OCTOBER, 1902. 
THE MIGRATION OF BIBDS. 
189 
wore on they gradually disappeared, and upon the approach of 
spring they were nearly all gone, having been eaten by the birds. 
Upon further investigation these birds were found distributed in 
certain localities throughout the state as far north as Polk 
County, in some instances in groups of over a hundred. In the 
majority of cases the acorns were found with the birds (34). 
These cases show rather strikingly that the influence of tem- 
perature upon migratory birds cannot be universal. The fre- 
quency, however, with which like phenomena have occurred, and 
the wide range of species that have been observed under similar 
conditions, might lead us to believe that temperature, per se, has 
no influence whatever. In fact, when we consider that many of 
our warblers, fly catchers, vireos, and others, leave for the south 
at times when they might still enjoy from one to two months of 
mild weather before the approach of winter, it would seem that 
even among the more delicate species temperature has a less 
determined influence than is generally supposed. 
In the examples given above we have in each instance found 
that the locality in which these species were discovered was con- 
nected with some unusual source of food supply. The term un- 
usual should be emphasized because the phenomenon we are dis- 
cussing is an unusual phenomenon and we may expect an unusual 
cause. When two phenomena are thus uniformly associated we 
may be justified in making one the cause of the other. The im- 
portance of food as a factor in migration at once becomes evident. 
According to their food habits birds may be classified as animal- 
feeders and vegetable-feeders, according as thev show a prefer- 
ence for animal or vegetable food. From a migratory point of 
view they are classified as summer residents and winter residents, 
the former including those that migrate south at the approach 
of winter and the latter those that remain in the north throu^s^h- 
out the year. Thus, as winter residents we have the ruffled 
grouse, the crossbills, the pine grosbeak and the linnets as exam- 
ples of vegetable-feeders ; and the creepers, tits, nuthatches, 
hawks and owls as examples of animal-feeders. Among the sum- 
mer residents are the thrushes, fly-catchers, cuckoos, snipes, 
plovers, warblers, vireos, swallows, the meadow lark and the bob- 
olink as animal-feeders ; and the red-winged blackbird, pigeons, 
ducks, geese and a few of the sparrows as vegetable-feeders. It 
is a noticeable fact that in winter examples of strictly vegetable- 
34. Winkenwerder. Notes on the Winter Habits of the Red-headed 
Woodpecker. Bull. Wis. Nat. Hist. Soc, vol. 2, N. S., p. 69. 
