Birds need to eat a lot to meet those absurdly high energy requirements like flight, egg production and their unique way of breathing. To maintain this rapid metabolic rate, birds need an efficient digestive system, so let’s take a look beyond the beak.
Birds forage for food soon after dawn to replace energy lost just by staying alive overnight, and in the evening to refuel after a day’s frenetic activity, but a bit like us, they are opportunistic and will feed at any time of day if food is available. They will need to convert that food into the component fats and proteins as soon as possible, not just to survive those winter nights or prepare for a long migratory flight, but for literally everything a bird does. Their core temperatures are a few degrees higher than ours, and that tiny furnace needs to be kept stoked to keep blood flowing and muscles working. Relaxation does occur, but is scarce, as predators can lurk behind any corner; birds can rarely fight back so fly they must, and that requires tons of energy. Rapid digestion also helps new chicks put on body mass very quickly, exceptionally helpful if you need to fly to Africa just two months after you’ve been born.
Rapid digestion also helps new chicks put on body mass very quickly.
With their specialised beaks, birds pull their food into small pieces, and pass those pieces through their mouths to their digestive system. Birds can’t chew their food – if you’ve ever watched them at feeders you will see they peck off tiny amounts, or will fly away with a larger morsel and then chip away at it in the relative safety of tree branches. Likewise, with raptors, they will tear off smaller chunks from the larger carcass, often gripping the food between their talons. Some birds, like shrikes, will impale their food on a sharp branch and tear it off from there, and some will bash it against rocks to break it up, like the bearded vulture.
To swallow, birds tip their heads back moving the food to the throat, and using their tongues and saliva they transfer the food through the pharynx, a small section connecting the mouth to the inner systems. This is where things get interesting. Food passes through the oesophagus, the same fundamental passageway that our food goes through, but instead of leading straight to the stomach, a bird’s food goes first to something called the crop, an organ that sits below the bill in the thoracic cavity. Digestion for a bird will of course depend on the species we’re looking at, and whilst there are the same basic steps across most species, certain anatomical elements like size and shape of internal organs will vary. One of the main variants you’ll actually be able to see is the crop. This is often hidden internally on many species, occasionally seen as a raised mound of feathers when it is full, and if you are ever lucky enough to hold a bird you may be able to feel a lumpy, gritty sort of sac in their neck. It can also be somewhat alarmingly obvious on other birds. The frigatebird, for example, has a huge bright red crop that it also uses in courtship dances, inflating it and using it as an acoustic instrument to show off and attract a mate worthy of such a protuberance. Owls don’t have crops at all, and swallow their food whole – more on that later.
Not considered part of the actual digestive system itself, the crop acts more as a holding area. Being able to consume a large amount at once but not digest it straight away is advantageous in many situations, such as when feeding out in the open, a big risk to many birds. Instead of returning time and again to take small amounts, they can make off with, say, a whole flower seed head, leaves and stem and all, and then keep that in their crop and allow it to digest slowly as passage through the oesophagus permits. Parent birds also store partially digested food in their crops which they then lovingly regurgitate back to their chicks at feeding time.
If you are ever lucky enough to hold a bird you may be able to feel a lumpy, gritty sort of sac in their neck.
Once food has left the crop, it passes into the first of two chambers that make up a bird’s stomach. The proventriculus is the first chamber and this is basically an acid-dousing section, helping to break down the solid mass of the food. Depending on the diet of the species, this acid can just kick things off or sizzle a meal to within an inch of its composite structure – the tougher and bonier the meal, the more acid it will produce.
The second chamber is a word some of us are probably more familiar with – the gizzard. Prized by some as a delicacy, this is a larger, tougher and more muscular organ than all those before it, as it needs to do a lot of work. This is where all the grinding down of food takes place, helped along by the soaking in acid in the previous chamber. Gizzards can’t do all the work in some cases though – some substances such as seed kernels, rice, and other grains need more than just muscles rubbing them together, and you’ll often see ground feeding birds like pigeons pecking at gravel and grit as well. This mineral matter helps the gizzard grind and smash the food into smaller particles, enabling greater absorption of the vital nutrients within.
The gizzard will reduce in size for birds who eat soft foods such as fruits and insects during the summer, only to become bigger and stronger in the winter when seeds and grain are back on the menu.
A notable adaptation of the gizzard is that of the grebe – by swallowing their own feathers, they create a filtering system that settles in the base of the gizzard before passage into the small intestine. This traps any small indigestible fish bones that have managed to slip through and will cause untold damage if they pass any further, which are then regurgitated out later on.
When as much of the valuable nutrients have been absorbed by the lining of this magnificent stomach, the rest is then passed into the small and large intestine, where any remaining water and nutrients are extracted, and microbial assistance prepares any final matter into faeces for eventual expulsion.
As before, the intestines are different sizes in different species – for those who ordinarily consume fruit and insects, easily absorbed soft food, they will be short – the intestine of a swift, for example, who only eats insects in the summer, is only approximately three times its body length, whereas birds who eat plants, fish, and tougher flesh can have intestines that are twenty times their body length. Berries can pass through a thrush’s intestine in less than half an hour, but rougher food usually needs half a day or more.
Birds don’t urinate: the process of digestion completes when the remains pass through the large intestine, where any nitrogenous waste from the food extracted earlier is reintroduced as uric acid from the kidneys. This combines with the solid matter that is left and forms a whitish paste which then exits the bird via the rectum or avian vent, known as the cloaca. This is also the opening from which eggs pass, and sperm from the male bird enters for fertilisation. Another example of a bird’s economical organ functions, conserving energy.
Something stuck in my throat
There are some birds like owls and other birds of prey who consume a large amount of indigestible material. Items like small animal skulls containing teeth that can’t be broken down by the acid and the muscle action of the gizzard are compressed into a pellet, and mashed together with feathers and fur. Once formed, this pellet is then passed back into the proventriculus. Falcons and hawks, having stronger acid in their first chamber, produce a smaller and easily managed pellet, but owls have weaker acid so it will be composed of more material, and bigger. Owls also lack crops – this means the pellet actually blocks the oesophagus meaning they can’t eat whilst they are digesting and forming a pellet. Not keen on losing energy they can’t replace by flying, and as they usually take their food home to consume it, you can often find owl pellets on the ground at the base of trees, meaning their nest is just above your head. Soaking a pellet in some water (wash your hands afterwards, kids) will often reveal a partial or full skeleton of some poor departed mouse. If you ever see an owl looking a bit uncomfortable and hungry at the same time, keep watching, as it will likely be ready to cough up a big one.
You can often find owl pellets on the ground at the base of trees.
As a bird’s digestive tract is designed to extract the most nutrients as quickly as possible, it is important that if we leave any food out for them, it is also packed with the best nutrients. This is why bread is not good for ducks and geese – ignoring the urban myth of bursting birds, they will eat it, and no doubt enjoy it, but as it is essentially just carbohydrates, it’s not good for them. All that expended digestive energy will be wasted, and replaced by nothing. The food we offer birds must promote their health, otherwise we are just adding to their downfall. As always, planting trees and shrubs that will provide the fruits, nuts and seeds essential to the diet of your resident birds and known migrants is the best thing you can do for birds, as not only are you providing sustenance, but also protection from predators, a home for new life, and a place to stop and relax, well-fed, if only for a while.