1921.] 



The Scarcity of Swallows. 



527 



to show a preference for damp and marshy areas, and may be 

 attracted by the wealth of insect Ufe there met with. Swifts, 

 on the other hand, are far less confined and seem to seek food 

 ever Li wide area. Also they hawk at higher elevations than the 

 ilirundines and may consequently feed on different insects. In 

 Scotland, they feed commonly over moors on the high ground 

 where the swiillow and its congeners never go, miles distant from 

 their nesting places. 



The food supply, aerial insects, is liable to considerable varia- 

 tion as a result of weather changes — high winds, heavy rain, and 

 frost. The last two, in particular, seem likely to be the most 

 potent — the rain bv " washing down," and the frost by cutting 

 down " and so killing off insect life. Consequently the group is 

 end-angered through possible food shortage as a result of these 

 weather fluctuations. 



The insect Kfe which supplies the food of these specialised 

 birds does not appear to be defined beyond " winged insects." 

 It is clear, however, that the insects themselves Avill be affected 

 by weather conditions, and that to some extent the species pre- 

 sent in any given area will be dependent on the vegetation grow- 

 ing there, and on the moisture in the soil and atmosphere. 

 Drought may cause a falling-off in the number of insects, though 

 this will affect different species in different ways, and changes 

 in the vegetation of an area — the cutting down of w'oods, or 

 even the change from arable land to pasture — will probably have 

 an effect on the number and species of the insect population. 

 Consequently the matter of food supply for the birds depends 

 on the weather and vegetation, which vary and interact the c-ne 

 on the other. 



Migration. — The four species under consideration pass through 

 the dangers of migration twice each year in their journeys to and 

 from these islands. Ever since the reality of these great flights 

 was accepted, it has been a matter of surprise that the migrant 

 species should be capable of making so prolonged an effort. 

 Recent investigations tend to overthrow the older conception of 

 a thousand miles flight in a night, but nevertheless these move- 

 ments, even if taken in stages, indicate an endurance quite un- 

 expected. In migration itself, the overseas flights are naturally 

 the most dangerous, and it is during these journeys that weather 

 changes can cause great havoc in the ranks of the travellers. 

 Weather can affect migrants in two important respects, the one 

 in regard to the actual physical effort, the other in regard to food 

 supply when land is reached. The species in question travel for 



