72 VESPERTILIONID^— NYCTALUS 



temperature, and other atmospheric conditions, rather than 

 the actual hour of sunset, probably determine the time at 

 which the Bats emerge, and the duration of their flight. Rain, 

 if not heavy, does not incommode them whilst feeding, but 

 if the night be cold and windy few or none will be seen. It 

 is probable that individual Noctules do not always resort to 

 the same den throughout the summer, for the numbers which 

 emerge in the evening are not constant, and even on con- 

 secutive evenings, when the atmospheric conditions appear to 

 be identical, the number sometimes varies considerably. On 

 the other hand, it is possible that on some evenings the whole 

 strength of the colony does not turn out, and that some of the 

 Bats remain in the den all night." 



Mr Oldham found that the time of first appearance varied 

 from twenty-eight minutes after sunset on 21st April to seven 

 minutes before sunset on 14th August. The hour of vespertinal 

 emergence thus becomes progressively more diurnal as autumn 

 approaches, until in the second week of September I have 

 observed them to appear at about 6.30 p.m., and the October 

 and November flights are, according to Mr Millais, all rather 

 diurnal than vespertinal. "The Bats," writes Mr Oldham, 

 " leave the den in rapid succession — on August 27th twenty- 

 two emerged within a minute — but their return is much less 

 regular. This is probably due to the varied success of 

 individuals in obtaining food. During August and September, 

 at any rate, on fine still evenings, the duration of the vesper- 

 tinal flight is sometimes less than an hour ; on August 27th 

 a Bat entered the den fifty-five minutes, and on Sept. 4th fifty- 

 seven minutes, after the first had emerged. On each evening, 

 however, the flight in some cases lasted at least an hour and 

 a half; and on Sept. 5th some of the Bats were absent for more 

 than two hours. It is probable that on wet and windy evenings 

 the duration of the flight is even less than an hour, but I have 

 no data to prove this. It is easy to count the Bats as they 

 leave their den in the twilight, but a difficult matter to make 

 sure of the number that return. They do not often enter the 

 hole immediately on their arrival, but dash round and among 

 the trees, and in many cases pitch several times for an instant 

 on the tree-trunk near the hole. Their advent is proclaimed 



