Principios de Taxonomia

6.11 Are Migratory and Sedentary Birds Able to Crossbreed?j145
range and breed in the north of Europe or Asia, as well as the south. In the south and
also in the west, for example, in the Mediterranean region and in Ireland and Great
Britain, a sufficient food supply is often available throughout the entire year. For this
reason, the birds can stay in the breeding habitat in winter and do not need to give up
their territories, remaining sedentary. Populations of the same species, however, also
breed in the north and in the east. Here, food shortages in winter would cause the
animals to starve if they did not leave their breeding grounds during the winter
months. These are migratory birds.
Most bird species, however, do not leave their breeding grounds when food
becomes scarcer, but depart significantly earlier. Most migratory birds leave Central
Europe in July, August or early September, when food is still abundant. Thus, they
reach their distant winter quarters in due time. From this observation, it follows that
the migratory behavior is not controlled environmentally by hunger or cold, but by
genetic factors that respond to day length, for example. The birds are not free to
decide whether they will migrate in fall or stay at home; instead, they are controlled by
their genes. At least in the case of long-distance migratory songbirds that often are not
guided by their parents during migration, all traits that distinguish migratory from
sedentary birds must be genetically controlled.
The differences between migratory and sedentary birds are enormous. As in the
other cases in which many traits distinguish different populations of a species,
the question arises as to whether these traits are controlled by just as many
genes (Helm, 2009). Regarding the many differences between migratory and
sedentary birds, it is difficult to understand how these organisms would be able
to crossbreed. If there are many genes involved, it can hardly be imagined that
the sedentary populations of a bird species would be able to crossbreed with
the migratory populations of the same species because the genetic incompatibilities
of the different alleles should be too large. The alleles for migratory or sedentary
traits would recombine in the hybrids and lose their genetic linkage. The offspring
of such crosses would show a mixture of traits belonging to migratory and
sedentary birds.
Indeed, experimental crossings have shown that hybrids from migratory and
sedentary birds exhibit properties that are intermediate in regard to the different
traits (Pulido et al., 2001; Helm, 2009). The many crosses between migratory and
sedentary birds show that at least some traits that distinguish migratory from
sedentary birds were inherited in an intermediate manner (Berthold and Querner,
1981; Helm, 2009). The hybrids resulting from such crosses have some alleles for
migratory behavior as well as some alleles for sedentary behavior. They obviously are
hampered in fertility and fitness.
The crosses between migratory and sedentary birds have, however, been conducted
under captive conditions. The generation of phenotypic intermediates
appears to contradict the realities seen in nature, where environmental influences
interfere with genetic constitutions (Helm, Fiedler, and Callion, 2006). In nature, the
clinal transitions between migratory and sedentary birds appear to contain few
intermediates. Instead, birds that leave the breeding grounds in fall live alongside
birds that stay behind during winter.