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 w?s 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 sufficient, 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 dep=nded 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 ]?ugonm californ[ca or on some other insect. The species of birds plainly seen to eat these butterflies were the Brewer blackbird (t?uphag'us cyanocephalus), western kingbird (Tyrannus verlicalis), and western meadowlark (Sturnella neglecta). By far the most efficient destroyer of the butterflies was the Brewer blackbird, (?ul)hagus cyanocel)halus). 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. BeZween 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 town. On different occasions this flock was seen feeding along the railroad track in a meadow, When
