Download feeding habits and adaptations

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
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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
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Oral cavity
Esophagus
Crop
Two-chambered stomach
– Proventriculous
– Gizzard
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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
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Foraging
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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
–
A birds foraging time must double when net energy gain is
reduced by half
• Birds alter behaviors based
on food availability
–
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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