Nov., 1911 
RELATION OF BIRDS TO AN INSECT OUTBREAK 
201 
abundance of food for either the larva or adult also has its influence. It will be seen, 
therefore, that the numbers of individuals from year to year depend on many 
factors, and that it is impossible to pick out any one as the factor. Probably it 
was a coincidence of several factors that caused the species to be so abundant 
this year. 
In the investigation two methods were used, observation in the field and ex- 
amination of stomach contents. Circumstantial evidence that a bird fed on the 
insects was not considered sufiicient, so that unless the bird was actually seen to eat 
a butterfly or unless remains of butterflies were found in the stomach, the bird was 
not incorporated in the list of species known to feed on the insect. Doubtless if 
more time could have been spent in the field and more stomachs collected, the 
number of species acting as checks would have been found much larger. It is to 
be regretted that the field investigation was not begun sooner so that the kinds of 
birds feeding on the larvae and pupae might have been determined. In the vicin- 
ity of Sisson, Siskiyou County, where, August 20 to 25, the investigation was 
carried on, the larvae had all pupated and hatched into butterflies. 
The writer is indebted to Professors C. A. Kofoid and C. W. Woodworth for 
valuable suggestions in the preparation of this paper and to Mr. J. Grinnell of 
the Museum of Vertebrate Zoology for a critical reading of the manuscript. 
Field observation can seldom be depended on to furnish information as to 
the kind of food taken by a bird. In this investigation, however, the insects 
concerned were so large that there was no difficulty in determining positively 
whether the birds were feeding on Eugonia calif ornica or on some other insect. 
The species of birds plainly seen to eat these butterflies were the Brewer blackbird 
lEtifhagtis cyanocephalus) , western kingbird {^Tyrannus verticalis) , and western 
meadowlark {Sturnella neglecta) . 
By far the most efficient destroyer of the butterflies was the Brewer blackbird, 
{Euphagus cyanocephaliis) . From early morning till evening on every day 
during my stay at Sisson, great numbers of Brewer blackbirds could be seen con- 
gregated along the damp places in the road or in the meadows where the butterflies 
gathered, busily engaged in catching these insects. It was only in the near vicin- 
ity of the town that these birds were seen, but large flocks, in many cases num- 
bering over a hundred individuals, were scattered about the small valley. Three 
particular flocks were closely watched. 
A flock of some twenty-five individuals could nearly always be found in the 
near vicinity of the depot. They spent most of their time catching butterflies 
along the track, or about the damp places in the street just north of the depot. 
Between 11 and 12 o’clock on August 20, several of these birds, feeding in the 
road, were seen to take an average of five butterflies each minute. The method of 
capture was often quite crude. The bird seldom flew after an insect but simply 
walked along and attempted to pick it up. When a butterfly flew away, the bird 
either ran after it or attempted to catch another one. Occasionally a bird succeeded 
in swallowing a butterfly whole, but more often the insect was held with the feet 
while the body was torn from the wings. In places the ground was strewn with 
the discarded wings. Several times a bird was seen to catch a butterfly only to 
have it escape a moment later badly injured. One blackbird, either having had its 
fill or being attracted by another one of the insects, was seen to crush a butterfly in 
its bill and then drop it. Doubtless, therefore, these birds killed more than they 
really consumed. 
Another large flock made its headquarters just west of towm. On different 
occasions this flock was seen feeding along the railroad track in a meadow. When 
