MIGRATION, HOMING AND NAVIGATION IN BIRDS Flashcards
Genetic control of migration in birds
- Whether to migrate
e.g. Robins in Germany: 80% migrate, 20% do not. Breeding experiments showed that
migration tendency is under polygenic control - Direction
e.g. East and west populations of white stork migrate in different directions If young are
swapped they migrate in direction of source population - Distance (clock involved, timing hours travelled as shown with caged birds).
- Timing of migration
Timing of seasonal migration
This is primarily due to an endogenous circannual rhythm, entrained by photoperiod.
May also be modified by external factors such as weather, and by energetic condition of
bird.
Two processes are involved :
Two processes involved in timing of seasonal migration
(1) Preparation for migration (initiated by photoperiod) characterised by increased fat
deposition and migratory restlessness (Zugunruhe).
Restlessness is related to daylength but may also be affected by weather- most restless when
weather is suitable for flight.
(2) Initiation of migration itself is triggered by accumulation of sufficient fat and appearance
of favourable weather conditions
Mechanisms of long distance orientation in birds
Orientation by landmarks, Sun compass, Polarised light, Star compass, magnetic compasses and map, odour map
Orientation by landmarks
Migrating birds may use coastlines or river valleys. Landmarks are used in familiar territory,
and may be especially important in home areas at each end of the migration. Also important
in finding suitable stopover locations.
Homing pigeons fitted with frosted lenses (cannot see images) can find their way from 20
km away to a short distance away from the loft, showing that they use something other than
visual landmarks to find the way.
European starlings were displaced 150 km east. Juvenile birds migrated parallel to the
undisplaced birds (and for the same distance), and ended up 150 km east of their normal
destination, indicating the use of a compass. Adults compensated for the displacement,
showing that they were using more than just compass sense.
Sun compass
The sun rises in the east and sets in the west. Therefore, it is a source of information on
compass direction for animals
Evidence that birds use the sun for direction comes from experiments with caged birds.
Starlings kept in circular cages from which they could only see the sky flew in the direction
of their normal migration as long as they could see the sun.
Obviously, the sun does not remain static. In further experiments, caged birds were trained
to seek food in a certain compass direction. They sought the food in the correct direction
irrespective of the time of day. This indicates that birds use an internal clock to compensate
for the apparent movement of the sun- i.e. they use a time-compensated sun compass. This
was confirmed using birds whose internal clock had been shifted by 6 hours- there was a 90o
shift in direction they chose.
Birds that migrate at night may also use the sun- by taking a bearing on the sun at sunset and
flying in a constant direction until sunrise when a new bearing can be used.
Polarised light
Light rays entering the atmosphere become scattered resulting in polarised light.
Even if the sun is obscured, birds can use the polarisation pattern of light from the sun as an
indicator of its direction, as long as there is some clear sky.
Star compass
Knowledge of the positions of constellations in the night sky has allowed humans to
navigate. Most famously, Polaris (the north star or pole star) is always roughly over the
north pole.
Some birds migrate at night. Experiments with caged warblers (nocturnal migrants) showed
lack of orientation indoors, but directed movement (north in spring, south in autumn) when
allowed see the night sky. Emlen recorded footprints of caged indigo buntings to show their
preferred direction. This was the same under a natural sky and a planetarium, but when the
planetarium sky was shifted by 180o the buntings exhibited restlessness to the south instead
of north in spring.
Birds do not compensate for apparent movement of stars during night, but use the
constellations that rotate the least (those around the pole star in the northern hemisphere).
Birds inherit the ability to use a stellar compass but not the actual patterns of stars. These
have to be learnt during a sensitive period between leaving the nest and starting the autumn
migration
Magnetic compasses and map
Some birds maintain correct direction even when sun or stars are not visible. For example,
caged European robin shows restlessness in the correct compass direction without visual
cues. Wiltschko placed Helmholtz coils around the cage to alter the direction of magnetic
north, and the robins altered their preferred direction accordingly.
Experiments in which magnets or Helmholtz coils are placed on the heads of pigeons
showed that they use a magnetic compass in homing, but only if the sun is not visible.
Rather little is known about the magnetic sense in animals. Some birds appear to have two
magnetoreceptors of which one is associated with the visual system. It is hypothesised that
this one provides information on compass orientation, and a second type provides
information on location (map sense). This is possible as the field intensity and orientation of
the magnetic field lines vary systematically around the globe, and birds can detect this (they
use an inclination compass rather than just a polarity compass such as those usually used
by humans).
The magnetic sense of animals is not well understood. It is probable that birds have
magnetoreceptors containing crystals of magnetite, each of which acts as a tiny magnet, and
as the crystals twist to align themselves with the earth’s magnetic field they stimulate
sensory cells associated with them.
Odour map
The use of an “odour map” as a navigation aid by birds is controversial and relates mainly to
homing in pigeons. Earlier work showed that pigeons which could not smell (e.g. nostrils
plugged) were not good at homing when released at an unfamiliar site. These studies were
criticised – perhaps the treatment reduced the pigeons’ motivation to fly. When the wind
(with its associated odours) was experimentally reversed at the pigeons’ home loft, birds
that were released at a distance from home flew in the opposite direction to home. It is
suggested that the birds learn the smells associated with home as well as foreign odours
carried by the wind
Which sense is used depends on age and circumstances
Many bird species have several systems for orienting and the use of available cues follows a
hierarchy
Young pigeons:
- magnetic compass (exclusively on 1st flight)
- learn to use other cues (sun compass by 3month)
