BIO 554/754

Digestive System:
Food & Feeding Habits 

Hit 'Reload' or 'Refresh' to View Again!

An updated version of these notes can be accessed from a new "Avian Biology' page


Because of their high metabolic rates, birds must consume more food in proportion to their size than most animals. For example, a warbler might eat 80 percent of its body weight in a day. As a group, birds consume just about any type of food you can imagine, including amphibians, ants, buds, crustaceans, fish, fruit, grass, insects, larvae, leaves, molluscs, nectar, other birds, pollen, reptiles, rodents, roots, sap, seeds, suet, snails, wax, & worms. To meet their metabolic needs while remaining as light as possible (to be efficient flyers), the digestive system of birds has to be both as light as possible and as efficient as possible. Weight has been minimized by the loss of teeth &, in many birds, limited jaw musculature.

Ligaments & muscles on the skull of a Steller's Sea Eagle.
Muscles that close the jaw include the adductor mandibulae externus, adductor mandibulae posterior,  & pterygoideus.
The depressor mandibulae opens the jaw.
(Source: Ladyguin 2000)

The need to keep weight as low as possible also means that, except perhaps prior to migration, there is a limit to the amount of fat a bird can store. 'Efficient' means that birds must locate, ingest, & digest food as quickly and efficiently as possible.

Retention time (in hours) for fluid & particulate digesta markers in the gastrointestinal tracts 
of representative reptiles, birds, & mammals (Based on: Stevens and Hume 1998).
Species Body mass
Fluid retention time 
Particle retention time 
Broad-tailed Hummingbird1
3.3 gm
Rock Ptarmigan
460 gm
Sooty Albatross
2.5 kg
Rockhopper Penguin
2.5 kg
38 kg
176 kg
            1McWhorter and Martinez del Rio (2000)

In general, typical mean retention times are 30 - 50 minutes for avian nectarivores, 40 - 100 minutes
for granivores, and 15 - 60 minutes for frugivores (Karasov 1990, Klasing 1998).

Here's a typical avian digestive system:


The major components of the avian digestive system are the alimentary canal plus several accessory structures. The 'canal' includes the oral cavity, pharynx, esophagus (which includes a crop in some birds), stomach (proventriculus & gizzard), small intestine, & large intestine. The large intestine then empties into the cloaca. Important accessory structures include the beak, salivary glands, liver, & pancreas.

A bird's bill consists of a bony framework covered by a tough layer of keratin. The keratin layer is continuously replaced throughout the life of a bird & is just as continuously worn down by eating and manipulating hard objects. The cutting edges of the beak are the tomia. The bill plays a critical role in food acquisition &, of course, bill morphology varies with food habits:

Flamingos use a series of projections, or lamellae, to filter tiny food items from debris in the water.

Swifts are aerial insectivores & use their wide gape to help capture flying insects.

Eagles (and hawks) are diurnal raptors & use their hook-like bills to tear apart large prey.

Shovelers use their spatula-shaped bills to filter food from mud & water.

Crossbills use their 'crossed-bill' to extract seeds from pine cones.

Herons use their bills to spear small fish and amphibians.

Avocets sweep their long
up-curved bills from side-to-side through the water to capture small  invertebrates
(or use it like a forceps to pick up prey).

Woodpeckers use their chisel-like bills to chop away wood & expose insects and insect larvae.

Wrens use their thin, probing bill to capture small insects. 

Curlews use their long bill to probe mudflats for small invertebrates.

Hawfinches are seed-eaters & use their bills to crack open large, hard seeds.

Macaws use their strong hook-like bills to feed on nuts. 

Mallards & other waterfowl use their bills to filter small invertebrates from mud and water.

Skimmers use their elongated lower mandible to skim the surface of the water & capture small fish and invertebrates.

Finches do not simply bite the seeds; instead; the lower mandible is moved toward the tip of the bill in a slicing motion. When most of the coat has been cracked or removed, the lower mandible is moved from side to side to remove the rest of the shell, thus releasing the kernel. Some large finches also have raised hard surfaces in both the upper and lower jaws that function as anvils whereby large seeds can be held firmly while the lower mandible slices and cracks the sides of the seed. As tricky as nutcracking sounds, most birds accomplish it rapidly, shelling small seeds in a few seconds and large finches can crack open and devour a large seed or nut in less than twenty seconds.

© Gregor Yanega, University of Connecticut
Big mouths get hummingbirds an in-flight meal - Hummingbirds have bendy lower beaks to help them catch insects (Yanega and Rubega 2004). The flexibility allows long-beaked birds to open their mouths wide enough to hunt on the wing. Hummingbirds use their long, narrow beaks to probe flowers for nectar, but they also need insects for essential nutrients. It wasn't clear how they could catch them; birds that hunt flying insects usually have short beaks to help them open their mouths wide. - Helen R. Pilcher, Nature Science Update

Feet and talons

While not part of the digestive system in an anatomical sense, some birds, like hawks and owls, use their feet and talons to capture prey. Typically, raptor prey are killed by the talons of the contracting foot being driven into their bodies; if required, the hooked bill is used to kill prey being held by the talons.


Talons of (left > right): Harpy Eagle, Golden Eagle, Bald Eagle, 
Great Horned Owl, Red-tailed Hawk, & Peregrine Falcon

Falconiformes (hawks & falcons) and Strigiformes (owls) differ in morphology, talon force, & hunting behavior -- Ward et al. (2002) examined the hindlimb morphology of six raptors (listed below) to determine if resource partitioning might be explained, at least in part, by morphological differences. One difference is that the digit pattern in Strigiformes is zygodactylous (see photo to the right), a pattern that may reduce the chance of prey escaping by maximizing the area of the foot & may allow owls to better subdue larger prey than similar-sized hawks. The morphology of owls (shorter & wider tarsometatarsus; see photo below right) also appears to be associated with a stronger grip, while the hindlimbs of hawks & falcons (relatively long and gracile) seem adapted for high-velocity movements. 
     The force produced by talons may be related to time of activity. Owls hunt when light levels are low so if an attacking owl misses its prey, relocating it may be difficult. Hawks are diurnal hunters and can use visual cues during and after an attack. If unable to subdue prey initially, they can relocate prey visually and catch it.  Given the morphological differences and hunting behaviors of these raptors, how well do those characteristics relate to prey-size selection? 
  • Great Horned Owls can take relatively large mammals such as porcupines and skunks, plus large birds like pheasants and quail 
  • Barred Owls prey mainly on medium-sized mammals, including mice and squirrels, as well as amphibians. 
  • Eastern Screech-Owls prey on insects, small birds, and small mammals. 
  • Red-tailed Hawks subsist primarily on rodents and larger mammals such as skunks and rabbits.
  • Red-shouldered Hawks, like Barred Owls, subsist mainly on medium-sized mammals such as squirrels and chipmunks, but also prey on frogs and salamanders.
  • American Kestrels, like Eastern Screech-Owls, eat mostly insects and small mammals. 
In sum, differences in grip force & the hunting
behavior of owls and hawks suggest at least a 
partial basis for resource partitioning in the eastern 
deciduous forests of North America. Each raptor has a unique force production, along with a different time of activity, that would allow for a degree of prey specialization. 

Great Horned Owl foot

Tarsometatarsi of a similarly-sized hawk & owl. 
(A) Red-tailed Hawk. (B) Great Horned Owl. 

Bristles are stiff and hairlike, consisting of a central rachis  without vanes, and provide both protective and sensory functions. Bristles occur most prominently around the eyes ("eyelashes"), the lores, the nostrils, and around the rictus (corners) of the mouth. Not all birds have bristles. Rictal bristles are prominent in many insectivorous birds, particularly aerial insectivores like nightjars (Order Caprimulgiformes) and flycatchers (Family Tyrannidae), and are used as sensory organs to help locate and capture prey, much like mammals use whiskers. Birds that chase insects through the air with open mouths, such as nightjars, are also thought to use the bristles as a funnel to help direct insects into the mouth. The photo to the right shows the rictal bristles of a Hooded Warbler.


The avian tongue:


Detailed view of the horny tip (left) of the Guadeloupe Woodpecker tongue in vivo position (Villard and Cuisin 2004).

     Goose tongue -- The dorsal surface of the tongue of Middendorff's Bean Goose (Anser fabalis middendorffii) has an anterior region that extends for five-sixths of its length plus a posterior region. Large conical papillae (indicated by arrowhead to the right) are located in a row between the anterior and posterior regions. On both sides of the anterior region, lingual papillae are compactly distributed, and small numbers of large conical papillae are found between the lingual papillae. The dorsal surface of the tongue is covered by numerous fine processes, which help hold food on the tongue's surface. 
     The taste buds of birds may be located in the upper beak epithelium, in the anterior mandible, and the mandibular epithelium posterior to the tongue. Some taste buds are also located ventrolaterally on the anterior tongue. -- From: Iswasaki (2002).

Surface structure and histology of the dorsal epithelium of the 
tongue of Middendorff's Bean Goose. (a) Macroscopic dorsal view of the tongue. Arrows show lingual hairs on the lateral sides). (b) Scanning electron micrograph of the lateral side of the tongue. Lingual papillae (arrows) are compactly distributed on the tongue, and large conical papillae (arrowhead) are scattered among them. Scale bars = 10 µm (a) 
& 500 µm (b). (From: Iwasaki 2002).


Energy and nitrogen balance in a hummingbird -- Keeping fit and healthy on a low-fat, fiber-free diet isn't easy, but despite the nutritional disadvantages of life on a liquid lunch, hummingbirds flourish by supplementing their nectar intake with tiny arthropods. But the beneficial snacks come at a high metabolic price; flies don't sit still, so hummingbirds work hard chasing their protein. Just how much nitrogen a hummingbird extracts from the protein in its diet, or the amount of effort needed to gather it, wasn't clear, so López-Calleja et al. (2003) began tempting the tiny birds with nitrogen laced nectar and found that although their protein requirements were relatively meager, the tiny creatures' metabolic demands were colossal: 43 kJ day-1!
     López-Calleja et al. (2003) trapped almost 40 Green-backed Firecrowns in central Chile, before transporting them to an aviary in Santiago ready to test out their metabolism. Back in the lab, the team prepared nectar solutions with different concentrations of amino acids to see how much protein the birds needed to maintain a stable body weight. By filming the birds as they sipped from feeders, they measured the amount of energy and nitrogen that the birds consumed. To calculate the bird's nitrogen uptake, they also needed to know how much waste nitrogen the birds lost. So, they collected all of the birds' feces, making sure that none dried out, and measured the nitrogen content. Not surprisingly, the birds that were fed small amounts of protein began losing weight quickly, even though they were able to sip as much high-energy nectar as they wanted. However, the birds that were fed 1.82% nitrogen or more, held their weight. López-Calleja et al. (2003) calculated that the tiny aeronauts need at least 10 mg nitrogen per day to maintain a stable body weight, or else they waste away.
     What does that translate to in terms of flies? López-Calleja et al. (2003) provided the birds with 500 fruit flies to snack on while offering them either an unlimited nectar supply, a restricted nectar intake, or no nectar at all. After five days of access to flies and nectar, the birds were fit and healthy, catching around 150 flies a day, sufficient to supply them with 5% nitrogen. The birds that had a reduced nectar supply also maintained a stable weight, although they went into torpor overnight to conserve energy. But the birds fed flies alone began losing weight, no matter how hard they worked to feed themselves. Fernández, one of the co-authors was surprised that `flies are not a complete food source for hummingbirds'. She suspects that although the flies should supply all of the hummingbirds needs, the birds simply have to work too hard to catch flies to rely on them as their soul food source. -- Kathryn Phillips, Journal of Experimental Biology

Flush–pursuit foragers use exaggerated and animated foraging movements to flush potential insect prey that are then pursued and captured in flight. The Myioborus redstarts comprise 12 species of flush–pursuit warblers found in montane forests of the American tropics and subtropics. All members of the genus have contrasting black-and-white tail feathers that are exposed by spreading the tail during foraging. Mumme (2002) examined plumage pattern and tail-spreading behavior to see how they affected flush–pursuit foraging performance of the Slate-throated Redstart (Myioborus miniatus) in Costa Rica. Although flycatching was the most common foraging tactic used by Slate-throated Redstarts, flush–pursuit prey attacks occurred more frequently following hops in the spread-tail foraging posture than hops in more typical warbler-like posture, suggesting that tail-spreading behavior assists in startling and flushing potential insect prey. The hypothesis that the white tail feathers enhance flush–pursuit foraging was tested by means of a plumage-dyeing experiment. After locating nests, Mumme (2002) captured the male and female and assigned one member of each pair to the experimental treatment group; its mate served as a control. For experimental birds, a permanent marker was used to blacken the white tips of the three outer retrices. For sham-darkened controls,the naturally black tips of the three inner retrices were also ‘‘blackened.’’ Experimental birds with darkened tail feathers were significantly less successful in flush–pursuit foraging, showed a significantly lower overall rate of prey attack, and fed their nestlings at a significantly lower rate than did their sham-darkened mates. For experimental birds, only 7.6% of hops in the spread-tail posture were followed by an attack on a prey item, compared to 20.9% of hops for controls. These results indicate that white tail feathers are critically important in startling potential prey. 

Buccal, or oral, cavity:




Owls set beetle trap with dung -  Levey et al. (2004) compared what Burrowing Owls ate when there was a typical litter of dung at the entrances to their nest burrows with their diet when the dung was removed. The owls ate 10 times more beetles when the dung was present, suggesting the waste did not build up by accident. Burrowing Owls make their nests in small tunnels, and place a variety of debris, including dung, at the entrance.  After finding that Burrowing Owls also had a high concentration of dung beetles in their diet, Levey et al. (2004) proposed that the owls might be using dung as bait to attract the beetles. To test this hypothesis, they cleared all nest entrances at two colonies of owls of debris, then one owl colony had a typical littering of dung applied while the other was left bare. After four days each entrance was again completely cleared and the situation was reversed. Analysis of the owls' waste clearly showed that when dung was present, the owls feasted on ten times more dung beetles. As Levey says, "this experiment demonstrates that tool use makes a difference to a wild animal". Although it may be tempting to conclude the owls are clever enough to devise this trap, Levey explained: "I don't believe these burrowing animals are aware of the link between the dung they bring in and the beetles they catch". Instead, the baiting may simply have evolved, as owls who happened to collect more dung had a better diet and therefore bred more successfully.  -- Peter Wood, BBC News Online 

The avian stomach is divided into 2 parts:


A Price Worth Paying -- Birds don't need teeth to grind their food; they solve the mashing problem with a powerful gizzard. But not all gizzards are equal. In fact, Red Knots' gizzards grow larger when the birds put on weight preparing for migration. But they also change size throughout the year. What causes such changes in gizzard size?

   van Gils et al. (2003) served knots that had large and small gizzards (as determined by ultrasonography) a selection of hard intact molluscs and soft mollusc meat and filmed the birds as they ate. Knots with large gizzards consumed far more molluscs with shells than the birds with smaller gizzards. van Gils et al. (2003) also offered the birds a shell-heavy diet, but even the birds with the largest gizzards needed to feed for 16 hours a day to sustain their weight! Birds with smaller gizzards simply couldn't feed fast enough. By allowing them to crush more shell per gizzard-full, larger gizzards gave birds the edge. 

   Thus, even though it is energetically costly for the knots to maintain a larger gizzard, when the bird needs to get the most out of its crunchy diet, it's a price worth paying. So, the birds' gizzards enlarge as they fatten for migration. van Gils et al. (2003) also found the knot's gizzards enlarged when the molluscs begin shrivelling (as their winter food supply dwindles). Because the molluscs' shells stay the same size as the molluscs shrink, the amount of shell a bird must process to eat its fill also increases. But with their larger gizzards, the birds can still make the most of even the crunchiest winter diet!  -- Kathryn Phillips, Journal of Experimental Biology

Frequency distribution of gizzard mass 
of free-living Red Knots (N=920).

Canary stomach

Long-term preservation of stomach contents in incubating King Penguins -- Male King Penguins (Aptenodytes patagonicus) are able to store undigested food in their stomach for up to 3 weeks during their incubation fast. Such an adaptation ensures hatchling survival if their mate's return is delayed. Using small electronic recorders, Thouzeau et al. (2004) studied the change in gastric pH, motility and temperature during the first week of food storage. The pH could be maintained at values as high as 6 throughout the incubation fast, a pH unfavorable for avian gastric proteinase activity. Gastric motility was markedly reduced for most of the incubating birds, with lower motility probably associated with a better conservation of stomach content. Stomach temperature was maintained at around 38°C. The fact that stomach temperature of incubating birds did not show a daily rhythmic fluctuation as seen in non-breeding birds could be due to temperature constraints on embryo development. Thus, this study demonstrates substantial adjustments of pH and gastric motility in incubating King Penguins, which may contribute to the inhibition of digestive gastric processes. Mechanisms underlying these adjustments are probably complex, including a combination of neuronal, humoral, and/or hormonal factors.


    Small intestine:

Gastrointestinal tracts of a carnivorous hawk, an omnivorous chicken, and 4 herbivorous birds.
Note larger size of crop in omnivore and herbivores, and particularly in hoatzin. Ceca are small in hawks and
relatively large in grouse. Although ceca are relatively small in hoatzin, emu, and ostrich, an expanded foregut
(hoatzin), a much longer midgut (emu), or a much longer colon (ostrich) compensates for this (From: Stevens and Hume 1998).

GI tract of an Ostrich chick
(P = proventriculus; G = gizzard; S1 = small intestine;
Ca = caeca; L1, L2, & L3  =  sections of large intestine)

Glucose transport in birds -- In contrast with regulation of intestinal glucose transport in mammals, amphibians and fish, intestinal glucose transport does not change with dietary carbohydrate in most birds. This is interesting, because the diets of many birds change with seasons, and the levels of carbohydrate in those diets also vary with season. Nevertheless, intestinal glucose transport rates do not vary with dietary carbohydrate levels in American Robins, House Sparrows, and Yellow-rumped Warblers. The absence of dietary modulation of glucose transport in birds may be due to the predominance of passive glucose transport, probably occurring through the paracellular pathway (i.e., between cells rather than through cells via active transport). If transport were largely passive and dependent on transepithelial concentration gradients, then there would not be any need for specific changes in carrier-mediated (active) transport. For example, passive absorption of nutrients such as fat-soluble vitamins is not subject to modulation by diet.

     Over-reliance on the passive pathway provides metabolic advantages and ecological constraints. It does provide birds with an absorptive process that can deal with rapid and large changes in intestinal sugar concentrations. The passive pathway is also energetically inexpensive to maintain and modulate. However, passive absorption through the paracellular pathway is dependent on concentration gradients. In the absence of a transport system that selects which materials to absorb, this non-discriminatory pathway may also increase vulnerability to toxins, and thus constrain foraging behavior and limit the breadth of the dietary niche of the birds. Another problem is that when luminal sugar concentrations are lower than those in plasma, glucose may diffuse back into the lumen. -- Source: Ferraris (2001).

    Large intestine:


Avian geophagy -- In birds, geophagy (the intentional consumption of soil) is known for geese, parrots, cockatoos, pigeons, cracids, passeriforms, hornbills, & cassuaries. Brightsmith and Muñoz-Najar (2004) observed ten species of psittacids, three species of columbids, and two species of cracids consuming soil from banks of a river in Peru.  They found that preferred soils were deficient in particles large enough to aid in the mechanical breakdown of food and help digestion. Percent clay content and cation exchange capacity (CEC), both predicted to correlate with adsorption of toxins, did not differ between used and unused sites as had been found in a similar study. Instead, preferred soils were more saline and had higher concentrations of exchangeable sodium. This suggests that the choice of soils at their study site was based primarily on sodium content.  Experimental evidence has shown that soils are capable of adsorbing biologically relevant quantities of toxins in vitro and that soil consumption by parrots does reduce the absorption of toxins in vivo. Brightsmith and Muñoz-Najar (2004) did not find evidence that parrots choose soils with greater CEC or clay content, the characteristics that correlate with the capacity to adsorb toxins. Instead, they found that birds chose soils with higher concentrations of sodium. These two findings are not mutually exclusive but instead suggest that there may be a set of conditional rules for soil selection. In situations in which sodium concentrations are variable, the birds appear to choose soils that are highest in sodium (this study). In areas in which sodium concentrations are uniformly high, birds may choose the soils that have the largest ability to adsorb dietary toxins.


Why are bird feces white? Unlike mammals, birds don't urinate. Their kidneys extract nitrogenous wastes from the bloodstream, but instead of excreting it as urea dissolved in urine as we do, they excrete it in the form of uric acid. Uric acid has a very low solubility in water, so it emerges as a white paste. This material, as well as the output of the intestines, emerges from the bird's cloaca. 

Accessory organs:

Avian Pancreas tissue

Useful links:

Albatrosses at Work: Food Storing

Determining Diets

Diet and Nutrition

Digestive Physiology of Birds

Eating Feathers


The Avian Digestive Tract

The Digestive System

Literature Cited:

Brightsmith, D. J. and R. A. Muñoz-Najar. 2004. Avian Geophagy and Soil Characteristics in Southeastern Peru. Biotropica 36: 534-543.

Duke, G.E. 1994. Anatomy and physiology of the digestive system in fowl. Pages 46-49 in Proc. 21st Annual Carolina Poultry Nutrition Conference, Dec. 7-8, Charlotte, N.C.

Ferraris, R. P. 2001. Dietary and developmental regulation of intestinal sugar transport. Biochem. J. 360:265-276.

Iwasaki, S. 2002. Evolution of the structure and function of the vertebrate tongue. Journal of Anatomy 201:1-14.

Karasov, W.H. 1990. Digestion in birds: chemical and physiological determinants and ecological implications. Stud. Avian Biol. 13: 391–415.

Klasing, K.C. 1998. Comparative Avian Nutrition. CAB International, New York, NY.

Ladyguin, A. 2000. The morphology of the bill apparatus in the Steller's Sea Eagle. Pp. 1 - 10 in First Symposium on Steller's and White-tailed Sea Eagles in east Asia (M. Veta and M.J. McGrady, eds.). Wild Bird Society of Japan, Tokyo.

Levey, D. J., R. S. Duncan, & C. F. Levins. 2004. Animal behaviour: Use of dung as a tool by burrowing owls. Nature 431: 1038-1039.

López-Calleja, M. V., M. J. Fernández, and F. Bozinovic. 2003. The integration of energy and nitrogen balance in the hummingbird Sephanoides sephaniodes. J. Exp. Biol. 206:3349 -3359.

McWhorter, T. J. and C. Martinez del Rio. 2000. Does gut function limit hummingbird food intake? Physiological and Biochemical Zoology 73:313-324.

Mumme, R. L. 2002. Scare tactics in a neotropical warbler: white tail feathers enhance flush-pursuit foraging performance in the Slate-throated Redstart (Myioborus miniatus). Auk 119:1024-1035.

Pauw, A. 1998. Pollen transfer on birds' tongues. Nature 394:731-732.

Stevens, C. E. and I. D. Hume. 1998. Contributions of Microbes in Vertebrate Gastrointestinal Tract to Production and Conservation of Nutrients. Physiological Reviews 78: 393-427.

Thouzeau, C., G. Peters, C. Le Bohec, and Y. Le Maho1. 2004. Adjustments of gastric pH, motility and temperature during long-term preservation of stomach contents in free-ranging incubating King Penguins. Journal of Experimental Biology 207: 2715-2724.

van Gils, J. A., T. Piersma, A. Dekinga, and M. W. Dietz. 2003. Cost-benefit analysis of mollusc-eating in a shorebird. II. Optimizing gizzard size in the face of seasonal demands. J. Exp. Biol. 206:3369 -3380.

Villard, P. and J. Cuisin. 2004. How do woodpeckers extract grubs with their tongues? A study of the Guadeloupe Woodpecker in the French West Indies. Auk 121:509-514.

Ward, A. B., P. D. Weigl, and R. M. Conroy. 2002. Functional morphology of raptor hindlimbs: implications for resource partitioning. Auk 119: 1052-1063.

Yanega, G. M., and M. A. Rubega. 2004. Hummingbird jaw bends to aid insect capture. Nature 428: 615.

More Lecture Notes:

    I - Introduction to Birds

    II - Bird Flight  I

   III - Bird Flight II

    IV - The Geography of Birds I

    V - The Geography of Birds II

    VII - Circulatory System

Back to BIO 554/754 syllabus