152 



The Canadian Field-Naturalist 



[Vol. XXXIV. 



to the destruction of hosts. In eating the pest at 

 this time birds arc almost sure to devour even more 

 parasites than hosts and by doing so they will 

 actually help to preserve the latter by keeping down 

 the surplus and so make rcom for those that remain. 



One other pcin; m.ust be taken into consideration 

 in connection with the part birds play in suppress- 

 ing insect pests and that is while they may destroy 

 numbers of parasites in eating the hosts they must 

 necessarily prevent many of the hosts from deposit- 

 ing eggs thus enabling egg parasites and other egg 

 enemies to concentrate upon those remaining. It 

 might be contended on the other side, that birds are 

 equally apt to destroy parasites in consuming insect 

 eggs such blame being especially aimed at the 

 Chickadee but I doubt very much whether the few 

 useful insects destroyed in this way could compare 

 with the value done by the birds in destroying 

 thousands of insect eggs. Further, we must re- 

 member that many insect eggs are placed in the 

 ground or in crevices, etc., where birds cannot get 

 at them but where parasites can. 



As a further point in the birds favour it may be 

 pointed cut that parasites are only present within 

 the bodies of their hosts for a limited period of the 

 hcsts' life and, therefore, by eating the host before 

 the latter becomes infested, birds are of unquestion- 

 able value to man; moreovei, by this means they 

 provide for a concentration of parasites upon the 

 hosts that survive. 



It will be noted that I have written nothing about 

 hyperparasites in this paper, that is parasites which 

 infest parasites. These complicate the whole ques- 

 tion but to include them would not, I think, show 

 birds in an unfavourable light. 



Turning now to the part which birds play in ac- 

 tually devouring useful insects such as tachmid 

 flies, syrphid flies, lady-beetles and other insects, 

 we find that the birds by this habit actually reverse 

 the arguments that have been used above but there 

 is this in extenuation. With the exception of those 

 I have mentioned and a few more, most of the 

 useful insects (especially parasites) are small while 

 (he noxicus ones arc more often large and so easily 

 detected. It wculd seen therefore, that far fewer 

 useful insects are taken than harmful ones and this 

 point is amply borne out by the examination of 

 bird stomachs, as a glance through the bulletins of 

 the U.S. Biological Survey will show. 



One of the strong points in favour of the doc- 

 trin': of insects ccntrolling insects is illustrated in 

 fuch pests as the hessian fly and western wheat- 

 j'em sawfly which are small and consequently little 

 p/fcct'd by birds. Naturally if these are kept in 

 check by parasites there is not much reason why 

 larger ones should not be. But the evidence is by 



ro means conclusive that they are. With the 

 hessian fly meteorological factors are of importance 

 at least in some parts of the insects' range and this 

 probably applies to the sawfly also. The relation 

 of humidity to insect prevalence is, indeed, a very 

 important question which, however, requires a sepa- 

 rate article to do it justice. 



There are unquestionably times when even 

 severe insect outbreaks are controlled locally 

 through the actions of birds, a well known example 

 of which occurred in Utah many years ago when 

 a locust infestation was cleaned up by gulls. We 

 need not, however, go so far afield for similar evi- 

 dence of the usefulness of gulls. 



During the years 1919 and 1920, a serious out- 

 break of grasshoppers, formerly called locusts, oc- 

 curred in scuth-west Manitoba which threatened 

 large areas of growing grain and required the 

 united efforts of government officials and farmers 

 to keep it in check. This outbreak extended from 

 Saskatchewan far to the eastward but in this ex- 

 tension there was a notable gap most marked in 

 the districts in which Boissevaine. Whitewater 

 and Ninga were situated. Since the soil is very 

 similar over all this territory and offers equal in- 

 ducements for grasshopper breeding the absence of 

 the insects over it in destructive numbers might 

 seem rather extraordinary, but I believe can be ex- 

 planed as follows: North of the villages mentioned 

 above is a large marshy lake upon which a great 

 many gulls and terns congregate and doubtless 

 breed. In any case the birds make this lake their 

 resting place and from it issue forth each day in 

 quest of food. In the spring time before the small 

 hoppers appear, the gulls may be seen in close at- 

 tendance of the ploughman when they are often 

 accompanied by black terns and frequently by 

 crows and blackbirds all of which vie with each 

 other in picking up the grubs and other insect life 

 exposed by the plough. Later, when summerfallow- 

 ing is under way and hoppers have attained suffi- 

 cient size to be seen easily the gulls again devote 

 much of their time to following the plough only now 

 they spread out further afield and obtain a glorious 

 feast of the grasshoppers which are endeavouring 

 to make their way from the ploughed land to new 

 feeding grounds. 



At a still later date when harvest is beginning, 

 the gulls and their allies take to the grain, fields and 

 roadways wandering up and down as if they im- 

 agined the waving grain were water and the grass- 

 hoppers the small fry swimming near the surface. 

 But be that as it may the results are much the 

 same. Many millions of grasshoppers have been 

 eaten by the time the gulls take their departure and 

 incidentally the farmer has reaped a far larger 



