Principios de Taxonomia

122j 5 Diversity within the Species: Polymorphisms and the Polytypic Species
Although this observation first appears to support the position that timidity is a
result of varying experiences with humans, many data also support a predetermined
genetic disposition. A newly hatched chicken can distinguish between a sparrow and
a harmless dove by their flight silhouettes without having learned of this difference
from their parents or other conspecifics. A sandpiper (Calidris spec.) from northern
Siberia that arrives in the Wadden Sea off the German North Sea coast in August and
which was born during the same summer can distinguish a human or a dog from a
sheep grazing on a dyke without having had previous experiences with humans, dogs
or sheep.
Advocates of the position that birds timidity toward humans is solely based on
negative experiences do not have arguments for why the European White Stork
(Ciconia ciconia), within living memory, has been fearless of humans, while its close
relative, the Black Stork (Ciconia nigra), is extremely fearful of humans. In several
breeding areas, both species are not persecuted, and persecution on the migration
routes and in the hibernation regions is not different between White Storks and
Black Storks.
When the cities that were built over the past century or more began covering
increasingly large areas, surpassing rural areas and forests with food availability and
protection against predators, several bird species moved from their ancestral
habitats. Birds moved into cities to an increasing extent, where they found a larger
food supply and better protection from predators. Examples of species that moved to
the cities include the Grey Heron (Ardea cinerea), several duck species, the Coot
(Fulica atra), the Moorhen (Gallinula chloropus), the Woodpigeon (Columba palumbus),
the Mistle Thrush (Turdus viscivorus) and the Carrion Crow (Corvus corone).
Remarkably, the Woodpigeon still maintains its name from past times, although it
has conquered the cities and urban regions in most of Europe and no longer primarily
occupies forests. The urbanization of the Woodpigeon occurred in northwest
Germany in the 1950s and 1960s. Before this time, this pigeon was a shy forest
inhabitant. Among hunters, the saying this is a woodpigeon signified a special
honor, as a successful shooting of a Woodpigeon signified rarity and the hunter s
ability. Today, Woodpigeons nearly feed out of one s hand.
Yet, why did it take so long for pigeons to conquer cities? The lesser danger of being
shot in cities and a larger food supply was certainly available for a much longer time
than the last half century. Even today, there are regions in Europe in which the
Woodpigeon is not urbanized. For example, such is the case in south and southeast
central Europe, where hardly any urban Woodpigeons can be found (Bezzel and
Kooiker, 2003).
The geographical differences in the timing of urbanization of different bird
populations illustrates that this process cannot be solely based on learning processes.
Therefore, timidity toward humans is most likely based in part on a genetic
disposition, and this disposition appears to be polymorphically anchored in a
population. A genetic disposition to fearlessness would explain why this behavior
arose in different places at different times. An increase or decrease in timidity
towards humans is subject to a selective advantage. Consequently, a single or small
number of alleles can displace other alleles among a population. The clear selective