SkarpChapter 3 of 3
How Birds Combine Instinct, Learning and Changing Clues
Explore how inherited migratory directions guide young birds, how experience can improve route correction, and how birds recalibrate conflicting celestial and magnetic cues when clouds, winds or unfamiliar terrain make navigation difficult.
1. Migration Needs More Than One Answer
Two Navigation Tasks
Migration can occur in many directions. A compass supplies a heading, while a map helps a bird estimate its position and correct a route after displacement.
A Flexible System
Sun, stars, magnetic information, odors, landmarks, wind, and memory can all contribute. Their usefulness changes with conditions.
2. Instinct Supplies a First Draft
A Built-In Starting Rule
An inherited program can set migration timing, a general heading, and sometimes a sequence of headings or travel intervals.
Vector Navigation
Vector navigation means following a programmed direction for a programmed period. It is useful, but limited after major displacement.
3. Young Birds Learn the Sky
A Moving-Sky Lesson
Young indigo buntings use the apparent rotation of stars to learn a directional reference, rather than relying only on fixed star pictures.
Why Planetariums Matter
When experiments shifted the apparent center of stellar rotation, the birds shifted their orientation too.
4. The Magnetic Compass Is an Inclination Compass
Not a Needle Compass
The magnetic field vector angles downward in the Northern Hemisphere and upward in the Southern Hemisphere. A bird's polarity-insensitive inclination compass reads the field-line axis, helping distinguish roughly poleward from equatorward.
Useful Under Clouds
When stars and the Sun are hidden, magnetic information can still help a bird preserve a migratory heading.
5. When Cues Conflict, Birds Recalibrate
Calibration, Not Blind Obedience
Birds can compare celestial and magnetic signals, then recalibrate a compass when the environment provides a stronger reference.
The Sunset Horizon
Polarized light near the sunset horizon appears especially important for aligning compass systems.
6. Experience Improves Route Correction
A Magnetic Map Signal
Reed warblers responded to a simulated magnetic displacement; disrupting trigeminal input removed that corrective response.
Odors Can Matter Too
Displaced adult gulls needed intact olfactory nerves to correct effectively toward their usual migration corridor in one experiment.
7. Thought Exercise: A Cloudy, Windy Night
Choose the likely strategy
Picture a migrant flying at night. Thick cloud hides the stars. A crosswind has pushed it west of its usual corridor.
- Hold a heading: magnetic compass information can help it avoid drifting into a completely random direction.
- Estimate displacement: an experienced bird may compare magnetic, odor, landmark, or other learned map cues with expectations.
- Correct cautiously: if the signals are uncertain, it may maintain a broadly suitable direction until stronger cues return at dawn, sunset, or a familiar stopover.
The key idea: wind creates a navigation problem; a compass maintains direction; a map-like system supports correction.
8. Check Understanding
One question
Use the distinction between a compass and a map to identify the best explanation.
Why might an experienced migrant correct after being blown far off its normal route while a first-time migrant continues in a general seasonal direction?
- Experience can add map-like information for estimating displacement, while the young bird may rely mainly on an inherited directional program.
- Experienced birds no longer use inherited migration programs.
- Young birds cannot detect magnetic fields or celestial cues.
Show Answer
Answer: A) Experience can add map-like information for estimating displacement, while the young bird may rely mainly on an inherited directional program.
A first migration can be guided by an inherited vector: a general direction and timing rule. Experience can add learned information that supports true navigation and route correction. Both young and experienced birds may use celestial and magnetic compass cues.
Key Terms
- map
- A navigation system that helps an animal estimate its position relative to a destination, route, or familiar area.
- compass
- A system that provides direction, such as a heading toward poleward, equatorward, eastward, or westward.
- calibration
- Adjusting one orientation system by comparing it with another trusted environmental reference.
- true navigation
- The ability to determine a corrective direction toward a goal after displacement from a familiar route or area.
- trigeminal input
- Sensory information carried by branches of the trigeminal nerve; it has been implicated in magnetic map responses in some migratory songbirds.
- vector navigation
- Following an inherited or learned direction for an approximate amount of time or distance.
- inclination compass
- A polarity-insensitive magnetic compass that uses the axis formed by the tilt of Earth's magnetic field lines to distinguish roughly poleward from equatorward.