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 caterpillar or 
the pupa. With this in view, almost half of the adults of Eugonia califoniica 
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 w’ork 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 w^eek, 
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 in 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 e.xtraordinary 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, Eugonia califoniica , 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 OE 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, show^ed the following results: 
1. The insect which became a pest was a butterfly, Eugonia califoniica, 
the larval form of which feeds upon snow brush or buck brush ( Ceanot/iiis cordu- 
latus, Ceanotlius velufinus ) . 
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 califor- 
nica, the Brewer blackbird {Eiipliagus cyanocephahis) , western meadowlark 
