56 CIRCULAR 363, U. 8S. DEPARTMENT OF AGRICULTURE 
shared by some of the gulls also, although here the evidence is not so 
conclusive. Herring gulls (Larus argentatus smithsonianus) banded 
as chicks at colonies in the Great Lakes have scattered in all directions 
after the breeding season, some having been recovered well north in 
Canada. 
These movements may be considered as migration governed only by 
the availability of food, and they are counteracted in fall by a directive 
migratory impulse that carries back to their normal winter homes in 
the south such birds as after the mating period have attained more 
northern latitudes. They are not to be compared with the great 
invasions of certain birds from the North. Classic examples of the 
latter in the eastern part of the country are the periodic flights of 
crossbills. Sometimes these migrations will extend well south into the 
Carolinian Zone. 
Snowy owls are noted for occasional invasions that probably are 
caused by a shortage of the lemmings and rabbits that constitute their 
normal food in the North. At least nine notable flights of these birds 
occurred during the period 1876 to 1927. In the great flight of 1926- 
27 they were noted as far south as Iowa, Ohio, West Virginia, and 
North Carolina. 
In the Rocky Mountain region great flights of the beautiful Bo- 
hemian waxwing (Bombycilla garrula pallidiceps), are occasionally 
recorded. The greatest invasion in the history of Colorado orni- 
thology occurred in February 1917, at which time the writer estimated 
that at least 10,000 were within the corporate limits of the city of 
Denver. The last previous occurrence of the species in large numbers 
in that section was in 1908. 
Evening grosbeaks (Hesperiphona vespertina) likewise are given to 
performing more or less wandering journeys, and curiously enough, in 
addition to occasional trips south of their regular range, they t ravel 
east and west, sometimes covering long distances. For ecm, 
erosbeaks banded at Sault Ste. Marie, Mich., have been recaptured on 
Cape Cod, Mass., and in the following season have been retrapped at 
the banding station. Banding records demonstrate that this east- 
and-west trip across the northeastern part of the country is sometimes 
made also by purple finches (Carpodacus purpureus). 
PERILS OF MIGRATION 
The period of migration is a season full of peril for birds. Untold 
thousands of the smaller migrants are destroyed each year by storms, 
in unfamiliar habitats, and through attacks of predatory birds, 
mammiais, and reptiles. If each pair of adult birds should succeed in 
raising two fledglings to maturity, the population of migratory birds 
would have a potential annual increase of 100 percent and the world 
would soon be heavily overpopulated with them. It is evident, 
therefore, that there is no such increase, and that the annual mor- 
tality from natural causes is heavy enough to keep it in check. 
STORMS 
Of the various factors limiting the abundance of birds, particularly 
the smaller species, storms are the most potent. Special sufferers 
are those birds that in crossing broad stretches of water are forced 
by a storm down within reach of the waves. Such a catastrophe was 
