INTRODUCTION. 9 
grain fields gave way to extensive orchards, which gradually crept 
up the hillsides and into the canyons, other species of birds began 
to utilize the new kinds of food and also the safe nesting sites 
afforded by orchard trees. Species that previously attracted little 
attention soon increased in numbers because of the increased food 
supply, ‘additional facilities for nesting, and the protection afforded 
by man, who killed or drove away their natural enemies. Asa result, 
some of them suddenly became of great economic importance, owing 
to their increased numbers and destructive habits. 
MIGRATION OF BIRDS. 
Owing to its extent and varied topography, California is rich in 
birds, both in species and individuals. Here altitude and topog- 
raphy, as well as latitude, govern climate. This fact leads to many 
peculiarities in distribution and complicates the study of birds in 
their economic and other relations. The movements of birds, too, are 
more complex than in the eastern part of the United States. The 
regular migration north in the spring and south in the fall, which is 
the rule over the greater part of the country, is here supplemented, in 
the case of many species, by a migration from the mountains, where 
they breed, to the valleys, where they winter. Besides the regular 
migrations, at times remarkable incursions of a single species take 
place. Such was the flight of mountain tanagers Women ludo- 
viciana) in the valleys in May, 1896. In several parts of California 
these birds appeared in immense numbers in localities where pre- 
viously they had been rarely observed. Their appearance coincided 
nearly with the ripening of the cherry crop, to which in some places 
they did much damage in spite of the fact that great numbers of 
them were shot. 
CAUSES OF DEPREDATIONS BY BIRDS. 
The failure of customary food supply sometimes leads birds to 
forage upon crops which they do not commonly eat. This may be 
the explanation of the depredations of robins in the fall and winter 
of 1900-1901, when thousands of these birds pillaged the olive 
orchards in Santa Clara Valley, the region about Santa Barbara, and 
other parts of California. In that year it was as much as the olive 
growers could do to save part of their crop. Since then no case of 
excessive loss of olives has been reported, though occasionally some 
damage has been done. 
The amount of damage inflicted by birds upon a crop often depends 
upon the surroundings. In the case of orchards in the midst of a 
treeless plain depredations are mostly confined to such birds as nest 
in them, but they may be visited and damaged by others during 
