Digestion and Food Habits

The special problems of bird digestion

• High metabolic rate!– Digestive system must be efficient but

lightweight• Hence, most can’t afford a lot of heavy anatomy.

Meta

bolic R

ate

Sb

Birds

Mammals

Ectotherms

• Gannets conserve energy while feedingVideo

Entering water about 2.9m/s (not much deceleration)

Diving between <1-10m (passive momentum to surface – sometimes aided by wing or foot movement)

General OverviewCarnivore mammal Herbivore mammal

No teethLimited jaw musculature

• A few muscles control a lot of jaw movement

Position of the depressor mandibulae

Retention time (in hours) for fluid & particulate digesta markers in the gastrointestinal tracts  of representative reptiles, birds, & mammals (Based on: Stevens and Hume 1998).

Species Body massFluid retention

time Particle retention

time 

Iguana - <48 207

Broad-tailed Hummingbird1 3.3 gm 1.2 -

Rock Ptarmigan 460 gm 9.9 1.9

Sooty Albatross 2.5 kg 6.3 15

Rockhopper Penguin 2.5 kg 3.8 17

Emu 38 kg 3.9 4.7

rabbit 2.1kg 39 27

pig 176 kg 39 48

General Overview (cont.)

• Alimentary Canal– Oral cavity– Esophagus– Crop– Proventriculus– Gizzard– Small Intestine

• D, J, I– Cecum– Large Intestine– Cloaca

• Accessory Organs– Beak– Salivary Glands– Liver– Pancreas

• Bill Morphology• Bony structure• keratin

• Multiple functions…

http://www.naturalhistorymag.com/master.html?http://www.naturalhistorymag.com/0904/0904_biomechanics.html

Bristles• Function

– Protect eyes– Increase gape width in insectivores

• Usually coupled with wide bill Hooded warbler bristles

American redstart

Morphology CCA 1

-2 -1 0 1 2

Att

ack

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-1

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Morphology Contrasts CCA 1

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Pounce

Sally

HG

R2 = 0.8*

R2Cont = 0.9*

Longer TarsusWider bill

SA (“o”) and NA (“n”) Flycatchers

Morphology CCA 1

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Att

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Morphology CCA 1

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Morphology Contrasts CCA 1

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Sally

HG

R2 = 0.8*

R2Cont = 0.9*

Longer TarsusWider bill

SA (“o”) and NA (“n”) Flycatchers

Tongue• Functions

– Aids in gathering and swallowing food – Usually no tastebuds on dorsal surface

• Epithelium of mandibles– Not muscular (xs) but reinforced by the hyoid apparatus – Morphology of the avian tongue varies with food habits

• How much processing must be done?• What kind of processing is done by the tongue?

– Fish-eating species:• typically have small, undifferentiated tongue • fish are often swallowed whole

– Woodpeckers• long, extensible• Barbed at tip

Tongues

Am. Robin (generalized)

Bananaquit (tubular - nectarivore)

White-headed Woodpecker (spear/probe)

Trogon (muscular frugivore)

Shearwater (fish eater)

N. Shoveler (food-strainer)

Golden-fronted WP

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

Nectarivores

Distal tip

Oral Cavity

• Taste buds

• May be modified anatomically based on diet

– Fringes of anseriformes • Inter-digitates with tounge frills

– Ridges of finches and blackbirds

Esophagus

• Features– Connects oral cavity and stomach– Larger, Muscular (peristalsis), expandable– Lined with mucous glands– Nutritive food production

• Sloughed epithelial layers

– Inflatable• Visual cues• Sound production

vocal

BLNI

Esophagus

• Mucous Glands

mammalian avian

lumen

lumen

Crop

• Expanded portion of the esophagus• Stores, softens foods• Regulates food flow through digestive tract• May become greatly modified for fermentation

– Kakapo– Hoatzin

keelcrop

Stomach

1. Proventriculus “glandular stomach”– anterior– receives food from the esophagus – secretions:

• Mucus• HCl• Pepsinogen + HCl Pepsin (proteolytic

enzyme)– Can dissolve bone rapidly

• Petrels - Oil by-product storage – Nutrition– defense

Band-rumped storm petrel

Stomach (cont.)

• Gizzard– Posterior– Muscular (analogous to mammalian molars)– Keratinized– Grinding– Reverse

Peristalsis

“re-treat” food

Red KnotsAerts Island, Canada

(Morrison et al. 2005 - Condor)

Time period after arrival at breeding grounds

Diet – molluscsBody mass decreasesDigestive organ mass increases

Frugivore stomachs

Intestine

• Chief organ of digestion and absorption

• Receives bile and pancreatic secretions

• Small and large intestine

• Length related to assimilation efficiency

EA = assimilation/ingestion X 100

omnivorecarnivore

herbivores

Large intestine

• Relatively short

• Absorb water

• Contains out-pocketings (“caeca”)– Some species – histologically similar to rest of

intestine– Symbiotic bacteria/fungi, lymphatic activity– Others – “lymphoid caeca” – lymphatic only

Cloaca• Receives

– Wastes from digestive– Materials from other systems:

• Reproductive• Urinary

• 3 main divisions1. Corprodaeum – waste from intestines2. Urodaeum – N waste (uric acid) and sperm/eggs3. Proctodaeum – ejection site – closed by anus

• Bursa of Fabricus– Dorsal wall of cloaca in young birds– Site of B-lymphocyte production– Atrophies with age

Accessory Organs

• Liver– Hormone and bile production– Filter toxins

• Pancreas– Exocrine: Produces digestive secretions– Endocrine: Insulin and glucogon (islets)

• Regulates blood sugar levels

Feeding Behavior and Energy Balance

• Influenced by:

– Anatomy (morphology)– Food availability– Decisions…

– Search/locating, attack (pursuit), handling, assimilating/digesting conversion

aspects of the behavior

Profit = Energy Gain – Energy Cost

Foraging Time

Searching behavior also an example of a form-function relationship…

• Search time can be predicted by morphology – WHY?

Body Size

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Bigger the bird the longer the search time

Longer legs = shorter search times

Corbin 2002

Searching position

• Related to familiar food– Why would a bird search in particular places

or for particular food items?

s.i. III

Prey abundance

# of

pre

y ta

ken

satiation

• R. MacArthur 1959

• When to give-up searching?

– Searching specialists– Searching opportunist

Searching position

Profit = Energy Gain – Energy Cost

Foraging Time

O.F.T. – centered on adjustments

Chickadees eating mealworms in the lab

Aerial hawkerreturning to perch

Poor habitatRich habitat

Chick provisioning questions

Habitat quality

Tim

e aw

ay

Search Time

• Give-ups are always longer than attempts. Or, at any given perch the attempt time is shorter than the give-up time. Hence, this suggests a threshold whereby if a kingfisher doesn’t attempt a capture within a certain timeframe, it will move to a different perch.

Corbin and Kirika 2002

Search time

White wagtailsPrey size reflects “optimal” decision making

Prey available

Prey taken

Prey handling efficiency

Handling Time

Attack and/or Foraging Time

• Related to foraging strategy

Sit-and-wait Active

Attacking Searching

• Depends upon

energetic payoff

• Is also related to morphology (past lecture material)