TIme migration for photoperiods
- To time their migrations correctly, birds must tune their internal clocks to different photoperiods, depending on latitude.
Northern home hypothesis
- migratory species evolved migration by shifting their non‐breeding ranges from ancestral high latitudes towards the tropics
Southern home hypothesis
- migratory species are descended from tropical, sedentary ancestors
- migration evolved as these species shifted their breeding grounds from the tropics to the temperate zone.
What is ecological mismatch and how might it affect migration?
- Many migratory birds have evolved to synchronize their breeding timing (solid red curve) with the period of greatest food availability for their offspring (dotted blue curve) so that they can breed successfully (shaded red area represents frequency of successful breeders).
- If environmental conditions shift such that peak food availability occurs earlier than peak breeding, early breeders will raise the most offspring successfully, but much of the population will breed too late to provide optimal levels of food to their young.
Orientation vs. Navigation
- orientation: Judging compass directions accurately
- navigation: Estimating absolute position and planning a route
Visual landmarks to navigate
- coastlines, mountain ranges, rivers
Geomagnetism
- geomagnetic fields of the earth provide a map of horizontal space, just as gravity and barometric pressures give info about vertical space
- example: bar magnet interferes with homing pigeon ability to return to its loft on overcast days
Magnetite
Magnetite-based magnetoreceptors located in the upper beak close to the skin
Function appears to be recording magnetic intensity and thus providing one component of the multi-factorial ‘navigational map’ of birds
How do cryptochromes function?
Light excites one of the electrons in a cryptochrome molecule, and the molecule and its associated neuron somehow are able to detect the effect of the magnetic field on the quantum‐mechanic spins of these solo electrons before they return to their stable paired state.
These cryptochromes may be able to produce a visual pattern in the retina that allows birds to “see” direction akin to a magnetic compass
Solar compass travel
- more fluttering activity on a cloudy sky than a clear day
How do birds used polarized light?
- Light waves in which the vibrations occur in a single plane
- indicates sun’s position
- traveling in sun’s pathway
Celestial compass: how do night migrating birds use stars?
The Indigo Bunting migrates at night between its summer range in the eastern United States and its winter range in Central America
Buntings in a state of migratory restlessness orient by the stars at night, even when confined to a funnel-like cage placed under a planetarium
Used orientation of the constellations that were within 35 degrees of the North Star
Orientation Guides (other)
Odors
Sounds
How does warm air and cold air interact?
- warm air and cold air in the same direction produce warm front
- cold air and warm air pushing against each other cause cold front
How does warm front influence spring migration?
- behind the warm front, we get clear skies, warm temperatures, and south winds
- in full force
How does cold front influence fall migration?
- creates clear skies rapidly, temperatures drop, and winds from northwest
- in full force
How does weather in migration influence mortality rates?
Bad weather
Collisions with man‐made structures
Predation
Disease
Habitat loss
