FEEDING HABITS AND ADAPTATIONS

FEEDING HABITS AND ADAPTATIONS

• Birds show a plethora of adaptations related to the food the eat. – Ranging from the unspecialized crow to the highly specialized hummingbirds – And others like the HOATZIN and SNAIL KITE

• Generally, most birds feed on a variety of foods and diets can vary substantially seasonally

• Hummingbirds specialize on flower nectar and their bills are adapted to their host plant species

• For example, the bill of a song sparrow is well adapted for a winter diet of seeds but is also good for catching insects in summer

• The hoatzin of South America feeds primarily on leaves

• The long bills of some shorebirds can probe below the surface but are also good at grabbing prey items on the surface

• The snail kite of south eastern US feeds exclusively on freshwater snails it extracts from the shell via a unique, hooked bill

BILL STRUCTURE • There are many bill adaptations depending on food source, with some bills adapted to general foraging and others highly specialized • The bill is the most versatile part of the digestive system

BILL STRUCTURE • Four major features make up the general morphology of bird bills – Upper mandible (maxilla) – Nasofrontal hinge – Lower mandible – Rhamphotheca - sheath

• The primary functions of the bill are to expose, seize/kill, and prep food items for swallowing

Cranial kinesis • Kinesis = movement • Ability to move upper jaw up and down through cranial action

Cranial kinesis • • • •

Increase size of mouth opening Faster jaw closure Can raise upper jaw yet keep head and mouth in same axis of orientation Upper jaw moves up & down at nasofrontal hinge --strip of flexible bone or a joint.

• All birds have it; some more than others • Shared feature w/ some reptiles

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FILTER FEEDERS

FILTER FEEDERS

• The varied lengths and curvatures of shorebirds determine which prey they can reach

• Flamingoes and waterfowl

• Slight changes in bill dimension influences foraging rate

• Lower mandible has a “trough” within which fits the tongue – This creates a suction to pull water into the bill – When depressed in the groove, water flows in via large lamellae in the upper mandible while excluding large objects – When elevated, water is forced out through a succession of smaller lamellae – Effectively, flamingos feed upside down

• Plovers feed on small invertebrates near surface • Longbilled curlews can reach deep burrowing prey

FILTER FEEDERS • Large, course filter of the greater flamingo strains out invertebrates and seeds at about 4-6 mm in size • In contrast, the extremely fine filter of the lesser flamingo can filter microscopic alga and diatoms • Hence the two species can be found together on rift valley lakes in east Africa

SEED EATERS • Many finches have hard ridges and groves on the upper mandible and anterior palate that hold the seed in place while it is cut open by the sharp edge of the mandibles • Crossbills have finely crossed bill tips specially adapted to extract seeds from conifer cones

• Highly specialized bill in flamingos

SEED EATERS • Seed eaters exhibit extreme variability • Doves swallow seeds whole and grind them up with the gizzard • Jays, titmice and chickadees hold seeds between their feet and hammer them open via their bills • Most passerine seedeaters crack and husk seeds open with their powerful bills – Includes such species as grosbeak and cardinals that can crack cherry pits

SEED EATERS • Bill size can be so important that it can drive natural selection • Galapagos finches is best example • Large billed species survive better during drought when small food is limiting • Small billed species survive better during wet years when small seeds are plentiful • Population changes are cyclic with El Nino events

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NECTAR FEEDERS • Hummingbirds and Sunbirds probe flowers and extract nectar through capillary-like tongue tips. • Bill forms in these groups tend to match the length and curvature of the preferred flowers • Bill morphologies have coevolved with the flowers

FISH-EATING BIRDS

FLESH-EATING BIRDS • Diurnal predators and nocturnal owls have strong, hooked beaks and sharp talons • In contrast, vultures that feed on carrion, have weak feet • Owls differ from hawks in that they swallow prey whole, have crop, and regurgitate a pellet after every meal • Hawks tear prey apart to get at the flesh and bypass fur, feathers, and bones

Avian Digestion

• Pelicans, cormorants, frigate birds, albatross, mergansers, loons • In many species the upper mandible is hooked as an adaptation to hold fish and/or the teeth are serrated as in mergansers and cormorants • The Osprey has a spiny tubercules under its toes as an adaptation to grasp fish

Parts of the Digestive System • • • •

Oral cavity Esophagus Crop Two-chambered stomach – Proventriculous – Gizzard

• • • •

Liver Pancreas Intestine Cloaca

MOUTH OR ORAL CAVITY • Generally, the avian mouth is roofed with a hard palate • The lower section of the mouth is often membranous, and in some species like pelicans, it is modified into an extensive pouch • The mouth has many small mucous glands that aid to moisten and swallow food

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MOUTH OR BUCCAL CAVITY

TONGUE

• Salivary glands are abundant in the pharynx where they secrete a starchdigesting enzyme

• The typical avian tongue is small, sharply pointed in front, and covered with papillae in rear

• Many swifts use dried saliva to cement their nests together

• In parrots, the tongue is very muscular, but in most species the tongue has few muscles

• The swifts of the east indies builds its nest entirely from saliva and are shipped to China for “Bird’s nest soup”

• Avian tongues are controlled by muscles of the hyoid framework

• Woodpecker tongues secrete two types of saliva: – Sticky for prey capture – Regular digestive enzymes

TONGUE

ESOPHAGUS

• The tongue varies from tubular, semi-tubular, or brushy in nectar eaters to long and barbed in woodpeckers

• The esophagus extends from the pharynx to the stomach

• Penguins have backward directed hooks to hold fish, lamallae aid in filter feeding in waterfowl, and in pelicans, ostrich, and hornbills the tongue is very small – vestigial

• Usually has mucous glands and is muscular

• Birds swallow food quickly, so there are few taste buds on the tongue

CROP Storage and place for seeds to soften Most pronounced in spp. that eat large meals (carnivores) or eat meal quickly for delayed digestion (e.g. seeds) High fat-high protein “crop milk” secreted from inner walls by pigeons, doves, flamingos, and male Emperor Penguins;

• Insect-eaters and species that beak food with beaks have a narrow esophagus while it is distensible in species swallowing while prey

THE STOMACH • All birds have two types of stomachs: – An anterior glandular stomach called the Proventriculus – A posterior muscular stomach called the Gizzard

• The gizzard in birds in the functional equivalent of molars in mammals • The proventriculous is comprised of mucous glands and nearly all digestive glands

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GIZZARD • The gizzard is strong and rigid with striated muscles usually arranged in bands

GIZZARD • In Carnivorous birds the gizzard acts as a trap for sharp bones and other indigestible fragments – Teeth, cellulose, chitin

• Mucous epithelium lining the gizzard secrete a keratinous fluid that hardens into horny plates that serve as the grinding surface

• These items are rolled into a ball and regurgitated as a “pellet” • Most common in owls but reported in >300 spp.

• The grinding action of the gizzard is facilitated by acquisition of grit

• Owls can not digest bone; hawks can.

LIVER

INTESTINE

• The 2-lobed liver is the largest organ in the body

• Intestine is chief organ for digestion and absorption of food

• The liver serves to: – store excess sugars and fats – Synthesize proteins – Create bile – Excrete waste from blood

• Generally, the intestine of meat and fruit eating birds is short, thin-walled and broad

CAECA

• The intestine of seedeaters is long, thicker, and small in diameter

CLOACA

• Caeca are a pair of dead-end sacs located toward the posterior end of the intestine

• The cloaca is a common area for discharge from the kidneys and intestine

• In primitive birds (ostrich, crane, galliformes) the caeca are very large

• The cloaca is also the connected to reproduction and excretion

• Chief function of the caeca is the absorption of water and digested proteins

• Structure where digestion, excretion, and reproduction meet

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DIGESTION IN STOMACH • Primary digestion occurs in the glandular stomach where gastric gland secrete a strong enzyme that breaks down proteins • Stomach fluid is very acidic with a PH of 0.7-2.3 • The avian stomach is very efficient. – For example, the lammergeyer of the Mediterranean can digest an entire cow vertebrae in less than 2 days

DIGESTION IN THE INTESTINE • Once food passes into the intestine it mixes with the digestive juices from the liver and pancreas • Bile produced by the liver neutralizes stomach acids and emulsifies fats to prepare foods for further digestion • Once digested, food is absorbed into the blood stream through the epithelial lining of the intestine • Digestion is rapid: – A shrike can digest a mouse in 3 hours – Thrush can void seeds from berries in 30 minutes

Foraging

Getting food – foraging 1. 2. 3. 4.

Foraging •

Each phase requires time and energy and has risk.

•

Acquiring food is driven by the following relationship:

Components (phases) of food-getting Searching Prey selection Pursuit and capture Manipulation (or handling and storage)

Foraging • ONLY NET ENERGY GAIN MAKES OTHER BEHAVIORS POSSIBLE

–

Profit = (Energy gain-Energy spent)/Foraging time

• Example: As nectar availability increases, time spent foraging decreases

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A birds foraging time must double when net energy gain is reduced by half

• Birds alter behaviors based on food availability

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The more time spent foraging the less time available for other behaviors

• Breeding, molting, development, migration, all conducted during time periods with higher food availability

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Prediction from optimal foraging theory • Selection should favor foraging styles (innate, learned, or both) that on average over time yield a return that exceeds the energy expended in the effort or increases net energy gain • Foraging must be time-efficient so as not to interfere with time required for other life activities • Foraging must be done in way that reduces risk.

Feeding Behavior and Energy Balance • Foraging Influenced by:

Profit =

Energy Gain – Energy Cost Foraging Time

– Anatomy (morphology) – Food availability – Decisions…

ENERGY AND GAIN • Birds tend to choose food with the highest energy profit • EXAMPLE: • White wagtail prefers mediumsize flies even though large flies provide more energy • Medium-sized flies provide more profit / forage time because larger flies take longer to catch, subdue, and swallow relative to energy gain

Foraging and Anatomy • Avian body form diversity is low, but beak diversity is high Birds have access to every kind of habitat, in 3 dimensions • Elimination of teeth - jaws can change shape depending on preferred prey items • Rhamphotheca relatively easily modified

RESPONSE TO FOOD AVAILABILITY • AREA RESTRICTED SEARCH - If food is concentrated, success is increased by remaining in the area

RESPONSE TO FOOD AVAILABILITY • Rufous-sided towhees in oak-hickory forests where food was abundant spent 2840% less time foraging than in pine-oak forests where food was scarce

• CHANGE OF VENUE – Theoretically, a bird should move to a new foraging area when the payoff declines below average in a specific patch • AVOID SITES - Where food is already harvested birds should not forage

• Anna’s hummingbirds of western US spent 14% of the day foraging in winter versus 8% during breeding season when nectar rich flowers were abundant

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FOOD CACHES • Found in at least 15 different families • Acorn Woodpeckers • Crows: Scrub-jays, etc. • Chickadees • Shrikes • Hoarding food for future use

FOOD CACHES • Clark’s Nutcracker exhibits communal storage behavior where flocks carry pine seeds and hazel nuts to shared cache • Allows them to store 23 times more energy reserves than they need to survive winter

FOOD STORAGE • Another group of food storing birds cache seeds in the ground • Blue Jays alone disperse 15 species of woody plant seeds • In Wisconsin, Jays were estimated to disperse 150,000 viable beechnuts in 27 days

SUMMARY • Birds have specialized adaptations of locomotion, bill structure, digestion, and foraging behavior driven by high energetic demands • The avian digestive system is specialized to process unmasticated food • Foraging behavior driven by energetic constraints • Maximize energy gained from eating • Minimize energy cost of finding and catching food • Foraging time ~ rate of energy gain or loss

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