INTRODUCTION. 
xiii 
them to fearch for and find their concealed food. To thefe 
tribes belong the Crane, the Heron, the Bittern, the Stork, the 
Spoonbill, the Woodcock, the Snipe, and many others. 
Without the means of conveying themfelves with great fwift- 
nefs from one place to another, birds could not eafiiy fubfift : The 
food which Nature has fo bountifully provided for them is fo ir- 
regularly diftributed, that they are obliged to take long journies 
to diftant parts in order to gain the neceffary fupplies ; at one 
time it is given in great abundance ; at another it is adminifter- 
ed with a very fparing hand ; and this is one caufe of thofe mi- 
grations fo peculiar to the feathered tribe. Befides the want of 
food, there are two other caufes of migration, viz. the want of 
a proper temperature of air, and a convenient fituation for the 
great work of breeding and rearing their young. Such birds as 
migrate to great diftances are alone denominated birds of paf 
fage ; but moll birds are, in fome meafure, birds of paffage, al- 
though they do not migrate to places remote from their former 
habitations. At particular times of the year moll birds remove 
from one country to another, or from the more inland diftri&s 
toward the fhores : The times of thefe migrations or flittings 
are obferved with the moll aftonifning order and pun&uality ; 
but the fecrecy of their departure and the fuddennefs of their 
re-appearance have involved\ the fubjeff of migration in general 
in great difficulties. Much of this difficulty arifes from our not 
being able to account for the means of fubfiftence during the 
long flights of many of thofe birds, which are obliged to crofs 
immenfe trails of water before they arrive at the places of their 
deftination : Accuftomed to meafure diftances by the fpeed of 
thofe animals with which we are well acquainted, we are apt 
to overlook the fuperior velocity with which birds are carried 
forward in the air, and the eafe with which they continue their 
exertions for a much longer time than can be done by the 
ftrongeft quadruped. 
Our fwifteft horfes are fuppofed to go at the rate of a mile 
in fomewhat lefs than two minutes, and we have one inftance on 
record of a horfe being tried, which went at the rate of near- 
