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Avian digestion

I. Introduction A. Shapes II. Taxonomy of food habits B. Grinding Function A. Herbivores C. Crop Milk B. Carnivores D. Other Functions A wonderful bird is the pelican, III. Mouth and Pharynx V. Stomach His bill will hold more than his belly can. A. Bill A. Proventriculus He can take in his beak B. Palate B. Ventriculus Food enough for a week, C. Tongue VI. Small Intestine But I'm damned if I see how the hell he D. Salivary Glands A. Duodenum can. IV. Esophagus and Crop B. Function A. Storage --Dixon Lanier Merritt (1879-?) The Pelican (1910)

I. Introduction

Digestive system needs to be

„ as light as possible

High metabolic rates require large „ extremely efficient amounts of fuel

„ warbler might eat 80 percent of its body weight in a day!

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Loss of teeth and minimum jaw musculature

Problem for birds – need to keep low body weight Thus, little fat storage need to locate, ingest, digest food as quickly and efficiently as possible

Stellar’s Sea Eagle

Major components of avian digestive system II. Taxonomy of food habits

• oral cavity Many birds are generalists but many are also specialists • pharynx • esophagus (+ crop) Specializations are evident through the entire alimentary • stomach (proventriculus, canal. ventriculus) • small intestine • large intestine • cloaca

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As a group birds consume about any kind of food A. Herbivores

„ ants „ nectar Granivores „ buds „ pollen Frugivores „ crustaceans „ roots Nectivores „ fish „ sap Folivores „ fruit „ snails

„ grass „ wax

„ insects „ etc.

„ leaves

B. Carnivores III. Mouth and Pharynx

Raptors A. Bill Insectivores Function – seize, kill, prepare food for swallowing Piscivores Mollusc-eaters Correlation between food and food handling machinery is often obvious

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A. Bill

Heron - Stabbing

American Avocet - Forceps

Blue-winged Teal – Filter

Bill size can be important for “niche partitioning” A. Bill

Structure – bony framework covered by tough layer of keratin (sheath) - rhamphotheca

rhamphotheca continuously replaced as worn

Bone

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edges of the bill are especially hard and Bill shape sharp and are called 'tomia', singular typically doesn’t 'tomium' vary between sexes

B. Palate

Rictal bristles – sensory function, Mouth roofed with hard palate prey capture Floor membranous choanal cleft no soft palate salivary glands taste buds

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C. Tongue

Typically small, covered with a cornified epithelium, sharply pointed

Parrots – very muscular •barbed Most species – few intrinsic muscles

Poorly supplied with taste buds

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Tactile corpuscles widely • tubular distributed on tongues of •brushy woodpeckers and finches

Parrots - high # on bill

D. Salivary Glands

Piscivores – few Serous – produce enzymes Granivores – dense Mucous – produce polysaccharides Woodpeckers

Density of glands depends on diet Usually present in pharynx Also secrete amylase

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Swifts and Swallows Edible Swiftlet

Hong Kong largest consumer of bird nests, http://news importing about a hundred tons every year, at a Many use dried saliva to cement their nests price of about U.S. $25 million.

Suppliers are Indonesia, Malaysia, Myanmar, Singapore, southern parts of India, Sri Lanka, Thailand, and Vietnam.

IV. Esophagus and Crop

Salivary glands enlarge ~ 50x Esophagus – tube connecting oral cavity and stomach during nest-building • produces peristalsis • large in diameter (relative to other verts) Chimney Swift ~ 12x • usually provided with mucous glands and somewhat muscular Gray Jay – secrete sticky saliva • lined with heavy epithelium for caching food

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Large structure for food storage –Crop A. Storage

Food can be stored ~ 1 day+ Seed-eating finches - side of esophagus has paired out - pockets for storage (swallowed or regurgitated for nestlings) If amylase in saliva some digestion occurs in crop Geese pack full of food

B. Shapes

C. Grinding Function

Variable in shape across taxonomic groups Hoatzin – crop serves as grinding organ

Muscular, numerous glands, contains bacteria Fusiform – Trochlidae, Emberizidae Bilobed – Columbiformes Single Sac – Falconiformes, House Sparrows

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D. Crop Milk

Estrogen Æ Growth of lining Æ Prolactin Æ Shedding Provides food for young

25-35% lipid Stratified epithelial lining proliferates under effect estrogen 10-15% protein 70% water

E. Other Functions

Greater Prairie Chicken – anterior end modified Mourning Dove – as resonating chamber • proliferation of lining begins after 8 days incubation

• sloughing 14 – 18 days Air forced into sac from trachea via pharynx • continues to 16 days after hatching

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Flamingoes and male Emperor Penguins Male E. Penguins incubate alone and Oily esophageal secretion females don’t return to nest ~ 7 days after hatching

59% protein 28% lipid Male has to provide some food

Chicks double body weight on this alone!!!

V. S t o m a c h

Birds possess two kinds of stomachs: Anterior glandular stomach Posterior muscular stomach

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A. Proventriculus

Spindle-shaped glandular stomach – avian innovation Initial site of protein digestion Not found in reptiles • HCl (pH 0.2 – 1.2) •proteases Size related to bulk of food intake • chitinases Large – piscivores, frugivores, granivores

B. Ventriculus

Strong muscular organ (granivores) Fruit eaters – glands secrete mucous only, ventriculus has Prepares food for digestion (analog of teeth) appearance of proventriculus

Mucous glands present in walls – secrete koilin lining

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Functions as principle barrier to indigestible material • In granivores may contain grit •feathers • lined with tough, abrasive, keratin layer (koilin) – cutica gastrica •hair •bone • teeth

Seasonal change in in structure of stomachs corresponding to seasonal change in diet avian gastrointestinal tract - distinct reverse peristaltic movements (Duke Bearded Tits 1994). Winter diet – seeds, stomach 0.88 – 1.2g allows material in the gizzard to reenter Summer diet – insects, stomach 0.5 – 0.6g the proventriculus for additional treatment with acid and pepsin.

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VI. Small Intestine A. Duodenum

Chief organ of digestion and absorption Histologically – not clearly differentiated from rest of stomach Once food enters intestine undergoes complete 1st loop after stomach chemical ateration via enzymes

Contents mixed and absorbed by intestinal mucosa

Anatomy

„ Small intestine Receives – bile ducts, „ duodenal loop pancreatic ducts and „ ilium secretions from glands in mucosa „ generally shorter than mammals

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Secretions B. Function

„ Amylase

„ Saccharidases 1. Enzymatic activity – carbohydrases, lipases, proteases, and nucleases „ Peptidases 2. Primary site of absorption – by time product „ Lipase reaches large intestine it is completely absorbed „ Maltase, isomaltase

„ Sucrase

„ Enterokinase

„ Secretin, CCK, vasoactive intestinal peptide

3. Length variable – varies with diet, 4. Retention time related to diet Long and tightly coiled– granivores, omnivores Long – granivores Short – carnivores, frugivores Short and slightly coiled – raptors, insectivores frugivores

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C. Lining

Villi contain blood vessels, lymph vessels that absorb Lined with villi – projections that increase surface area nutrients for transport available for absorption.

VII. Large Intestine

Primary function is for water & electrolyte Contains minimum amount of non-digestable waste reabsorption • wastes voided as quickly as possible No relation between diet and structure • wastes sometimes voided in other ways Relatively short organ (e.g. pellets)

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VIII. Intestinal Caeca

Has out-pockets called caeca Posterior end of intestine occur pair of caeca

Size variable Large – Folivores (Cranes, ducks, geese) Small – Falcons, woodpeckers, insectivores, passerines, piscivores Absent - Parrots

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IX. Cloaca

Functions: A. Structure and Function 1. Absorption of water Receives waste from large intestine and materials from urinary and reproductive systems 2. Absorption of non-protein nitrogen 3. Digestion of carbohydrates and proteins 4. Microbial breakdown of cellulose 5. Microbial synthesis of vitamins(?)

B. Bursa of Fabricus

Divided into 3 sections Located on dorsal wall of cloaca • Coprodaeum – receives waste from large intestine Lymphatic pocket prominent in young birds but atrophied • Urodaeum – receives urine from kidneys (via in adults ureters), sperm and eggs from gonads • Proctodaeum – stores (temporarily) and ejects materials; closed posteriorly by muscular anus

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Serves as the area where B – lymphocytes (wbc that produce antibodies) are generated.

Once produced B – lymphocytes migrate to lymphoid tissue in other parts of body.

Farmer and Breitenbach (1966) inoculated 2 wk old Now - Go take the Test! chickens with malaria ½ had Bursa Fabricus removed

Controls recovered, but 11 of 15 bursectomized died

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Motility

„ Food in the mouth stimulates salivation and swallowing

„ Food in the esophagus or crop is directed by the gizzard contraction state

„ Coordinated gastroduodenal contractions

„ thin muscles-> duodenum-> thick muscles- > proventriculus

„ about 3 cycles per minute

Motility Regulation of feeding

„ Refluxes of GI contents „ Body weight and age

„ gizzard to proventriculus „ Environmental temperatures „ small intestine to gizzard „ Day Length „ cloaca to colon or rectum „ Activity „ Egestion „ Reproductive cycle „ Passage rate „ Food quality

„ Water availability

„ Food energy content

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Digestibility

„ In birds, because of the combined urine and fecal excretion, metabolizability is usually determined rather than digestibility

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