1921.] 



The Scarcity of Swallows. 



529 



of industry (can we measure the bad influence of coal smoke?), 

 the changes from forest to clearings, pasture to arable and the 

 reverse — to realise that it is actually surprising that the changes 

 are so small. 



It has been established that in spite of the recollections of the 



oldest inhabitants," there has been Httle or no change in the 

 average weather conditions of Great Britain within the records 

 available ; consequently, in spite of fairly general opinion, 

 w^eather as a sole cause must be ruled out. 



The eftect of weather on average food supply is accordingly im- 

 probable, but from what has been stated regarding the homing 

 instincts of these birds, it can be seen that a series of unfavour- 

 able summers might bring about a decrease which would take 

 some years to remedy. Any lack of food reacts on the adults 

 themselves, rendering them less vigorous and causing a reduc- 

 tion in the brood, and the brood in turn is affected by the pre- 

 vailing shortage. Two or sometimes three broods are normally 

 reared in a season, and obviously smaller broods, and the possi- 

 bility that only two broods, and in some cases but one brood, will 

 get away, are conditions which would tend to result after bad 

 weather, while the condition of both adults and young will be 

 less favourable for their southward journey. Under average 

 conditions, the annual increase just about balances the appalling 

 losses of life during the remainder of the year, so that a succes- 

 sion of cold wet summers — or even one (19-20 for instance) — 

 leaves a decided mark on our bird population. This loss must 

 • be made up by the birds normally nesting in this country, since 

 the number of strangers which turn aside and become as it were 

 new settlers cannot be large. 



Further, it is possible that the gradual draining of many parts 

 of Great Britain is responsible for more permanent changes. To 

 take a definite area, the Carse of Gowrie. bordering the Tay 

 Estuary, has undergone considerable changes within compara- 

 tively recent times. At one time marshy, now well drained, 

 not so long ago one of the leading grain-gi'owing areas, now 

 largely given to grazing, even within the last twenty years the 

 reduction in numbers of swallows has continued steadily. At one 

 time, the villages were the homes of hundreds of house martins ; 

 now only a few pairs return. On the other hand, in certain of 

 the glens in the Grampians, the numbers appear to be much the 

 same as in years past. In these glens, little or no change has 

 come about in the character of the country except where hold- 

 ings have been abandoned, and on the whole, numbers seem to 

 be fairly steady. D 



