^8 

 YEAR 



Territorial 66, 

 Trelease Woods, Illinois" 



I I I I I I I I I I I I I 



FIG. 17-2 Yearly variations in populations of the house wren. 

 The data for northern Ohio are for total males and females on 

 a 15-acre estate, most of the males being banded and captured 

 at their nest-boxes. The data for Trelease Woods, in central 

 Illinois, are for territorial males censused on a 55-acre tract by 



the spot-map method, tvlean temperatures are for the winter- 

 ing range only, based on monthly weather data for Tampa and 

 Jacksonville, Florida; Savannah, Georgia; Montgomery and 

 Mobile, Alabama (Kendeigh and Baldwin 1937, Kendeigh 1944). 



in this curve occur in 1918, 1926, 1940, and 1958. 

 Information from observers indicated that low popu- 

 lations were widespread in eastern North America 

 both in this species and in many other song and game 

 birds during these years. There is considerable evi- 

 dence that these conspicuous variations in abundance, 

 as well as some less pronounced, were the result 

 of severely low winter temperatures. It is of interest 

 that, in England, severe winters causing high mor- 

 tality among such song birds as thrushes, blackbirds, 

 and tits were recorded in 1111, 1115, 1124, 1335, 

 1407, 1462, 1609, 1708, 1716, 1879, 1917 (Elton 



1927). To birds that feed on the ground, the depth 

 of snow is as critical a factor as low temperature in 

 determining the number that survive. 



Among mammals, fluctuations in the population of 

 the common hare in Denmark have been correlated 

 with the varying effects of summer rainfall, spring 

 temperatures, and the number of days of frost during 

 the winter (Andersen 1957). 



Catastrophes may occur with practically any type 

 of animal life. The severe winter of 1917-18, for 

 instance, produced a marked reduction in the num- 

 bers of many species of marine invertebrates in the 



39 '41 '43 '45 '47 



WINTER ('29 = 1928-29, etc.) 



FIG. 17-3 Fluctuation In the average snow depth In the winter 

 In southwestern Finland and population fluctuations of the par- 

 tridge during the autumns following. The dotted line shows the 



depth, 



Increases (soil' 

 s) in populatloi 



1956). 



des) and 

 ally occur 



236 Ecological processes and dynamics 



