The opposing theory is simpler in some respects and supposes that
the ancestral home of all birds was in the Tropics and that as all
bird life tends to overpopulation there was a constant effort to
seek breeding grounds where competition would be less keen. Species
that strove for more northern latitudes would be kept in check by
the ice and forced to return southward with the recurrence of winter
conditions. As the ice retreated, vast areas of virgin country became
successively suitable for summer occupancy, but the winter habitat
remained the home to which the birds returned after the nesting
season. It is a fact that some species spend very little time on
their breeding grounds; the orchard oriole (_Icterus spurius_),
for example, spends only 2½ months in its summer home, arriving in
southern Pennsylvania about the first week in May and leaving by the
middle of July.
Both theories assume that migration is an ingrained habit, but
neither is supported by positive biological data. Both have been
criticized also on geological grounds, and neither can be accepted
without qualification. It is apparent, however, that whether the
ancestral home of any species was at the northern or at the southern
limits of its present range, or even in some intermediate region,
the search for conditions favorable for breeding in summer and for
feeding in winter has been a principal factor underlying the origin
of migration.