20 USEFUL BIRDS. 
birds eat a great many caterpillars containing parasites, 
though birds will reject any caterpillars that show signs of 
weakness or disease. The question then arises, Is the bird 
doing harm by eating caterpillars or other larve containing 
parasites? The bird certainly ends the destructive career 
of the larva at once. The parasites would have ended it 
eventually ; but had it been left to them, it might have gone 
on for some time in its destructive career, doing as much 
injury as if not parasitized ; the parasite merely destroys it 
in time to prevent it from propagating its kind. So far the 
evidence is in favor of the bird. The. question remains, 
-however, whether the bird and its young would eventually 
destroy more caterpillars than would the progeny of the 
parasites had they not been eaten by the bird. This question 
evidently is unanswerable. Birds act as the primary check 
on the increase of destructive insects; parasitic insects are 
the secondary check provided by nature to operate in con- 
junction with the birds, or to supplement the regulative 
action of birds where the number of birds is insufficient to 
check the increase of insects. 
‘Birds sometimes kill many of the imagoes of parasitic 
insects in flight, where such insects are numerous. At first 
sight, this would seem to condemn the birds; on further 
study, it seems probable that this is often a harmless habit. 
Where parasitic insects are found in great numbers, it is 
probable that the birds destroy mainly the surplus flies, 
which otherwise, failing to find hosts for their young, would 
merely live out their time and die without issue were they 
not killed by the birds. Such harm as birds do in killing - 
primary parasites may be offset by the killing of secondary 
parasites by birds, for this acts as a protection to the pri- 
mary parasites. 
Certain predaceous bugs feed not only on insects but also 
on vegetable food. They also attack other predaceous or 
useful insects. Birds, by preventing their undue increase, 
may prevent excessive injury to both useful plants and 
insects. 
All reasoning from known premises leads to one conclusion 
