Food & Feeding Habits
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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, carrion, 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)
Muscles involved in jaw closure for a Cooper’s Hawk. A. Lateral view showing adductor mandibulae group, including
adductor mandibulae externus pars profunda (AMEP), adductor mandibulae ossis quadrati (AMOQ), adductor mandibulae
externus pars rostralis (AMER), adductor mandibulae pars ventralis (AMEV). B. Antero-lateral view showing the
pseudotemporalis group, including adductor mandibulae caudalis (AMC), pseudotemporalis profundus (PSP),
pseuodotemporalis superficialis (PSS), pterygoideus pars dorsalis (PTD), and pterygoideus pars ventralis (PTV)
( Sustaita 2007).
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).
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:
A kingfisher using its bill to capture prey!
Great Egret (slow motion)
Golden Eagle vs. a goat
Red-tailed Hawk eating a red squirrel
The Bird That Walks On Water – video powered by Metacafe
Foraging Wilson’s Petrels
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.
(Check the Gannet videos at ARKive).
Aerial insectivores — Swifts depend on flying efficiently and maintaining high speed. Hawking insectivores, like flycatchers, depend on perches
located near prey, but they must be able to accelerate rapidly and be very maneuverable. Swallows combine these two strategies; they are fast, maneuverable
and able to accelerate when necessary (Warrick 1998).
Feet and talons
Although 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 (check this short video of an owl trying to eat a moth).
Talons of (left > right): Harpy Eagle, Golden Eagle, Bald Eagle,
Great Horned Owl, Red-tailed Hawk, & Peregrine Falcon
(Short commercial before the video, sorry about that)
The raptor digital tendon locking mechanism — Digital tendons form a mechanical-locking mechanism in many birds that must maintain a degree of grip force, including perching, hanging, tree-climbing, and raptorial species. In raptors, powerful hindlimb muscles produce a strong grasp, and a tendon locking mechanism (TLM) helps sustain grip force. The components of the digital TLM include a ‘textured’ pad on the ventral surface of each flexor tendon that contains thousands of minute, rigid, well-defined projections called tubercles (see figure below). The neighboring portion of the surrounding tendon sheath contains a series of transversely running plicae (folds) that often have a proximal slant (i.e. towards the base of the toe). When the flexor tendons are pulled taut, and the digits flexed, the tubercle pad moves proximally over the stationary plicae on the sheath. When resistance to digital flexion is met, the locking elements intermesh and engage and the friction produced prevents slippage of the tendons. This permits digital flexion to be maintained with little or no muscular involvement (Einoder and Richardson 2006).
Action of the avian digital TLM: (1) digital extension, (2) digital flexion. This shows the movement of the talon (a), flexor (e) and extensor (d) tendons,
ungual phalanx (b), and the movement of the ventrally located tubercle pad (f) relative to the stationary plicated sheath (g) and phalangeal bone (c)
(From: Einoder and Richardson 2006).
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) .
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.’
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.(Source: www.westol.com/~banding/Pictorial_Highlights_080511_2002.htm)|
The avian tongue:
- can aid in gathering and swallowing food
- usually not very muscular but reinforced by the hyoid apparatus
- morphology of the avian tongue varies with food habits:
- Fish-eating species – typically have small, undifferentiated tongue because fish are often swallowed whole
- 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).
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:
- 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).
- 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)
- 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.
Note the distended throat of this American Kestrel
|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:
- 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.
Photomicrograph (210X) of longitudinal section of the gizzard showing folds of mucous membrane lined by simple
prismatic epithelium (P); simple tubular glands (Gs) in the lamina propria constituted by connective tissue (Lp);
secretion of glands (S) that are continuous with the cuticle (or koilin); (C), part of muscle layer (m), interpersed with bundles of
connective tissue (Tc) (From: Catroxo et al. 1997).
Photomicrograph (400X) of the koilin of an Eclectus Parrot (Eclectus roratus).
Note the regular, columnated structure of the koilin layer (K) and its
association with the glandular epithelium (E) of the ventriculus (From: De Voe et al. 2003).
(1) Section through inner lining of a chicken gizzard. A, koilin, B, crypts, C, glands that secrete koilin, D, epithelial surface, E, desquamated epithelial cells,
(2) Mucosa of the gizzard. A, koilin, B, secretion in gland lumens and crypts, and (3) Koilin layer. A, secretion column, B, koilin-layer surface,
C, horizontal stripe indicating a ‘pause’ in secretion of the koilin, D, cellular debris. (From: Eglitis and Knouff 1962).
|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).
Reversible size changes in the gizzards of adult Japanese Quail (Coturnix japonica) (a) and Red Knots (Calidris canutus) (b).
Quail were given a diet of alternatively low or high non-digestible fiber content (3% vs. 45%). Within 14 days, they showed a doubling
of the size of their gizzards. Red Knots have strong muscular gizzards for feeding on molluscs. With a change in diet from medium-small
mussels (Mytilus edulis) to a diet of soft food pellets, gizzard mass was reduced by almost 50% in about 8 days. A shift back to a mussel diet
induced about a doubling in gizzard mass in just a few days. As the knots were fed progessively smaller mussels (day 22 to day 46) that are easier
to crush, gizzard mass again declined. A switch back to a soft food pellet diet caused a further decline in gizzard mass. Finally, a switch back to a
mussel diet again cause a rapid increase in gizzard mass (From: Piersma and Drent 2003).
- 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.
|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.|
- chief organ of digestion & absorption
- receives bile from the liver and pancreatic juice from the pancreas
- divided into a small intestine & a large 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 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).|
- lined with numerous structures called villi (pictured below). Villi are projections from the intestinal wall that increase the amount of surface area available for absorption. Further increasing the surface area are the numerous microvilli of the cells lining the surface of the villi. Inside each villus are blood vessels that absorb nutrients for transport throughout the body.
Cross-section of the intestine (ileum) of a Spotted Tinamou (Nothura maculosa).
Villi are lined with columnar epithelium (EP), including goblet cells (arrows) that secrete mucus. The muscle
layer includes longitudinal fibers (MI) on the perimeter, circular fibers (Mc), and additional longitudinal fibers
at the base of the villi (muscularis muscosae; MM) (From: Chikilian and de Speroni 1996).
Intestinal microvilli (‘brush border’) of a (A) House Sparrow and (B) Savannah Sparrow. Scale bar = 0.5 µm
(From: Casotti 2001).
- relatively short
- primary function is to absorb water and electrolytes
- contains a minimum of non-digestible waste (to help keep birds as light as possible) because:
- has out-pocketings called caeca. Caeca are histologically similar to the small and large intestines and found in a wide variety of birds. They are best developed in some waterfowl, gallinaceous birds (like chickens & grouse), and ostriches. In these large ceca, food particles are acted upon by cecal secretions, bacteria, and fungi and nutrients can be absorbed. Some birds (e.g., Passeriformes, Falconiformes, Ciconiiformes, and Pelecaniformes) have very small ceca (see below) called lymphoid ceca. Lymphoid ceca are not important in digestion but contain lymphocytes (white blood cells) that produce antibodies (Clench 1999).
Avian ceca. (A) Little Cormorant, Phalacrocorax niger, (B) Cattle Egret, Bubulcus ibis, (C) Cotton Teal, Nettapus coromandelianus,
(D) Crested Serpent Eagle, Spilornis cheela, (E) Common Quail, Coturnix coturnix, (F) Indian Ring Dove, Streptopelia decaocto,
(G) Red-wattled Lapwing, Vanellus indicus, (H) Koel, Eudynamys scolopacea, (I) Spotted Owlet, Athene brama, (J) Indian Roller, Coracias benghalensis,
(K) Eastern Skylark, Alauda gulgula, & (L) Grey Wagtail, Motacilla caspica (From: Clench 1999).
At various times and under various conditions, ceca are the site for (1) fermentation and further digestion of food (especially for the breakdown of cellulose) and absorption of nutrients, (2) production of antibodies, and (3) the use and absorption of water and nitrogenous components (Clench 1999).
|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.||
Scarlet Macaws at a clay lick near the Manu River, Peru
(not a great video; some talking in background, but you can see macaws feeding on the clay lick)
- receives waste from the large intestine & materials from the urinary & reproductive systems
- divided into three sections:
- 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.
- 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)
Avian Pancreas tissue
Respirasi organisme autotrof VS heterotrof
1. Phylum Protozoa
Protozoa tidak memiliki alat pernafasan.
Pengambilan oksigen dilakukan secara difusi
melalui permukaan tubuhnya. Oksigen masuk ke
dalam mitokondria dan terjadilah proses
Oksigen dan karbon dioksida keluar-masuk
melalui membran sel secara difusi. Oksigen dan
karbon dioksida tersebut merupakan gas-gas yang
terlarut di dalam air. Contoh: Amoeba sp.
2. Phylum Porifera
Hewan phylum ini tubuhnya tersusun atas
banyak sel dan memiliki jaringan yang
Udara pernapasan dipertukarkan langsung
oleh sel-sel di permukaan tubuh atau oleh
sel-sel leher yang bersentuhan dengan air.
Contoh: Spongia sp.
3. Phylum Coelenterata
Hewan phylum coelenterata tubuhnya tersusun
atas banyak sel dan memiliki jaringan. Hewan ini
tidak memiliki alat pernapasan yang lengkap.
Alat bantu pernapasan berupa lekukan-lekukan
lapisan gastrodermal yang berada sedikit di bawah
mulut, yang disebut sifonoglifa. Namun sel-sel di
permukaan tubuh yang lain juga dapat melakukan
pertukaran gas dengan lingkungannya.
Contoh: Aurelia aurita, Hydra sp., dan Metrium sp.
Golongan cacing (Vermes) terbagi dalam tiga
Pada cacing pipih (Platyhelminthes) pernapasan
terjadi di seluruh permukaan tubuh melalui
difusi. Contoh: Planaria sp.
Pada cacing gilik tidak bersegmen
(Nemathelminthes) pernapasannya juga melalui
difusi lewat permukaan tubuhnya. Contoh:
Pada cacing gilik bersegmen (Annelida)
pernapasannya melalui permukaan kulit yang
selalu basah oleh cairan mukus. Contoh:
Protozoa, Porifera, Coelenterata,
dan cacing (Vermes)==>tidak memiliki alat pernapasan khusus, atau
alat pernapasannya tidak tampak jelas
Sistem respirasi berupa sistem
pernafasan trakea. Alat pernafasan
berupa pembuluh trakea. Udara
keluar masuk melalui spirakel
yang terdapat pada setiap sisi ruas
Aliran udara pernafasan : oksigen masuk melalui spirakel
menuju trakea. Selanjutnya menuju trakeolus dan terjadi
pertukaran gas dengan sel tubuh.
Mekanisme pernafasan : bila otot perut berkontraksi,
trakea memipih sehingga udara kaya CO2 dari dalam
tubuh keluar. Bila otot perut relaksasi, trakea ke posisi
semula dan udara luar kaya O2 akan masuk melalui
Kalajengking atau laba-laba (Arachnida) alat
pernafasannya berupa paru-paru buku (darat) atau
insang buku (air). Udara keluar masuk melalui
spirakel yang disebabkan oleh gerakan otot yang
6. Ikan (pisces)
Alat pernafasan berupa insang (branchia).
Tiap lembaran insang terdiri dari sepasang
filamen yang banyak mengandung lamela
(lapisan tipis). Pada filamen terdapat
pembuluh darah yang mengandung kapiler
sehingga memungkinkan terjadinya
pertukaran gas O2 dan CO2.
Inspirasi : O2 dari air masuk ke dalam insang
yang kemudian diikat oleh kapiler darah
untuk dibawa ke jaringan tubuh.
Ekspirasi : CO2 dari jaringan bersama darah menuju ke
insang dan selanjutnya dikeluarkan dari tubuh.
Ikan yang hidup di tempat berlumpur mempunyai labirin
yang merupakan perluasan insang berbentuk lipatan
berongga tidak teratur. Labirin berfungsi untuk menyimpan
cadangan oksigen sehingga ikan tahan pada kondisi
kekurangan oksigen. Misal pada ikan lele dan ikan gabus.
7. Katak (amphibia)
Katak dalam daur hidupnya mengalami metamorfosis atau
perubahan bentuk. Pada waktu muda berupa berudu dan
setelah dewasa hidup di darat.
Mula-mula berudu bernapas dengan insang luar yang
terdapat di bagian belakang kepala. Insang tersebut selalu
bergetar yang mengakibatkan air di sekitar insang selalu
berganti. Oksigen yang terlarut dalam air berdifusi di dalam
pembuluh kapiler darah yang terdapat dalam insang.
Setelah beberapa waktu insang luar ini akan berubah
menjadi insang dalam dengan cara terbentuknya lipatan
kulit dari arah depan ke belakang sehingga menutupi
Alat pernafasan berupa selaput rongga mulut, kulit dan paru-paru.
Alat pernafasan ini mempunyai lapisan tipis dan basah yang
berdekatan dengan pembuluh darah sehingga oksigen dapat
Selaput rongga mulut : bila faring rongga mulut bergerak, lubang
hidung terbuka dan glotis tertutup sehingga udara masuk rongga
mulut melalui selaput rongga mulut yang tipis.
Kulit : oksigen masuk kulit melewati vena kulit (vena kutanea)
kemudian ke jantun gdan selanjutnya diedarkan ke seluruh tubuh.
CO2 dari jaringan dibawa ke jantung dan selanjutnya ke kulit dan
paru-paru melalui arteri kulit paru-paru (arteri pulmo kutenea).
Paru-paru : terdapat sepasang paru-paru berbentuk gelembung
tempat bermuara kapiler darah. Katak tidak memiliki tulang rusuk
dan diafragma, sehingga mekanisme pernafasan diatur oleh otot
rahang bawah dan otot perut. Inspirasi maupun ekspirasi
berlangsung pada saat mulut tertutup.
9. Burung (Aves)
Bernafas dengan paru-paru yang berhubungan
dengan kantong udara (sakus pneumatikus) yang
menyebar sampai ke leher, perut dan sayap.
Kantong udara terdapat pada :
– pangkal leher (servikal)
– ruang dada bagian depan (toraks anterior)
– antar tulang selangka (korakoid)
– ruang dada bagian belakang (toraks posterior)
– rongga perut (saccus abdominalis)
– ketiak (saccus axillaris)
Fungsi kantong udara :
– membantu pernafasan terutama saat
– menyimpan cadangan udara (oksigen)
– memperbesar atau memperkecil berat
jenis pada saat burung berenang
– mencegah hilangnya panas tubuh yang
Inspirasi : udara kaya oksigen masuk ke paru-paru. Otot
antara tulang rusuk (interkosta) berkontraksi sehingga
tulang rusuk bergerak ke luar dan tulang dada
membesar. Akibatnya teklanan udara dada menjadi kecil
sehingga udara luar yang kaya oksigen akan masuk.
Udara yang masuk sebagian kecil menuju ke paru-paru
dan sebagian besar menuju ke kantong udara sebagai
Ekspirasi : otot interkosta relaksasi sehingga tulang
rusuk dan tulang dada ke posisi semula. Akibatnya
rongga dada mengecil dan tekanannya menjadi lebih
besar dari pada tekanan udara luar. Ini menyebabkan
udara dari paru-paru yang kaya karbondioksida ke luar.
Pernafasan burung saat terbang :
Saat terbang pergerakan aktif dari rongga dada
tidak dapat dilakukan karena tulang dada dan
tulang rusuk merupakan pangkal perlekatan otot
yang berfungsi untuk terbang. Saat
mengepakan sayap (sayap diangkat ke atas),
kantong udara di antara tulang korakoid terjepit
sehingga udara kaya oksigen pada bagian itu
masuk ke paru-paru.
10. Sistem Pernafasan Manusia
Alat pernafasan manusia terdiri atas bagian-bagian sebagai
– Rongga hidung (cavum nasalis) : di dalamnya udara
dibersihkan oleh rambut-rambut dan dihangatkan.
– Faring : dibawahnya terdapat pangkal tenggorok yang disebut
laring yang di dalamnya terdapat selaput suara.
– Trakea (batang tenggorok).
– Bronkus (cabang dari batang tenggorok).
– Bronkiolus (cabang dari bronkus) : bercabang lagi sampai
halus, dengan dinding semakin tipis dan pada brokiolus ini
cincin tulang rawan tidak terdapat lagi.
– Alveolus : dinding tipis, elastis, terdiri dari satu lapis,
mempunyai banyak pembuluh kapiler dan merupakan tempat
terjadinya pertukaran O2 dan CO2 .
– Paru-paru (pulmo).
Proses inspirasi dan ekspirasi diatur oleh otot diafragma dan otot
antar tulang rusuk (intercostalis).
a. Pernafasan dada :
Jika otot antara tulang rusuk berkontraksi maka tulang rusuk
terangkat sehingga volume rongga dada membesar. Akibatnya
tekanan udara di paru-paru mengecil sehingga udara luar
mempunyai tekanan lebih besar masuk ke dalam paru-paru,
maka terjadilah inspirasi.
Bila otot antar tulang rusuk relaksasi maka tulang rusuk
tertekan sehingga rongga dada mengecil. Akibatnya tekanan
udara di paru-paru membesar sehingga udara keluar, maka
b. Pernafasan perut :
Diafragma berkontraksi sehingga mendatar maka rongga dada
membesar. Keadaan ini menyebabkan tekanan udara di paruparu
mengecil sehingga udara luar masuk dan terjadilah
Bila otot diafragma relaksasi maka rongga dada mengecil,
akibatnya tekanan di paru-paru membesar sehingga udara
keluar maka terjadilah ekspirasi.