AuvcustT 16, 1918] 
ceptional when some are not to be found in 
fall and winter in favorable localities in our 
latitude. A comparatively small number are 
seen returning the following spring, often ac- 
companying flocks of north-bound warblers. 
This past season is remarkable in that prac- 
tically none came south. Two circumstances 
that have come to my attention have a signifi- 
cant bearing on this fact. In late winter at a 
locality on Long Island where the species can 
generally be found and it was absent, chicka- 
dees were observed feeding on pine seeds, one 
of the nuthatch’s favorite foods, showing that 
it would have fared well had it been present. 
Also, in the preceding summer, observers who 
visited the southern edge of its breeding range 
found it unusually scarce. 
We may place the movement of the red- 
breasted nuthatch with what I shall call 
centrifugal migrations. Species which possess 
such, of which there are a number of good 
examples, periodically attain a great abun- 
dance in their permanent range, and then sweep 
outward, as it were in waves. For purposes 
of discussion I will mention three other types 
of birds. The white-breasted nuthatch which 
occupies a broad area to the south of its red- 
breasted cousin, has, so far as is apparent, no 
migration. The song sparrow, although al- 
ways present over a large part of its range, 
has a very definite intraspecific migration, and 
many species have what I shall call a centrip- 
etal migration, that is, they return from the 
distant south to breed each year in a definite 
northern area usually unsuitable to their per- 
manent occupancy. 
The centrifugal type of migration is notable 
for two things, its futility and its wasteful- 
ness. By futility I mean that species do not 
seem to increase their permanent range by 
that method; rather, periods of abundance and 
migration are followed by periods of scarcity 
even in that range. Data enough has been 
gathered to at least partially explain this. 
Individuals swept south by the wave seem not 
to have the definite migration instinct which 
causes centripetal migrants to return to their 
- identical nesting localities over zones of lati- 
tude. One or more instances are at hand of 
SCIENCE 
169 
the red-breasted nuthatch remaining to breed 
in southeastern Massachusetts where (in the 
Cape Cod region) it becomes especially numer- 
ous during its incursions, perhaps from the 
abundance of its favorite pine seeds. Yet it 
is probably too much of a wanderer ever to 
establish a permanent colony there, even if 
the environment were satisfactory. Probably 
the majority of any wave of centrifugal mi- 
grants is utterly dissipated and lost, and a 
small minority find their way back to their 
permanent range. In fact there is little to be 
said in favor of centrifugal migration except 
that it is expedient, in fact the exigencies of 
the case may demand it. 
I will now enter a little further into the 
realms of hypothesis and present the most 
plausible view of the sequence of migrations. 
The centrifugal condition is the original one, 
with the species in a state of unstable abun- 
dance, followed by the elimination of centrif- 
ugal migrants and the permanent resident 
condition typified by the white-breasted nut- 
hatch, where the species is sufficiently ad- 
justed to conditions to maintain itself in un- 
varying though comparatively small numbers. 
The migratory tendency now begins to express 
itself in a definite way among the individuals, 
many of which have definite breeding and 
winter localities, the former perhaps (in the 
case of the song sparrow) in some garden, the 
latter in some swamp. The tendency is for 
these two localities to become separated by 
greater and greater distances of latitude until 
we have a well-marked intraspecific migration. 
As this process goes forward the range of a 
species may well break in the middle, leaving 
a centripetal migration in which highly devel- 
oped homing instincts in the individual bird 
take the place of the futile centrifugal “ wan- 
derlust” of the race in its initial condition. 
The maximum ability to colonize and expand 
would come with the stage in which the in- 
dividual had a definite migratory instinct to 
adjust to the season and yet was sufficiently a 
permanent resident to “hang on” in a good 
locality against adverse circumstances, a con- 
dition to my mind approximated among famil- 
iar species by the song sparrow, which has at 
