CHECKS UPON INCREASE OF USEFUL BIRDS. 361 
sprayed. Mr. Robert Ridgway noticed that birds decreased 
very much in numbers in a section of Illinois where practi- 
cally all the farmers began spraying their orchards ; but in a 
recent letter he expresses some doubt as to whether spraying, 
or a bounty crusade against the Sparrows, caused the dimi- 
nution of birds. The reduction of birds in such cases may 
perhaps be explained by the fact that the insects had been 
destroyed by spraying, leaving the birds without food. Mr. 
William Brewster has observed two instances where the 
spraying of shade trees caused a sudden decrease in the 
numbers of birds, and in both cases several dead birds were 
found. The stomachs of some of these birds are to be ex- 
amined for traces of arsenic, and this ought to determine 
whether they have been poisoned. Birds usually reject 
sickly insects,.and would not be likely to eat those which 
showed the effects of poison; but gipsy caterpillars will 
carry more poison in proportion to their size than would 
kill a man, and they will still appear healthy. It has been 
noticed in some cases that birds have avoided trees that 
have been sprayed with arsenate of lead, but in other cases 
they have not. This subject should be further investigated. 
THE NATURAL ENEMIES OF BIRDS. 
Under nature, the indigenous natural enemies of birds 
cannot be regarded as the cause of any material reduction in 
the numbers of the smaller species. Under man’s rule, how- 
ever, the conditions may be so changed that certain natural 
enemies of birds may become positively harmful. For this 
reason, if for no other, the bird protectionist should care- 
fully study the effect produced upon birds by their enemies. 
Any natural enemy of birds which becomes unduly numer- 
ous may prove seriously restrictive to their increase, and 
may require severe checking. 
Foreign species introduced and liberated in a new country 
may constitute a serious danger to bird life. Still, many 
people have deliberately introduced mammals and birds from 
other countries and liberated them here. Forturiately, per- 
haps, few of these attempts to saddle foreign species upon 
us have proved successful. There can be but little objection 
