Both are then passed through to the gizzard. Like the proventriculus, the gizzard is lined with mucus secreting cells. Differentially, these cells form a 1mm thick cuticle that lines the gizzard in protection against the acids produced by the proventriculus. Naturally, this cuticle goes through a cycle of being continuously worn away and regenerating itself. The sloughing of this membrane entirely has been observed, such in the instance of a male hornbill presenting its mate with essentially a sack full of fruit (gizzard membrane full) as she incubated their eggs, confined to the nest.(Bartlet, 1869; Flower, 1869; Murie, 1874). If allowed to build up, the cuticle can have adverse effects.
Birds have been recorded to swallow small stones, otherwise known as rangle, to scour the cuticle lining and wear it down to avoid such complications.So, if you come across stones in any pellets you find, don’t be alarmed! Acciptors (true hawks) such as the goshawk and sparrow hawk have not been recorded to use rangle, however this is mostly due to the difficulty in finding their pellets; it is not to say that they don’t use such a method. This method has also been replicated by falconers with captive birds to ensure the natural cycle of the gizzard cuticle.
*An interesting side note; this behaviour of eating rangle has also been documented in species such as crocodile, seals sea lions and several other non-bird species*
Back to the digestion….The soft parts of the prey are passed through to the intestine for further digestion and absorption. A grooved valve prevents any indigestible matter such as fur, feathers and in some instance’s bones, from passing through with the soft matter. Instead in remains in the gizzard, where its muscular wall compacts them together into pellets with contractions occurring every 20-30 seconds. Once the gizzard becomes full, the pellets are regurgitated, and thus indigestible food is removed.
Many birds also have what is known as a crop; an expanded muscular pouch found near the gullet or throat essentially forming an enlarged part of the oesophagus. In most species it is used to store food, such as that of the scavenging vultures who take full advantage in an available meal and gorge. However, in dove and pigeon species the crop can be used to produce its own milk which is used to feed their newly hatched chicks.
Birds such as falcons and hawks use the regurgitation of pellets, to cleanse their crop and remove unwanted bacteria. In fact, in falconry, birds are purposely given what are referred to as castings for the bird to regurgitate after a meal. These consist of either cotton or plumage such as rabbit fur or feathers. Not only do they provide the birds with their natural benefits, but they also provide an indication as to the health and condition of the bird once regurgitated. If cast out round and white, fairly dry and lacking odour then the bird is in good health. Any alternatives to this, particularly those that are produced with a slimy texture and dark in colour, indicate poor health.
Pretty interesting stuff, right?! With all that in mind, next time you are out walking keep an eye out for any pellets and get dissecting yourself. Near complete skeletons have been found within owl pellets, as they are less able to break down bones, whereas raptor pellets tend to contain more keratinous structures such as claws, beaks and feathers. So, who knows what you might find!
I hope you enjoyed learning a little more about pellets and digestion and feel inspired to keep an eye out next time you’re out for your nature stroll!
*Of course, be sure to thoroughly wash your hands after handling the pellets*