Since the 1970’s studies into the genetics of
birds have shown that migration is a genetically inherited characteristic.
Various experiments have that the genetics of the individual bird appears
to control
·
The direction (compass bearing) of migration
·
The basic urge to migrate which can be selected
for, to produce a migratory population (or against to produce a resident
population).
·
The duration of the migratory behaviour, which we
can equate with the distance travelled.
Experiments with blackcap hybrids showed that when
two populations were hybridised the offspring migrated in a direction
intermediate to the parental populations
The European blackcaps migrate over a 1400
angle to arrive in the Mediterranean. A
small number of birds have sporadically migrated to England – requiring an
additional 300 angle. This
can be explained in terms of a distribution curve with the majority of blackcaps
being within the 1400 degree angle of migration but a few individuals
extending that angle. With harsh
winters in England they have never really thrived but now a respectable
population of several thousand is developing.
What has caused the change? The
answer may lie partly in milder winters, improved habitat in the UK, along with
the British love of animals - the garden bird table. The net result is a growing population with a shorter
distance to migrate and an earlier return to the breeding grounds.
Working with robins and blackcaps it has been
possible through artificial selection to produce populations which show
increased or decreased migration behaviour.
These results suggest that several genes are involved (multi-locus
inheritance).
A factor sometimes overlooked is the duration of
migration. Here some light has been
shed by breeding experiments which show that there is a variation in the
duration of migration caused by inherited genetic factors.
This means that natural selection can not only select the direction but
the approximate distance to be migrated.
When considering migration behaviour, is it really
all down to genetics? The answer is
no. To find the genetic component
it is necessary to use young birds which have not made the migration previously.
The experiment with starlings conducted by
showed that young birds were using their genetic compass while older
(experienced) birds modified this with experience.
When flown to Switzerland the young birds continued on a compass bearing
while experienced birds altered course to reach the wintering grounds they had
previously visited.
Artificial
selection can increase and decrease the sedentary/migratory nature of a
population
Hybrid
birds show intermediate duration of the migratory behaviour (this would
influence the distance they would migrate)
Now that so much is known about migration in birds, is it possible to predict how migration has developed? With a knowledge of the genetics involved we can predict that a semi-resident population of birds could easily develop into a truly migrant population if a severe winter were to kill off the non-migrants. Similar disasters at the migrants winter destination could tip the scales in favour of the residents.
The
study of genetics has also shown that there are a variety of routes which can be
selected for genetically, as well as distances. This helps to explain the
growth in the UK population of European blackcaps. This genetic variation
is very important in allowing birds to cope with changes in habitat, food supply
possibly even continental drift.