block 5- migration Flashcards
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 overcast and so cannot use the sun compass
-ayelight scattering means that sunlight becomes polarised =When sunlight bounces off surfaces (like water, glass, or air particles), the light waves start to vibrate mostly in one direction.
-the strength of polarisation varies systematically with the angle of scatter. (max 90 degrees)
-the pattern of polarisation in any part of the visible sky can be used to infer where the sun is even if its not visible
-birds can see this and this is time-compensated just like the sun compass and provides backuo
star compass
- used in the night hut nocturnally migrating birds e.g. warblers
- starts appear to move because of earths rotation, the starts form recongnisable unchanging patterns online the sun (constellation)
-the starts nearest the axis of rotation move less or not at all
-birds learn patterns of constellations
-during critical developmental periods young indigo Buntings observe which part of the starry sky rotates least and learn start configurations and can use this later on to pinpoint north directly without having to observe moving stars
-if birds are prevented from seeing the nocturnal sky during critical period, the are unable to use the star compass as adults
-the star compass is not time-compensated as they arent looking at movement of stars they look at the ones not moving
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
-mechanism to infer magnetic fields from electromagnetic inductions. used by sharks,rays and other fish
-moving through the earths magnetic field induces tiny electric current that their electroreceptors can detect
how do radical-pair reactions work?
Blue or UV light excites a molecule in the eye.
An electron is transferred from a donor to an acceptor molecule, forming a radical pair (each with an unpaired electron).
The two electron spins can be:
Opposite spins = Singlet
Same spins = Triplet
The Earth’s magnetic field (its angle) affects how likely each spin state is.
This changes the outcome of the chemical reaction.
The brain detects these chemical differences 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
<5% of downstream migrants survive to return.
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.
The reason they can do this is that thyroid hormones play a role during smoult transformation. These hormones not only help them adapt to saltwater, but they also trigger the learning process of their home river’s smell.
The experiment with chemicals:
In another experiment, juvenile salmon were exposed to a chemical called morpholine, which is not naturally found in rivers. The salmon learned to associate this chemical with their home environment.
As adults, whenever they encountered this chemical in the water, they would stop migrating because they thought they were near their home.
Salmon that weren’t exposed to morpholine didn’t react to it at all.
sequential imprinting hypotheisis
Thyroid hormone levels increase as salmon develop.
These hormones drive the downstream migratory drive and open the first critical period to learn the smell of the natal tributary.
This is when salmon become imprinted on the scent of their home river.
🌀 Fluctuating Hormone Levels:
Thyroid hormone levels change based on:
Developmental factors.
Environmental factors during outward migration.
During each critical period, salmon learn new olfactory waypoints (key smells) along their migration route.
🔁 Returning to the Natal Tributary:
Adult salmon can follow this sequence of smells in reverse to find their way back to the natal tributary from the coastline.
-The hypothesis doesn’t explain how salmon navigate while in the ocean (during migration at sea).
warblers
Eurasian reed warblers use magnetic inclination and declination to navigate.
They can correct artificial displacements using a magnetic map.
Magnetic intensity not involved, only inclination and declination matter.
Warblers extrapolate the map beyond areas they’ve experienced.
homing and migration summary
Migration: Driven by innate (genetic) and learned mechanisms.
Urge to migrate and direction are genetically controlled.
True maps: Some animals may genetically encode a magnetic map.
First-time migrants: Use compass and time/distance navigation, leading to high mortality.
Experienced migrants: Use visual, olfactory, auditory, and geomagnetic cues to develop true maps.
Salmon: Use sequential olfactory and geomagnetic imprinting for navigation.
Cryptochrome molecules in birds are important for magnetoreception.
