Principios de Taxonomia

5.20 The Morphs of the Brood-Parasitic Cuckoo Female Cuculus canorusj125
combined the descendants of common ancestors into a group; he referred to species
under this classification as synepigonic species. This was, roughly speaking, the
anticipation of the cladistic species concept (Hennig, 1966) (Chapter 7).
5.20
The Morphs of the Brood-Parasitic Cuckoo Female Cuculus canorus
During the breeding season, the Eurasian Cuckoo (Cuculus canorus) ranges throughout
Europe and as far as Eastern Asia. Its most striking trait is brood parasitism. The
cuckoo is unable to build nests, to incubate its eggs or to raise its young. Instead, it
lays its eggs in the nests of alien bird species, with these being nearly exclusively
members of the suborder Passerines (Passeriformes). These host birds generally do not
recognize the fake brood. Rather, the host bird incubates the cuckoo s egg and feeds
the fledgling until it is fully fledged and no longer depends on the foster parent s care.
The cuckoo is extraordinarily relevant to the discussion of the species concept, as
every cuckoo female is genetically adapted to a specific host bird with respect to a large
number of traits. Every cuckoo female has only a single species of host birds in whose
nests it lays its eggs. A robin cuckoo lays its eggs only in the nests of robins. The
cuckoo female does not change hosts, even if it cannot locate any nests of its specific
host during the egg deposition period. The female offspring of a robin cuckoo lays its
eggs only in the nests of robins again in the following spring.
Just in Europe alone, there are approximately 90 different species of host birds to
which cuckoo females are adapted (Glutz von Blotzheim, 1994). Accordingly, there
are 90 genetically different cuckoo populations. However, not all species of host birds
are parasitized by cuckoos with the same frequency. Some host species are visited by
cuckoo females especially frequently; other species are visited by only a few cuckoo
females. This is an example of the equal distribution of the alleles in a polymorphic
population. Moreover, there are still other species of host birds for which the cuckoo
offspring has only a small chance to fledge successfully. There are many additional
species of host birds in Asia, bringing the total number of cuckoo host species and
cuckoo populations to over 200 (del Hoyo, Elliott, and Sargatal, 1997).
The strong adaptation to only one species of host birds is determined by numerous
heritable traits. The cuckoo female must be able to lay its eggs at the correct time.
European Robins (Erithacus rubecula), Black Redstarts (Phoenicurus ochruros) and
thrushes (Turdus spec.) are frequently visited hosts in Central Europe and breed at the
end of April. In contrast, reed warblers (Acrocephalus spec.) breed only at the end of
May. The cuckoo must be hormonally adapted to very different egg deposition times;
its eggs must be ready for deposition neither too early nor too late.
The cuckoo must locate and recognize the nest of the correct host bird. To this end,
it can at best rely only on memories from the previous year, when it was raised in such
a nest. However, except for certain reminders of its own place of birth, it of course
never received parental instruction regarding (1) how to locate a suitable breeding
biotope, (2) locating a potential host bird s nest, be it in a tree, in low shrubbery or on
the ground, or (3) distinguishing such a nest from the nests of other hosts. Cuckoo