Page:Condor13(6).djvu/39

 Nov.,1911 RELATION OF BIRDS TO AN INSECT OUTBREAK 207 struction of larva and pupa, but also to parasitism. After the butterfly is hatched there is usually but a slow decrease throughout the winter. From this it will be seen that the butterfly has a far greater chance to survive than the caterpi. llar or the pupa. With this in view, almost half of the adults of P-Jugonia calzfornica can be counted on to survive until egg-laying time. A much smaller percent of larvae or pupae could be counted on to survive till this stage owing to the greater death-rate. Any destruction of the butterfly, therefore, is an attack on the insect at a critical period in its life history. Consequently the work of five species of birds at this critical point might be more important as a check on the increase of the insect than the work of many more species during the larval and pupal stages. It appears also that in this particular case birds are among the very few natural checks on the butterfly, whereas parasites as well as birds probably play an important part as checks on the insect in its larval and pupal stages. If we consider the work of one Brewer blackbird, its value as a check becomes apparent. Suppose that one of these birds having fourteen hours a day in which to feed, takes an average of one butterfly a minute for eight hours out of the fourteen. Judging from observations made, this would not be extraordinary. By the end of the day it would have consumed 480 butterflies, by the end of the week, 3360, and by the end of a month, over 100,000. If, say, a third of the butterflies destroyed were females, probably a larger percent are females, the numbers of eggs so destroyed would number near 336,000. Such computations as this are of some- what doubtful value for they often seem so exaggerated that in the mind of the reader, the real facts are discounted. Its use here is simply to give some idea of the extent to which a bird might act as a check and probably did act as a check in this instance. One of the most striking things brought out in the investigation was the great difference iu the food habits of the red-winged and the Brewer blackbirds. Whereas the Brewer was found to feed almost entirely on the pests, the red-wing apparently paid no attention to the extraordinary abundance of insect food. The results of investigation show that a good percentage of the birds larger than the Say phoebe fed on the butterfly, Fugonia cahfornica, and this was without doubt a factor in the reduction of the insects. A comparison of birds taken at Sisson before the butterflies became abundant with those taken at the time of the investigation proved the fact that the birds varied their food ratios and took advantage of the abundant supply of this particular insect food. SUMMARY OF RESULTS The investigation instituted by the State Fish and Game Commission into the relation of birds to an insect outbreak in northern California during the spring and summer of 1911, showed the following results: 1. The insect which became a pest was a butterfly, Eugonia californica, the larval form of which feeds upon snow brush or buck brush (Ceanolhus cordu- lalus, Ceanolhus velulinus). 2. The great number of caterpillars and butterflies and the large amount of territory covered by the plague furnished an interesting example of an insect out- break. Since the relation of birds to any insect outbreak furnishes important in- formation as to their economic value as checks, the value of an investigation into the relation of birds to this particular outbreak was evident. 3. Five species of birds were found to feed on the butterfly, Eugonia calzfor- nica, the Brewer blackbird (Euphagus cyanocephalus) western meadowlark