RELATIONS OF BIRDS TO PARASITIC INSECTS. 87 
up the order Diptera, to which. belong the tachina parasites (such as 
were found in the stomach of one blue grosbeak), the data now avail- 
able show that probably less than 3 percent of the food of birds in 
general consists of members of this order. And there is no reason to 
believe that parasitic forms of this or other orders are specially sought 
for by birds. Since the dipterous parasites are confined to three or 
four families of the three score in the United States, it is not likely 
that the part of the 3 percent of bird food they furnish results in 
much injury to agriculture. 
Turning to that most important group, the Hymenoptera, it is to 
be noted that many of them are small, some being extremely minute 
(as is the case with a large group of the most effective parasites, the 
chalcis flies), and these are very rarely found in bird stomachs. A 
few, families of Hymenoptera (Chrysidide, Trigonalide, Sapygide, 
and Mutillide) are characteristically parasitic on other and useful 
members of the order. The destruction of any of these by birds (the 
cuckoo-flies are taken, at least occasionally) is a benefit. The same 
must be said of the capture of many species belonging to other fam- 
ilies which are guilty of the same practice. They parasitize their 
beneficial relatives, often destroying a large proportion of them. 
Moreover, even the species usually beneficial do not always stick to 
their rdles, but sometimes parasitize species having the same as their 
own normal relation to the host. All of these things tend to mitigate 
the injury done by birds that feed on Hymenoptera. The facts at 
hand show that, excluding ants (which are in no way concerned with 
parasites), Hymenoptera compose not more than from 3 to 5 percent 
of bird food in general. Of the insects composing this percentage, ac- 
cording to our identifications, nonparasitic forms outnumber the para- 
sites in the proportion of about 4 to 1. Hence at most not more than 
1 percent of the food consumed by all the members of our avifauna 
consists of parasitic Hymenoptera, taken as free-moving forms. 
Proceeding now to the main point at issue, whether great numbers 
of parasites in the egg or larval state are eaten by birds along with 
the usual prey, the following statements can be made. The parasites 
in insect eggs (which include very many of the large superfamily 
Proctotrypoidea, and some others) are absolutely safe from the ma- 
jority of birds, only the smaller titmice, warblers, etc., searching much 
for eggs. Parasites also in the cocoons and chrysalids are compara- 
tively secure, as only few birds consume these objects. The parasites 
of all minute insects, like egg parasites, are exempt from danger, ex- 
cept from a certain contingent of the smaller birds. The parasites 
living upon medium-sized caterpillars, beetle larve, and adult in- 
sects, however, run considerable risk of becoming bird food. But 
even members of this group are in a measure favored, since a good 
proportion of the caterpillars or other insects they infest are not 
