Principios de Taxonomia

6.10 The Adaptation of Breeding Times in Birds to the Annual Maximum Food Supplyj143
A fictive, speculative example follows: Songbirds often recognize their partners
by the quality of the song of the male. In many cases, the important components of
the song are not genetically determined; young birds must learn these components
from their fathers. If this education goes awry, the young bird may later chose a
mating partner of a different species. This behavior can erect a species barrier with no
genetic cause.
6.10
The Adaptation of Breeding Times in Birds to the Annual Maximum Food Supply
Many animals arrange their annual behavioral rhythm such that they rear their
offspring at the time of optimal food supply. Many birds can only feed their young
during a short period of time that consists of a few weeks out of the year because there
are not enough insects or other prey available during the remaining weeks.
In Europe, owls, ravens and many raptors start breeding in winter under very
unfavorable external conditions to optimally utilize the food supply for their nestlings
in the early spring; their prey is more easily found before the trees come into leaf.
Eleonora s Falcons (Falco eleonora), which breed on a few Mediterranean islands, only
begin their breeding period in the fall because they feed their young with small
songbirds. These prey, however, do not live at the falcons breeding ground, but
European and West-Asian migratory birds appear on the islands during late August
and September, where they exhaustedly take a rest during the crossing of the
Mediterranean Sea and can easily be caught by the falcons.
These examples of the unusual breeding times of some birds make clear that the
triggering signals for mating, egg deposition and the start of breeding cannot be the
optimal weather conditions, the temperature or the food supply because the most
favorable conditions do not exist during the period when courtship, nest-building and
egg formation must take place, but weeks later, when the hatchlings must be fed.
Obviously, a bird does not know at the time of nest-building and the start of breeding
what food supply will be available to it when its young will need to be fed a few weeks
later. How do the birds know in advance when this food optimum will be available?
The breeding time of most bird species is controlled in a circannual rhythm by the
day length (Helm, 2009). The day-night rhythm is the alarm clock that makes them
prepare for breeding. Courtship and mating behavior, as well as egg maturation in the
ovary, nest-building and many other behaviors are triggered hormonally. Hormonal
control is genetically anchored.
A current problem is that many bird species start to breed in different geographical
locations. At different geographical latitudes as well as on different vertical altitudes in
mountainous regions, the food optima occur at different times and are thus not at all
correlated with the same length of day and night for the birds of a given species that
breed in the north as for those in the south; the same problem applies to birds that breed
at different mountain altitudes. The north s lower temperatures enforce later breeding
times than are seen in the south, and the time of maximum insect supply is correlated
with another day-night ratio. As these differences in the control of hormonal regulation