8 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
parent birds (rarely both) frequently returns and nests in the same 
tree, bush, or box that held its nest in the previous season (fig. 1). 
One ordinarily thinks of the world of birds as quiescent during two 
seasons each year, at nesting time and in winter. For individual 
species this is obviously the case, but when the entire avifauna of the 
continentis considered 
itis found that there 
are at almost all pe- 
riods some latitudinal 
movements. 
MOVEMENTS OF SPECIES 
AND GROUPS 
Some species begin 
their fall migrations 
early in July, and in 
some parts of the coun- 
try distinct southward 
movements can be 
detected from then 
until the beginning or 
middle of winter. For 
example, many shore 
birds start south in the 
early part of July, 
while the goshawks 
(Astur  atricapillus), 
snowy owls (Nyctea 
nyctea), redpolls 
(Acanthis linaria), 
Bohemian waxwings 
(Bombycilla garrula), 
and many others do 
not leave the North 
until forced to do so 
= tedees ae ; sem by theadventofsevere 
FIGURE 1.—The bluebird may return regularly year after year to nest = 
in the same hole or box that was occupied in previous seasons, winter weather or by 
lack of the customary 
food. Thus, an observer in the northern part of the United States 
may record an almost unbroken southward procession of birds from 
midsummer to winter, and note some of the returning migrants as 
early as the middle of February. Purple martins (Progne subis) 
have been known to arrive in Florida late in January on their way 
north, and the northern movement may continue among late arrivals 
into the first week of June. In some species the migration is so 
prolonged that the first arrivals in the southern part of the breeding 
range will have performed their parental duties while others of that 
species are still on their way north. 
A study of these facts indicates that sometimes there exists a very 
definite relationship between what we may term northern and 
southern groups of individuals of the same species. A supposition, 
on which additional banding work is expected later to give definite 
facts, is that in the case of some species that have an extensive lati- 
