Principios de Taxonomia

78j 4 What are Traits in Taxonomy?
As a consequence, some species carry species-specific recognition signals only in
the overlap region and lack these signals where they do not meet each other. A welldocumented
example for this is the European species pair the Pied Flycatcher
(Ficedula hypoleuca) and Collared Flycatcher (Ficedula albicollis) (Saetre et al.,
1997). The Pied Flycatcher is widely distributed throughout central and northern
Europe to western Siberia, while the Collared Flycatcher is predominantly restricted
to the southeast of Europe.
Because of the great resemblance of both species, there is a problem of correct
partner choice, which relatively often results in species hybridizations (Chapter 6). In
the Flycatcher example, it becomes apparent that the species identification traits used
by us humans are not the same as those traits that birds recognize and distinguish
themselves. The recognition trait relevant to us humans is the eponymous white
collar of the Collared Flycatcher males that the Pied Flycatcher lacks. However, the
birds obviously cannot use this trait for species discrimination. The female flycatchers
use another trait for recognizing the correct sexual partner. Pied Flycatcher males
are pale brown colored on the top and distinguishable from the Collared Flycatcher
males, which are colored deep black on the dorsal side. Experiments have shown that
this plumage coloration trait is indeed used as species recognition trait by the Pied
Flycatcher females to correctly recognize their males and to distinguish them from
the Collared Flycatcher (Saetre et al., 1997).
Interestingly, in the vast regions of Western and Northern Europe where the Pied
Flycatchers occur alone, the gray plumage coloration does not occur. There the males
of the Pied Flycatcher are colored pitch-black similar to the Collared Flycatchers and
are accepted by female Pied Flycatchers because confusion with the wrong species is
impossible. In these regions, the Pied Flycatcher females do not need to distinguish
their males from those of the Collared Flycatcher. Consequently, the species
recognition trait of the pale brown back is missing there. The species recognition
trait of a pale brown back as opposed to a black back in the case of the Pied Flycatcher
has apparently developed under selective pressure to facilitate species recognition for
females only in the regions where overlap occurs.
A similar example of this phenomenon occurs in the species pair of the Western
Rock Nuthatch (Sitta neumayer) and Eastern Rock Nuthatch (Sitta tephronota) (Vaurie,
1951). The Western Rock Nuthatch occurs in southeastern Europe and southwestern
Asia, the Eastern Rock Nuthatch lives more to the east in Iran and further east into
Pakistan. Both species overlap in Iran, and only there do the two species differ
distinctly in some traits. In the regions of exclusive occurrence, the two species are
nearly indistinguishable. However, in the overlap region, the Eastern Rock Nuthatch
is larger than the Western Rock Nuthatch and most importantly has a larger and
longer beak. It is likely that the difference in body size and beak form in the region of
joint occurrence of the two species prevents their merging because it serves both
Nuthatch species as a species recognition trait for a correct partner choice. In
addition, the difference in beak size in the overlap region seems to lead to different
food preferences, thus avoiding competition. This does not make sense in places
where both species do not occur jointly and thus do not have to compete with each
other.