Fly Away with Me Physiology of Avian Migration

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Fly Away with MePhysiology of Avian Migration

• Notes on Assignment One
• September 16, 2005

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Factors to ConsiderQuestions to Ask

• Fact vs. hypothesis and working theories
• Why migrate?
• How do birds know when its time to migrate?
• Diversity of migration routes, distances
• Adaptations for flight and migration
• Migration triggers
• Specialized traits of migratory birds
• Role of glycogen stores
• Role of protein
• Role of lipid

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Factors to ConsiderQuestions to Ask

• Role of diet (especially pre-migratory diet)
• Pre-migration behavior
• Long-distance migrants vs. short-distance
  migrants differences?
• Stopover sites, stopover duration maximum
  migration distance without refueling
• Impact of global climate changes, pollution I.e.
  impact of phenology

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Why migrate?

• A means of exploiting distant food resources
  (when food supply becomes scare due to weather
  changes) and avoiding the physiological stress
  associated with cold weather fall
• Return to breeding grounds where food is
  plentiful, longer daylight hours, greater area
  over which birds can inhabit leads to breeding
  success spring

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How do birds know when its time to migrate,
where to go and how do they find their way?

• Cirrcadian clock?
• Temperature and daylight length changes?
• Changes in hormonal levels stimulated by
  environmental changes?
• Genetic control? Learned Behavior?
• Navigate by the stars? Earths navigational
  fields? Polarized light patterns?topography?
  Prevailing wind patterns?

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Diversity

• E.g. Artic Tern (Sterna paradisaea) annual
  round trip 30,000 km between opposite ends of the
  globe, from Artic breeding grounds to Antarctic
  seas birds are adapted to feed at sea allowing
  them to refule en route
• E.g. certain land and shore birds whose
  transoceanic flights must be accomplished
  non-stop e.g. Pacific Golden-Plover (Pulvialis
  fulva) travels for more than 100 hours to
  travel 5000-7000 km from northern Siberia and
  Alaska to Hawaii and other islands of Pacific
  Ocean

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Diversity

• E.g. Blackpoll warbler (Dendroica striata) 80-90
  hour over-water flight from New England/southern
  Canada to Caribbean would be metabolically
  equivalent to human running a 4 minute mile for
  80 hours!
• Ruby-throated hummingbird (Archilochus colubris)
  weighing about a penny, flies 1000 km flight
  across Gulf of Mexico from Yucatan Peninsula to
  southern coast of Us in 24 hours

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Adaptations to Flight Migration

• Feathers provide insulation necessary to
  maintain a high body temperature
• Feathers long flight feathers act as airfoils
  which help generate the lift necessary for flight
  well-developed pectoral muscles
• Stream-lined body lightweight skeleton (hollow
  bones) minimize air resistance reduce amount of
  energy necessary to become remain airborne

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Adaptations to Flight Migration

• Super-efficient circulatory respiratory systems
  I.e. large, hearts proportionately weighs 6x more
  than human heart rapid HR (e.g. song bird
  500/min) satisfies the metabolic demands of
  migration avian respiratory system is most
  efficient in animal kingdom 2 lungs special
  air sacs take up 20 of birds volume compared to
  5 in humans (unlike mammalian lungs, avian lungs
  remain inflated at all times, with the air sacs
  acting as bellow to provide the lungs with a
  constant supply of fresh air)

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Specialized Traits of Migratory Birds

• Longer, more pointed wings (further minimizes air
  resistance)
• Pectoral muscles tend to be larger composed of
  fibers which are more richly supplied with
  nutrient- O2-carrying blood vessels
  energy-producing mitochondria, making the
  pectoral muscle so migrants especially efficient
  at energy production use

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Specialized Traits of Migratory Birds

• Fly at high altitudes (e.g. songbirds may fly at
  500-2000 m Bar-headed Geese Anser indica fly
  over the Himalayas at 9000m!!) low level of O2
  available at these elevations 2 specialized
  adaptations in migratory birds blood 1.
  O2-carrying capacity of blood is enhanced by high
  concentration of RBCs 2. Some migratory birds
  have 2 forms of hemoglobin which differ in their
  O2 carrying abilities allowing migrants adequate
  O2 supply over a wide range of altitudes AND
  allows birds to adapt rapidly to varying levels
  of O2 availability

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Specialized Traits of Migratory Birds

• Many migrants that are typically not gregarious
  will flock together prior to, or during,
  migration this social behavior may result in
  improve predator avoidance, food finding,
  orientation some fly in formation, a strategy
  that improves aerodynamics reduces energy
  expenditure

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Specialized Traits of Migratory Birds

• Some species migrate only at night (many
  shorebirds and songbirds) possible advantages
  include decreased vulnerability to predators,
  reduced threat of dehydration or overheating, a
  greater likelihood of encountering favorable
  winds a stable air mass (rising hot air more
  variable wind directions occur during the
  daytime) they have time during the day to
  forage
• Some species migrate only during the day as they
  soar and glide on rising currents of air (which
  form only during the day as the suns rays heat
  the Earth) others fly during the day because
  they feed on flying insects

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Role of Lipids

• Dominant fuel source during flight (produces more
  energy per gram than CHO or protein)
• Produces water as a metabolic by-product (useful
  in avoiding dehydration)
• Can be mobilized at low temperatures
• Can be stored in the birds bodies without
  interfering with the birds aerodynamic shape
• Lipid is lighter/less bulky than protein/CHO
• Many factors controlling the relative NB of
  lipid, protein and CHO to flight migration are
  unclear
• Do fat birds use more lipid and less protein than
  thin birds?

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Role of Proteins

• Protein catabolysis likely occurs in both flight
  muscle as well as organ tissue during migration
  some of the reasons why are still unknown but
  some current research is focusing on water
  production, anti-oxidative purposes and adaptive
  changes in flight muscle size as possible
  mechanisms supporting the idea that protein loss
  could represent optimizing physiological
  flexibility

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Protein lipid utilization

• Differences in p l utilization across species
  I.e. some species demonstrate low level of
  proteolysis (low uric acid) high degree of
  lipid oxidation which others show a higher level
  of proteolysis lower lipolysis
• These differences may be related to diet of
  various species in particular, dietary
  adaptation prior to migration

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Protein lipid utilization

• Migratory fuel use splitting according to dietary
  preference, rather than phylogenetic
  relationships. Support the hypothesis that diet
  was a more important factor in migratory fuel-use
  than phylogenetic history

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Impact of Glycogen Stores

• A bird with large glycogen stores may catabolize
  less protein to maintain blood glucose levels
  during migration than a bird with small glycogen
  reserves
• Possibility that enzymatic activity reducing body
  tissues glucose requirements, and therefore
  conserving glycogen reserves thus body protein

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Pre-migration Behavior

• Timing of migration I.e. differences between
  spring and fall migration
• Pre-flight weight gain to increase fat reserves
  due to both behavioral physiological changes
  (increase in appetite food consumption
  hyperphagia begins 2-3 weeks before migration
  persists through migration aided by increase in
  efficiency of fat production storage for some
  birds can increase BW through fat deposition by
  as much as 10/day pectoral muscles become
  larger well supplied with enzymes necessary for
  oxidation of fat

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Specific Metabolic Changes associated with
Migration

• Digestive organs (e.g. small intestine gizzard
  liver) increase in size capacity with increased
  food intake energy demands (e.g.associated with
  exercise) converts more food energy into usable
  metabolic energy to fuel the increased energetic
  costs of exercising enlarged skeletal muscle (GIT
  organs are very metabolically active
  energetically expensive organs)
• recent research has shown 1. features of the gut
  (size, nutrient uptake rates, digestive enzyme
  activity) are modulated (varied) in response to
  changes in

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Specific Metabolic Changes associated with
Migration

• (cont) the quality quantity of food and 2.
  these adjustments are NB for permitting high
  feeding rate of migrants
• Smaller guts after migration

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Some Potential General Observations Conclusions

• Spring migrants minimize the time spent on
  migration, presumably to acquire good breeding
  territories
• Over evolutionary time, migrants have solved
  migrational challenges with a suite of
  morphological and behavioral adaptations e.g.
  many passerines migrants switch from a mainly
  insectivorous diet to a fruit-based diet
  preceding and during fall migration (associated
  with decreased catabolism of endogenous body
  protein) fruits are abundant (while insect
  numbers may be declining) high in carbohydrates
  lipids

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Some Potential General Observations Conclusions

• NB remember unlike lipid and CHO reserves,
  protein is stored as functioning tissue so it
  would not be advantageous to need/have to
  catabolize protein in flight muscles during
  migration
• In ecological terms, due to the billions of birds
  that migrate each season, this is a massive
  movement of biomass between ecosystems

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Some web sites of note

• Migratory Bird Center www.nationalzoo.si.edu
  (Fact Sheets on Neotropical Migratory Birds)
• Migration Takes Guts Digestive Physiology of
  Migratory Birds and its Ecological Significance
  www.wildlife.wisc.edu/faculty/Karasov/publications