528 



The Scarcity of Swallows. 



[Sept., 



the most part during the day, and no doubt refresh themselves 

 with such insects as cross their path during the flight, but cold 

 and wet weather must take a heavy toll of travel -weary birds, 

 as a result of exhaustion from want of food. 



Conditions differ somewhat in spring and autumn, hi spring, 

 the birds tend to follow the lines of the isotherms as the season 

 advances ; consequently disaster is less likely to overtake the 

 main body of migrants through unexpected cold. Exceptional 

 weather in May, however, may kill off many birds soon after 

 their arrival. In autumn, an early break up of the weather may 

 cause considerable losses among the young of the year making 

 their first journey south. In this connection, it is probable that 

 food supply and not temperature is the real crux of the matter, 

 as frequently the birds travel overland at least under severe con- 

 ditions, in wind and driving rain, at times when the real urgency 

 of departure due to cold does not press upon them. 



It has become increasingly evident that migrants (particularly 

 the species under consider ation") do not in general on arrival 

 (-occupy the first suitable area they come to in their journey, but 

 pass on to the particular place which was their nesting ground in 

 the previous year, or, in the case of young birds, the scene 

 of their upbringing. Further, there is every reason to believe 

 that the same wintering areas are occupied from year to year, 

 and that a constant route is traversed between the summer and 

 winter quarters. Admittedly these are matters hard to prove, 

 but the repeated return has been demonstrated frequently in 

 the case of the swallow and house martin, and the probability of 

 the succeeding ideas seems reasonable. 



Decrease of the Swallows. — The question of the gradual de- 

 crease of the swallows is a complex one, and the decrease is prob- 

 ably not due to immediate causes but may extend back over many 

 years. We are recording to-day the effect of some change in 

 conditions which has reacted unfavourably on the group. The 

 last tliirty years or so have been marked by several remarkable 

 extensions of breeding range, notably in the case of the starling, 

 the tufted duck, and the gr^at crested grebe, while other species 

 have become restricted. The great reduction in the number of 

 predatory birds and animals doubtless has had its effect in cer- 

 tain cases, but notwithstanding the study which has been given 

 to these variations, very little indeed has been elucidated as to 

 their real causes. We have only to consider the vast changes 

 which have been made in the conditions of this country — the 

 draining and improvement of agricultural land, the development 



