552 MIGRATION 
Drosselzug is regarded in many places nearly as the Twelfth of 
August or the First of September is with us. In most localities 
in Britain the new comers depart after a short sojourn, and are 
accompanied by so many of the home-bred birds that in some 
parts of the island it may be safely declared that not a single 
Song-Thrush can be found from the end of November to the end 
of January, while in other districts examples can always be seen. 
Much the same may be said of the Redbreast. Undeniably resident 
asa species, attentive scrutiny will reveal the fact that its numbers 
are subject to very considerable variation according to the season 
of the year. At no time do our Redbreasts collect in bands, but 
towards the end of summer they may be seen in the south of Eng- 
land successively passing onward, the travellers being mostly if not 
wholly young birds of the year; and so the majority disappear, 
departing it may be safely presumed for more southern countries, 
since a few weeks later the markets of most towns first in France 
and then in Italy are well supplied with this species. But the 
migratory influence affects, though in a less degree, many if not 
most of the Redbreasts that remain with us. Content during the 
autumn to occupy their usual haunts, the first sharp frost has a 
decided effect upon their distribution, and a heavy fall of snow 
drives them towards the homesteads for the larger supply of food 
they find there, while should severe and long-continued hard 
weather follow even these birds vanish, leaving only the few which 
have become almost domesticated. 
These two species have been here chosen as illustrative cases 
because they are at once plentiful and familiar, and want of space 
only forbids us from citing others, but we shall find on inquiry 
that there is scarcely a Bird of the Holarctic Region, whose habits 
are at all well known, of which much the same may not be said, 
or in other words, that every Bird of the northern hemisphere is to 
a greater or less degree migratory in some part or other of its 
range. Such a conclusion brings us to a still more general in- 
ference—namely, that Migration instead of being the exceptional 
characteristic it used formerly to be thought, may really be almost 
universal, and though the lack of observations in other, and especi- 
ally tropical, countries does not allow us to declare that such is the 
case, it seems probable to be so. Before proceeding, however, to 
any further conclusions it is necessary to examine another class of 
facts which may possibly throw some light on the matter. 
It must be within the experience of every one who has ever 
been a birds’-nesting boy that the most sedentary of Birds year 
after year occupy the same quarters in the breeding-season.! In 
some instances this may be ascribed, it is true, to the old haunt 
1 Two remarkable instances of this persistency may be noticed. The nest of 
a Falcon (Falco peregrinus) on Avasaxa—a hill in Finland somewhat celebrated 
