VARIATIONS IN THE NUMBERS OF ANIMALS 131 



distance in the community. Let us take the example of small 

 birds in England. At intervals of ten or more years (and 

 sometimes less) there have occurred in England very severe 

 winters accompanied by continuous frost and snow for several 

 months, which have killed off a very large proportion of the 

 smaller birds each time. Such winters, resulting in the death 

 on a large scale of small birds like thrushes, blackbirds, and 

 tits, are recorded as having occurred in the years mi, 1115, 

 1124, i335> 1407^ 1462, 1609, 1708, 1716,81a 1879,118 1917. 

 Undoubtedly there are many more which have escaped record. 

 In 1407 there was a very long and severe winter, with frost 

 and snow during December, January, February, and March. 

 Thrushes and blackbirds and many thousands of smaller birds 

 died from hunger and cold. In 171 6 numbers of goldfinches 

 and other species were destroyed. 



Recently there was a very severe winter in England (1916- 

 17) which caused much death among birds and lowered the 

 numbers so seriously that they have only just recovered again. 

 Actually, the death of these little birds is probably due to 

 starvation and not to cold acting directly ; for Rowan ^^ has 

 shown that a small Canadian finch (Junco hyemalis) can, if 

 provided with plenty of food, withstand blizzard temperatures 

 ranging down to —52° F. As long as there is fuel, the body 

 temperature can be kept up. It is probably the effect of 

 frost upon the rest of the environment which is the serious 

 factor in these cases. 



6. The net result of periodic bad winters is therefore a 

 periodic fluctuation in the numbers of many small birds. 

 Many species of birds vary in numbers from causes which are 

 largely unknown. Baxter and Rintoul^s have studied for 

 some years the fluctuations in numbers of breeding birds oh 

 the island of May in Scotland. They say : " Frequently it is 

 possible to explain an increase in numbers of a nesting species 

 by some change in environment, such as new plantations 

 which afford convenient nesting places. But on the Isle of 

 May no such obvious alterations in environment have taken 

 place. Nevertheless the species come and go there, they 

 increase and decrease, and the reasons are not by any means 



