Bird Digestion
Source: http://numbat.murdoch.edu.au/Anatomy/avian/fig4.1.GIF
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:
 
Foraging Wilson's Petrels
Sea-anchor 'soaring' by storm-petrels -- As described by Pennycuick (2008), storm-petrels have a unique method for
picking up food from the water's surface. They glide into the wind with their body clear of the surface, but with their webbed feet
in      the water, helping to 'anchor' them. Aerodynamic (or air) drag on on wings (friction drag) and body (parasitic drag) push
them backward through the water so that the feet tend to slowly move backward through the water, and that creates forward-directed
hydrodynamic (or water) drag. When this water drag balances the aerodynamic (air) drag, the storm-petrels remain in place, suspended above
the water's surface at a height that allows them to pick small food items from the surface (Photo source: Wikipedia ).
Pale-billed Woodpecker
( Mayflower Bocawina National Par k, Belize ) 
The beak's outer shell is made of hexagonal keratin tiles cemented together with an organic glue and piled in several staggered layers.
The interior of the beak is rigid "foam" composed of bony fibers and drum-like membranes sandwiched between outer layers of keratin. The "foam" is covered with overlapping keratin tiles, each about 50 µm in diameter and 1 µm thick,  glued together to form sheets.The closed, air-filled spaces reduce overall weight without loss of rigidity.
Toucan bills: strong but light -- Toucans  live in the jungle canopies of South America and feed on tree fruits growing at the ends of branches. They perch on sturdier portions of a branch and use their long beak to reach their food. So, the beak must be rigid enough to resist bending and twisting forces, but has to be light  or the bird couldn't get off the ground. Incredibly, a toucan's beak that comprises about 1/3 of its body length, only makes up about 1/20 of a toucan's mass. The lightweight strength of the toucan's beak is due to a matrix of bony fibers and drum-like membranes sandwiched between an outer layer of keratin. Seki et al. (2005) found that the secret to the toucan beak's lightweight strength is an unusual bio-composite. The interior of the beak is rigid "foam" composed of bony fibers and drum-like membranes sandwiched between outer layers of keratin.
 
 
The capillary ratchet mechanism
Surface tension transport of prey by feeding shorebirds: the capillary ratchet -- Recent work by Prakash et al. (2008) has revealed that phalaropes and other shorebirds take advantage of surface interactions between their beak and water droplets to propel bits of food from the tip of their long beaks to their mouth. They peck at the surface, picking up droplets of water with  prey  inside. Because their beaks point downward when feeding, gravity must be overcome to get those droplets from the tip of the bird's long beak to its mouth. This feeding strategy depends on  surface tension. As the beak  opens and closes, each movement propels the water droplet one step closer to the bird's mouth. Specifically, when the beak closes, the drop's leading edge moves toward the mouth, while the trailing edge stays put.  In this stepwise ratcheting fashion, the drop travels along the beak at a speed of about 1 meter per second. The efficiency of the process, called the "capillary ratchet," depends on  beak shape, and long, narrow beaks, like those of phalaropes, are best suited to this mode of feeding. - MIT News
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  the upper palate that function as anvils so 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.
Serin (Serinus serinus) with a seed positioned in its bill. Note how the tongue is used to hold the seed in position
(From: van der Meij 2005).
© 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
Photo source: http://www.homepages.mcb.net/wormwell/Gannet130604.htm
The plunge dive of the Cape Gannet -- Gannets have one of the most spectacular prey-capture behaviors of all marine predators,
plummeting from up to 30 m into the water, where they seize fish with their razor-sharp beaks. Ropert-Coudert et al. (2004) monitored
the biomechanics of plunge diving in 25 free-ranging Cape Gannets Morus capensis using a rapid-sampling acceleration and depth
recorder. Their data provide the first detailed description of this highly specialized foraging technique. They recorded no or a very low
deceleration when Gannets entered the water, which underlines the remarkable streamlining of this large bird. Birds use their momentum
to travel underwater at an average descent rate of 2.87 m/s (sd = ±1.53) before actively braking once they attain the desired depth (range:
0.3–9.7 m). Ropert-Coudert et al. (2004) showed that Gannets sometimes used either their wings or feet for underwater propulsion during
the course of 9.4% of the dives that had undulations in their depth profiles. After chasing prey, birds developed an upward momentum before
gliding passively back to the surface, making use of their buoyancy to complete the dive at the lowest possible energy cost.
Tarsometatarsi of a similarly-sized hawk & owl. 
(A) Red-tailed Hawk. (B) Great Horned Owl. 
The relation between rate of success and direction of movement for a food item that was pulled forward (a), backward (b) and sideways (c). Direction of prey progression – dotted arrow (1), direction of owl flight – dashed arrow (2), and direction to which the owl had to move its head or trunk – solid arrow (3). Owl picture from Knudsen (2002).
Movement and direction of  prey affect raptor success rate -- Shifferman and Eilam (2004)  tested a novel idea, that rather than maximizing their distance from a predator during close-distance encounters, prey species are better off moving directly or diagonally toward the predator in order to increase the relative speed and confine the attack to a single available clashing point. They used two tamed Barn Owls (Tyto alba) to measure the rate of attack success in relation to the direction of prey movement. A dead mouse or chick was used to simulate the prey, pulled to various directions by means of a transparent string during the owl's attack. Both owls showed a high success rate in catching stationary compared with moving food items (90% and 21%, respectively). Success was higher when the prey moved directly away, rather than towards the owls (50% and 18%, respectively). Strikingly, these owls had 0% success in catching food items that were pulled sideways. This failure to catch prey that move sideways may reflect constraints in postural head movements in aerial raptors that cannot move the eyes but rather move the entire head in tracking prey. So far there is no evidence that defensive behavior in terrestrial prey species takes advantage of the above escape directions to lower rates of predator success. However, birds seem to adjust their defensive tactics in the vertical domain by taking-off at a steep angle, thus moving diagonally toward the direction of an approaching aerial predator. These preliminary findings warrant further studies in Barn Owls and other predators, in both field and laboratory settings, to uncover fine predator head movements during hunting, the corresponding defensive behavior of the prey, and the adaptive significance of these behaviors.
Barred Owl primary - leading edge below and trailing edge above
The serrated leading-edge feather of an owl
(Norberg 2002)       .
Vortex generators on an airplane wing.
The silent flight of owls -- Noise is generated by vortices produced when air flows over a bird's wing and larger vortices produce more noise. Wings with  small saw-toothed projections (vortex generators), like those on the leading edge of owl wings, generate many small vortices instead of large vortices and produces less aerodynamic noise. In addition, the fringe feathers at the trailing edge of the wing (with fewer hooklets at the ends of the barbs) help to break up the sound waves that are  generated as air flows over the top of their wings and forms downstream wakes, and the soft down feathers located elsewhere on the wings and legs of owls  absorb the remaining sound frequencies above 2,000 hertz and make owls completely  silent to their prey. As a bonus, with high angles of attack and at slow speeds, vortex generators stick out of the stagnant air near the surface of the wing, and into the freely moving air outside the boundary layer. This surface layer is typically quite thin, but dramatically reduces speed of the airflow towards the rear of the wing. The vortex generators mix the free stream with the stagnant air to get it moving again, providing considerably more airflow at the rear of the wing and helping to prevent stalling. This process is referred to as 're-energizing the boundary layer.'
 
 
Eurasian Sparrowhawk
Unpredictable predators -- The use of space by predators in relation to their prey is a poorly understood aspect of predator-prey interactions. Classic theory suggests that predators should focus their efforts on areas of abundant prey, that is, prey hotspots, whereas game-theoretical models of predator and prey movement suggest that the distribution of predators should match that of their prey's resources. If, however, prey are spatially anchored to one location and these prey have particularly strong antipredator responses that make them difficult to capture with frequent attacks, then predators may be forced to adopt alternative movement strategies to hunt behaviorally responsive prey. Roth and Lima (2007) examined the movement patterns of bird-eating Sharp-shinned Hawks (Accipiter striatus) in an attempt to shed light on hotspot use by predators. Their results suggest that these hawks do not focus on prey hotspots such as bird feeders but instead maintain much spatial and temporal unpredictability in their movements. Hawks seldom revisited the same area, and the few frequently used areas were revisited in a manner consistent with unpredictable returns, giving prey little additional information about risk.
But why wouldn't Sharp-shinned Hawks focus their hunting on the areas with the most potential prey (bird feeders)? One possibility is that behaviorally responsive prey diminish the "hotspot" quality of feeders. Although feeder hotspots are sources of abundant prey, the individuals at such feeders generally benefit from group vigilance as a result of these higher densities. As a result, the vulnerability of the prey may actually be lower at feeders than at other locations. In addition, unpredictable movement may reflect a sort of "prey management" by predators, whereby predators spread their hunting activity over multiple areas in an effort to avoid inflating the antipredator behavior of their prey. This hunting strategy may be effective when prey are anchored to high-resource areas such as feeders and use antipredator behaviors, such as high vigilance, that reduce a predator's attack success if it attacks frequently and predictably.
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 may be used as sensory organs to help locate and capture prey, much like mammals use whiskers.  In addition, bristles around the mouth may help protect the eyes from food items a bird is trying to capture (Conover and Miller 1980). The photo to the right shows the rictal bristles of a Hooded Warbler.
Woodpeckers - the tongue     is long, extensible, & 'barbed' at the tip to facilitate the     capture     and extraction of prey (like insect larvae) from bark crevices (check     this skull     of a Golden-fronted Woodpecker ).
Source: Birder's World
Detailed view of the horny tip (left) of the Guadeloupe Woodpecker tongue in vivo position (Villard and Cuisin 2004).
(a) Skull of Great Spotted Woodpecker , (b) hyoid apparatus of a Great Spotted Woodpecker, and
(c) hyoid apparatus of a non-woodpecker ( Eurasian Hoopoe ) (From: Wang et al. 2011 ).  
 
 
     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).
Fig. 1. Dorsal view of the tongue of the Spotted Nutcracker (Nucifraga caryocatactes) . Arrows show two elongated processes of the apex. A, apex, B, body, R, root, LP, laryngeal prominence. Scale bar, 3 mm.
Fig. 2. Lateral view of the tongue of the nutcracker. Arrow shows elongated processes, pointed diagonally, B, body, R, root. Scale bar, 3 mm.
Nutcracker tongue -- Jackowiak et al. (2010) found that a characteristic feature of the Spotted Nutcracker's tongue is the unique structure of the anterior part of the tongue, which has two long and highly keratinized processes — a product of the renewable keratinized layer of the epithelium covering the ventral surface of the tongue. These dagger-like processes, which are a modified “lingual nail,” help nutcrackers  lever up and shell seeds, which are transported over the short sulcus-shaped body of the tongue. Another unique feature of the nutcracker's tongue is the groove separating the body from the root. Two rows of highly keratinized, mechanical, conical papillae are located at the junction of the body and the root. These papillae are mechanically protective elements for passing food particles in the form of seeds. The nutcracker tongue is an efficient tool resembling a lever that is helpful in shelling seeds.
Nectar feeders (hummingbirds, sunbirds,       spiderhunters, & honeycreepers) - tongues may have brushy tips that collect nectar and permit the bird to essentially lap it up (more detail provided below).
Source: http://www.zoobooks.com/newFrontPage/animals/virtualZoo/animals/h/humingbirds/head2.htm
 
Hummingbird tongues are fluid traps, not capillary tubes -- Hummingbird tongues pick up a liquid, calorie-dense food that cannot be grasped, a physical challenge that has long inspired the study of nectar-transport mechanics. Existing biophysical models predict optimal hummingbird foraging on the basis of equations that assume that fluid rises through the tongue in the same way as through capillary tubes. Rico-Guevara and Rubega (2011) found that  hummingbird tongues do not function like a pair of tiny, static tubes drawing up floral nectar via capillary action. Instead,  the tongue tip is a dynamic liquid-trapping device that dynamically traps nectar by rapidly changing their shape during feeding. High-speed video observations showed that an entire tongue transformation cycle occurs in as little as 1/20th of a second. In addition, the tongue–fluid interactions are identical in both living and dead birds, demonstrating that this mechanism is a function of the tongue structure itself, and therefore highly efficient because no energy expenditure by the bird is required to drive the opening and closing of the trap. These results rule out previous conclusions from capillarity-based models of nectar feeding and highlight the necessity of developing a new biophysical model for nectar intake in hummingbirds.
Links:
How the hummingbird tongue really works (with videos)
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. 
 
American Redstart
Close encounters with possible prey . . .  counterfeit predators -- You are a 12-gram, insectivorous, tropical rainforest bird, foraging in shady, tangled, dappled, rustling foliage where edible caterpillars and other insects are likely to shelter. You want to live 10–20 years. You are peering under leaves, poking into rolled ones, searching around stems, exploring bark crevices and other insect hiding places. Abruptly an eye appears, 1–5 cm from your bill. The eye or a portion of it is half seen, obstructed, shadowed, partly out of focus, more or less round, multicolored, and perhaps moving. If you pause a millisecond to ask whether that eye belongs to acceptable prey or to a predator, you are likely to be—and it takes only once—someone’s breakfast. Your innate reaction to the eye must be instant flight, that is, a “startle” coupled with distancing. The bird that must learn to avoid what appears to be a predator’s eye is not long for this world. Now, a safe few meters away, are you going to go back to see whether that was food? No.
You, like billions of other individuals and hundreds of other species for millions of years, have just been a player in an act of natural selection favoring mutations that lead to the multitudes of “false eye” color patterns, “eyespot” patterns, or “facsimiles of eyes” and “faces” adorning tropical caterpillars and pupae.  Associated body patterns often suggest other head and facial features, which in turn enhance the eye-like nature of the spots. None of these patterns exactly matches the eyes or face of any particular species of predator; but, even when quickly and partially glimpsed, all give the illusion of an eye or face. These false eyes are mimicking the eyes and faces of such predators of insect-eating birds as snakes, lizards, other birds, and small mammals, as perceived at close range by the insectivorous birds in their natural world. These color patterns, and the birds’ reactions to them, are the evolutionary footprints of predator/prey encounters as shallow as today and as deep as the first terrestrial vertebrate eyes - From: Janzen et al. (2010) .
Representative  caterpillar false eyes and faces
Buccal, or oral, cavity:
contains few mucous glands & taste buds
salivary glands are well-developed in many birds. In some, like woodpeckers, the 'sticky' saliva aids in capturing prey. In others, like swifts, saliva is used in nest building (see photo below).
salivary glands are reduced in aquatic species (because aquatic prey like fish require little additional lubrication to be easily swallowed).
Source: http://animal.discovery.com/fansites/jeffcorwin/carnival/flyingace/swiftlet.html
Esophagus:
tube that connects the oral cavity and the stomach. The muscular walls of the esophagus produce wave-like contractions ( peristalsis ) that help propel food from the oral cavity to the stomach.
large in diameter compared to other vertebrates, especially in birds that swallow large prey, e.g., cormorants, herons, & raptors
Anhinga swallowing a large fish
may serve for temporary storage of food:
temporary distension - fish-eating species, birds of prey (see kestrel video below), & some   fruit   eaters (like Cedar Waxwings)
Cedar Waxwings
crop
out-pocketing of the esophagus that's particularly well-developed in       seed-eaters       like pigeons & doves (Columbiformes) and gallinaceous birds (grouse       and pheasants)
specialized for production of 'milk' that pigeons & doves feed to       their       young. Crop       'milk' is rich in proteins, fats, & vitamins and is produced by       proliferation & sloughing off of epithelial cells that line the       crop.
 
Source: http://www.vetmed.ucdavis.edu/whc/Necropsy/Avian.html
The non-‘lactating’ pigeon crop (A) has a completely different appearance from that of the ‘lactating’ crop (B). The lactating crop is
more than     twice the size of the non- ‘lactating’ crop, with a thickened wall and two very obvious lateral lobes. When the ‘lactating’ crop is
opened (C) the     pigeon ‘milk’ is seen as a bed of close-packed discrete  rice-shaped pellets that are closely associated with the mucosal surface
of the  tissue (From: Gillespie et al. 2011 ).
Crop 'milk' -- Both male and female Rock Pigeons (Columba livia) produce  pigeon ‘milk’ that is fed to their young. Pigeons generally lay two eggs one day apart, which hatch 18 days after they    are laid. Two days before the first egg hatches, pigeon ‘milk’ begins to be    produced in the crop of the parent birds. A similar substance is produced by flamingos  and male Emperor Penguins. The normal function    of the crop is food storage. During the process of pigeon ‘lactation’, a curdlike    substance is regurgitated from the crop and fed to young pigeons. This 'milk’ is  60% protein and the remainder is mostly    fat (32-36%) with a small amount of carbohydrate (1-3%), in addition to the mineral  (calcium, potassium, sodium, and phosphorus) content. Pigeon 'milk' also contains IgA antibodies and antioxidants (carotenoids).
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 
Burrowing Owl
The avian stomach is divided into 2 parts :
Proventriculus -  also called the glandular stomach; receives food from the esophagus & secretes mucus, HCl, and pepsinogen. HCL and pepsinogen are secreted by the deep glands (see photomicrograph below). Pepsinogen is converted into pepsin (a proteolytic, or protein-digesting, enzyme) by the HCl.
Photomicrograph (50X) of a cross section through the proventriculus showing folds of mucous membrane (P);
deep proventricular glands (GP); capsule (connective tissue) around the glands (arrow head); muscle layer (m); serosa
(connective tissue) with blood vessels (S), and the lumen (L) (From: Catroxo et al. 1997).
Ventriculus   (or gizzard):
the avian equivalent of teeth
very muscular (but less so in birds that eat meat, insects, nectar, and   other 'soft' foods)
used primarily to grind & break-up food (such as seeds)
may, in seed-eating birds, contain grit (small stones ingested by the   birds   to help grind the food)
lined with a tough, abrasive keratin-like layer of koilin, known as the   cutica gastrica (or cuticle; see photomicrograph below). The cuticle is secreted by simple tubular glands (see photomicrograph below). Grinding action may, particularly in seed-eating   birds,   be assisted by grit and stones deliberately ingested.
Source: http://members.madasafish.com/~grahamwhite/download/gizzard.htm
The avian gastrointestinal tract, unlike that of mammals, executes distinct reverse peristaltic movements that are critical to optimal digestive function (Duke 1994). The gastric reflux allows material in the gizzard to reenter the proventriculus for additional treatment with acid and pepsin.
Ostrich (Struthio camelus) stomach. Note how particle size of material in the gizzard (ventriculus) is smaller than in the proventriculus due to the grinding action of the muscular walls plus small pebbles (gastroliths). The capacity to reduce particle size is related to the metabolic demands of a species. Therefore, particle size reduction is often considered the key digestive difference between ecto- and endotherms that allows endotherms to rely on shorter digesta retention times without losing digestive efficiency, and hence facilitate the high level of food intake necessary to meet their increased metabolic requirements. Oes, esophagus; Prov, proventriculus; Gizz, gizzard; SI, small intestine (From: Fritz et al. 2011).
Gizzards vs. teeth -- A gizzard system is  advantageous when the  organismal design demands that the site of particle size reduction be close to the center of gravity; as such, the use of a gizzard is usually linked to the primary characteristic of herbivorous birds, flight. In contrast, adaptations for chewing intrinsically increase the weight of the head. The use of the gizzard system has the potential advantages that intake rate is not limited by chewing, that no investment in dental tissue is necessary, and that dental wear is not a determinant of senescence as observed in mammals. The absence of age-dependent tooth wear might even be a contributing factor to the slower onset of senescence in birds as compared to mammals. On the other hand, the use of a gizzard requires the intake of suitable grit or stones—an action that represents, in the few studies where this has actually been quantified in birds, a relevant proportion of feeding time ( Fritz et al. 2011 ).
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.
Intestine:
chief organ of digestion & absorption
receives bile from the liver and pancreatic juice from the pancreas
divided into a small intestine & a large intestine
Small     intestine:
short & slightly coiled in meat-eating birds (e.g., raptors) but longer & highly coiled in herbivores (e.g., seed eaters) and omnivores (click on the figure for a larger version)
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 Hoatzins , Emus, and Ostriches, an expanded foregut
(Hoatzins), a much longer midgut (Emus), or a much longer   colon (Ostriches) 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)
coprodaeum - receives waste from the large intestine
urodaeum - receives urine from the kidneys (via the ureters) and sperm & eggs from the gonads
proctodaeum - stores (temporarily) and ejects material; closed posteriorly by the muscular anus
the bursa of Fabricius is located on the dorsal wall. The bursa is most prominent in young birds and serves as the area where B-lymphocytes (the white blood cells that produce antibodies) are generated (T-lymphocytes are generated in the Thymus). Once produced, the B-lymphocytes migrate to lymphoid tissue in other parts of the body & the bursa of Fabricius atrophies.
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. 
Position of model penguin during defaecation and physical parameters used to
calculate rectal pressure necessary to expel fecal material over a distance of 40 cm.
Pressures produced when penguins pooh -- Brooding Chinstrap (Pygoscelis antarctica ) and Adélie (P. adeliae) penguins do not leave their stony nest to defecate, but move to the edge of it, stand up, turn their back nest-outward, bend forward, lift their tail, and shoot The expelled material hits the ground an average of 40 cm away from the bird.  Meyer-Rochow and Gal (2004) determined that the pressures involved could be approximated if they knew the (1) distance the feces traveled, (2) density and viscosity of the material, and (3) shape, aperture, and height of the anus above ground. They determined these parameters and calculated that penguins generated pressures of around 10 kPa (77 mm Hg) to expel watery material and 60 kPa (450 mm Hg) to expel material of higher viscosity similar to that of olive oil (forces  well above those known for humans). How penguins choose the direction of defecation, and how wind direction factors into that decision, remain unknown.
Accessory organs:
liver - produces bile that is transported to the small intestine via the bile duct. Bile emulsifies fats (or, in other words, breaks fats down into tiny particles). Emulsification is important because it physically breaks down fats into particles than can then be more easily digested by enzymes (lipase produced by intestinal cells and the pancreas).
pancreas - produces pancreatic juice that is transported to the small intestine. This 'juice' contains a bicarbonate solution that helps neutralize the acids coming into the intestine from the stomach plus a variety of digestive enzymes. The enzymes help break down fats, proteins, and carbohydrates. The pancreas also produces the hormones insulin and glucagon which regulate blood sugar levels (cells that produce these two hormones make up the 'islets of Langerhans', one of which is represented by the light-colored, circular structure in the photomicrograph below).
Pigeon liver and pancreas (& other organs)
The Avian Digestive Tract
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