Principios de Taxonomia

5.12 Partially Migratory Birds – an Example of Genetic Polymorphismsj111
however, sedentary animals of the same species remain in their breeding habitat
during the winter. At least in the example of long-distance migratory birds, this
coexistence of migratory and sedentary birds is not due to voluntary decisions of the
individual birds but is genetically programmed (Berthold and Querner, 1981; Sutherland,
1988). The two populations of the same bird species are genotypically different.
The migratory disposition of most birds, that is, their motive for leaving their
breeding habitat, is not triggered by seasonal climatic changes, at least in the case of
long-distance migratory birds. Only for so-called cold fugitive species
( K€altefl€uchtlinge ) are decreasing temperatures or food shortage the immediate
triggers for the migration. Among these species are certain European birds, which
generallymigrateonlyshortormediumdistances,forexample,manyducks,geeseand
swans (Anatidae) and some herons (Ardeidae). Several individuals of these species
remain in their breeding habitat in the fall or pause in stopover regions on their way to
thesouthwestofthebreedinghabitat.Theselatterindividualsremaininthesestopover
regions until frosts and food shortages cause them to migrate further southwest.
In the case of most long-distance migratory birds, however, it is not simply food
shortages or the encroaching cold that causes the animals to leave their breeding
habitat for southern or western climes. These migratory birds leave their breeding
habitats during high or late summer, when the temperatures and food supplies are
still optimal. Consider the European Swift (Apus apus), which leaves central Europe in
mid July, when airborne insects are still available in sufficient amounts. These birds
do not migrate because certain living conditions have worsened or because they are
starving or cold. Instead, these migrations are controlled by the decreasing length of
the day. This migration behavior has a genetic basis. The birds become restless due to
an internal clock, not because of external compulsions.
The genetic disposition for migratory or sedentary behavior likely requires the
cooperation of several genes. Nearly nothing is known regarding these genes (Helm,
2009). However, in most cases, the genetic differences between migratory and
sedentary birds are likely not differences in structural genes but rather (1) differences
within regulatory elements (enhancers) of structural genes and (2) tissue-specific
activities of certain transcription factors (Chapter 4).
Migratory and sedentary birds are distinguished by a multitude of traits. A
migratory bird requires additional instincts. In migratory birds, genes are active
that control the instinctive migratory behavior twice a year at the proper time; these
genes are inactive in sedentary bird populations. Migratory birds also require a
different metabolism and anatomy.
In contrast to sedentary birds, migratory birds become restless in the late summer
and fly continuously in a particular geographic orientation. The direction of migration
and the migration distance are genetically determined. This is known because
many songbirds migrate at night without parental guidance. These birds must locate
the migration paths themselves, without the help of other individuals. Small songbirds
are short-lived and can hardly benefit from the experience of the previous year.
Instead, the bird must know instinctively whether it must migrate in the southwest or
southeast direction. The bird must also know when it has arrived in the wintering
grounds. A migratory bird must be genetically programmed to know how many days