844 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1919. 
phenomenon of westward extension in the bobolink, which species 
rigidly adhered to its ancient migration route though adding 1,000 
miles to its line of flight. 
Gradual expansion to the north can be found in the case of the 
greater spotted woodpecker in Great Britain and in the case of 
Passer moabiticus moabiticus. This latter bird, formerly confined 
to the south end of the Dead Sea, is now commonly found in the 
Jordan Valley at the north end of the Dead Sea and will doubtless 
extend to Galilee. 
Eastward expansion, though the example must be taken from out- 
side the Palaearctic region, is well illustrated by the gray parrot 
in equatorial Africa. This bird, formerly unknown much east of 
Uganda, has rapidly extended its range across the Mau Plateau and 
Rift Valley, and will ere long find itself on Mount Kenya and thence 
to the east coast of Africa. 
Gradual contraction of range from natural causes may be due to 
meteorological or climatic conditions. Gaetke (Birds of Heligo- 
land) quotes the erosion of the Heligoland cliffs as partly destroying 
the breeding haunts of the guillemot and razorbill. A cyclone in 
Mauritius almost exterminated the local species of martin. The 
sudden rising of water on an artificial lake in Baluchistan completely 
destroyed many dozens of nests of a grebe, together with many hun- 
dreds of their eggs, and the whole colony of breeding birds moved 
that night and have not since returned to that lake as a breeding 
species. 
Or contraction may be due to inability to establish a migratory 
habit, which we see after severe winters among some of our own 
resident forms; or to an insufficiently developed migratory habit, as 
with certain communities of redwings, fieldfares, and starlings, who 
perish in the south of England and Ireland in very severe weather 
rather than continue their passage to southwestern Europe, as do 
other communities of the same species who have developed an in- 
creased migratory line of flight. 
Or contraction may be due to expansion in range of some other 
species which becomes an evicting factor. The jackdaw is believed 
to have been largely responsible for driving the chough from the 
cliffs of southern and western England. The house sparrow, in ex- 
tending its range in Russian Turkestan, has supplanted the tree spar- 
row and has evicted the house and sand martin from many nesting 
haunts in England. The puffin has replaced the Manx shearwater 
in some of the islands of the inner Hebrides. 
Food supply will also contract the range of a species, though this 
is usually only a temporary inconvenience. 
Gradual contraction among nonmigratory species will eventually 
produce interrupted distributions, extermination, or isolation. Of 
