block 5- migration Flashcards
(28 cards)
migration
– Population-scale movements (all or subgroups) on specific
routes from origin to target (and back).
– ‘Directed locomotory activity’.= purposefully,goal directed movement of animals , not random
Homing
– Precise return to a specific ‘home’ location.
– May be part of migration, or shorter-term.
– E.g. swift returning to same nest box/site; ant returning to
its nest after foraging, salmon returning to its home
tributary after being at sea.
roles of experienced in migration
-first-time migrants ,must use relatively simple orientation systems based on info inherited or learned before departure.
-they usually follow experienced companions
-experiences migrants have previously learnt cues gradients and generated a map that they can use to correct even for displacement to unknown locations. thus can perform true naviagation
genetic control of migration
- the urge and general direction of bird migration is likely under genetic control
-* Blackcaps (Sylvia atricapilla) on the Cape Verde Islands off Africa are non-migratory. - When crossed with individuals from a
migratory European population, 40% of the
offspring were migratory.
tool to study migration
-observations of population distributions
- ID tagging
– Leg bands, wing tags, fin tags.
* Release track observations.
* Radar tracking.
* GPS tracking.
* Lab or semi-lab experiments
– Emlen funnels, circular cages.
– Manipulations of cues – sun, stars, magnetic field,
olfactory cues.
migration mechanisms
- Time-compensated sun compass
– All classes of vertebrates, insects, spiders,
crustaceans. - Polarised skylight compass
– Birds, fish, amphibians, invertebrates. - Star compass
– Birds. - Geo-magnetic compass
– All classes of vertebrates, insects, molluscs
crustaceans – even bacteria.
sun compass
-Animals use sun azimuth, not elevation, as compass cues
-suns position changes by 15 degrees per hours
-need an internal clock to extract copass information from the sun position
-this is a time-compensated sun compass = animals use their internal circadian rhythms to o adjust (or “compensate”) for the sun’s movement an figure out durectuoion
polarisation compass
Can be used when the sun is obscured (e.g., overcast conditions), making the sun compass unusable.
Rayleigh scattering causes sunlight to become partially polarized as it is scattered by small particles in the atmosphere.
When sunlight is scattered (especially by air molecules), the light waves vibrate predominantly in one direction—this is what we call polarization.
The degree of polarization varies systematically with the angle of scattering, reaching a maximum at 90 degrees from the sun.
This creates a predictable pattern of polarization across the sky, which can be used to infer the sun’s position, even when it’s hidden by clouds.
Birds (and some insects) can detect this polarization pattern using specialized vision. This forms a time-compensated compass, much like the sun compass, and acts as a backup navigation system when the sun itself is not visible.
star compass
Used at night by nocturnally migrating birds such as warblers and indigo buntings.
Because of the Earth’s rotation, stars appear to move across the sky, but they form recognizable, fixed patterns known as constellations—unlike the sun, which changes position throughout the year.
Stars near the celestial pole (the axis of Earth’s rotation) appear to move the least or not at all.
Birds learn the pattern of constellations, especially those near the north celestial pole, which helps them determine direction (true north).
During a critical developmental period, young birds (e.g. indigo buntings) observe which part of the night sky appears to rotate the least. They memorize the surrounding star configurations, which they later use to navigate.
If birds are prevented from seeing the night sky during this early learning period, they fail to develop a functioning star compass as adults.
The star compass is not time-compensated, because birds are not tracking the movement of stars, but rather using the fixed position of the star patterns near the pole to determine direction
why cant animal rely on just their compass
- sun and star compasses can help explain how animals determine their direction of migration, but cannot tell the animal where it is on earth
geomagnetic compass
A natural compass used by animals (like birds) to sense the Earth’s magnetic field.
Helps them figure out direction and position during migration.
Birds use:
Inclination (angle of magnetic field lines)
Declination (difference between true and magnetic north)
Possibly intensity (strength of the field)
Works like an internal GPS – even when it’s cloudy or dark!
geomagnetic compass:polarity
- Polarity (NOT used by birds)
Earth’s magnetic field flows from the South Pole to the North Pole.
This is like how a compass points north.
BUT: Birds don’t seem to use this “north/south direction” directly.
geomagnetic compasses:inclination
Inclination (USED by birds)
This is the main thing birds use.
It means: how steep the magnetic field lines are when they enter the Earth.
👉 Here’s the trick:
Near the equator, the lines are flat (0°).
Near the poles, the lines go straight down (90°).
🧠 Birds sense this angle to figure out if they’re:
Closer to the equator (flat field lines), or
Closer to the poles (steep field lines)
It’s like a magnetic map that tells them where they are north/south!
geomagnetic compaases:declination
. Declination (USED by some birds)
This is the difference between:
True north (where maps point)
Magnetic north (where compasses point)
🌍 These don’t line up perfectly everywhere.
🧠 Some birds can sense this tiny difference to adjust their direction more accurately.
geomagnetic compaases:intensity
Intensity (MAY be used)
This is how strong the magnetic field is.
It’s:
Stronger near the poles
Weaker near the equator
🧠 Birds might use this strength level to get clues about latitude (how far north or south they are).
what is the mechanism do birdss prefer to use inn migration?
-sun compaases
-use others uch as geo-magnetic fields when the sun is not out
-studys showed this
electromagnetic induction
-Some animals, such as sharks, rays, and certain fish, can detect Earth’s magnetic field using a mechanism based on electromagnetic induction.
As these animals move through the Earth’s magnetic field, it induces tiny electric currents in the surrounding conductive seawater (especially as it flows through the animal’s body or head).
These induced electric fields are detected by specialized sensory organs called electroreceptors (e.g., the ampullae of Lorenzini in sharks and rays).
This allows them to infer the direction of the magnetic field and use it for navigation.
how do radical-pair reactions work?
Blue or UV light excites a light-sensitive molecule (likely cryptochrome) in the eye.
This triggers the transfer of an electron from a donor to an acceptor molecule, forming a radical pair—two molecules, each with an unpaired electron.
These electrons can exist in two spin states:
Singlet state – electron spins are opposite
Triplet state – electron spins are the same
The Earth’s magnetic field influences the interconversion between singlet and triplet states by affecting the alignment of the electron spins.
This, in turn, changes the chemical reaction pathways and products.
The brain detects these changes, allowing the animal to sense magnetic direction.
Role of Cryptochrome
Cryptochrome is a light-sensitive pigment in the retina that forms radical pairs.
Activated by blue light – explains why birds need blue/UV light to navigate.
Cry4a is a special form of cryptochrome:
Found in double-cone cells of night-migrating birds.
Not linked to the circadian clock.
2.5x more active during migration season.
Likely aligned in a way that helps detect magnetic orientation.
Magnetite-based sensors
Magnetite (Fe₃O₄) is a naturally magnetic mineral found in many animals.
Acts like a tiny compass to help detect the Earth’s magnetic field.
🦠 In Bacteria:
Magnetite is grouped into magnetosomes.
These allow magnetotaxis – the ability to align and move along magnetic field lines.
🐦 In Birds:
Some evidence suggests magnetite is in cells in the upper beak.
These cells are linked to the trigeminal (ophthalmic) nerve, which may detect magnetic signals.
🧠 Current Understanding:
The trigeminal system may support the main magnetic sense (which is vision-based).
It could help birds build a more complete navigational map.
However, it’s still unclear:
If magnetite is actually involved in birds.
Which specific cells use it.
geomagnetic sense summary
Birds likely have multiple magnetic sensing systems, each serving different or complementary roles.
These may include: Retinal, Trigeminal, and Inner Ear (Lagena) systems.
Reptiles and fish share some of these systems and may have others.
📍 Compass vs. Map:
The retinal system (eye-based) likely gives birds a magnetic compass (i.e., direction).
But this alone can’t tell them where they are.
Birds likely build a navigational map using multiple cues:
Innate + learned information
Geomagnetic features:
Field strength
Inclination (angle of field lines)
Declination (difference between true and magnetic north)
Local geomagnetic anomalies (learned over time)
Olfactory cues (smell-based navigation)
Visual/geographic landscape features (learned landmarks)
coho salmon
Spawn in small streams on the Pacific coast of North America during autumn.
Adults die after spawning.
Life cycle:
Eggs hatch → Fry → Parr → Smolt over ~18 months.
During smolt transformation, they adapt from freshwater to saltwater.
Migrate thousands of km into the ocean.
Grow up to 90 cm and 7 kg in ~18 months.
At around 3 years old, they return to the coast near their natal stream.
Re-adapt to freshwater.
Rapidly become sexually mature.
Navigate upstream to the exact tributary where they hatched.
coho salmon homing
<Fewer than 5% of the animals (likely fish, such as salmon or eels) that migrate downstream—usually from rivers to the sea—survive long enough to return upstream later in life, typically to reproduce.
Of the returners:
~95% return to their natal tributary.
Others spawn in different rivers/tributaries (may help with colonisation or avoiding degraded habitats).
Larger populations = better homing accuracy.
Likely use collective navigation during their migration.
olfactory impriniting in salmon
Olfactory imprinting in salmon is the process where young salmon learn the specific smell of their home river (called the natal river). This learned smell helps them find their way back to that river later in life when they’re ready to spawn.
Key Points:
When they’re young:
If juvenile salmon are moved to a new river before they go through a process called smoult transformation (when they adapt from freshwater to saltwater), they will return to the new river when they migrate back. They don’t go back to their original river (their natal river).
After smoult transformation:
If the salmon are moved after this transformation, they will return to their original natal river, not the one they were released in.
Why this happens:
This learning (imprinting) happens during a critical period in their development. It’s kind of like how some birds “imprint” on the first thing they see after hatching. The learning is quick and permanent — once they learn the smell of their home river, it sticks.
esentially need enough time to imprint on their natal river when they are young. chatgpt explains if still confused
