21 
on the migration of birds. 
It is not at the commencement of this enlargement, nor until 
it is considerably advanced, that the birds are prompted to 
migrate ; and this is very wisely ordered ; for were they to set 
off, when first the testes and ovaria begin to grow tumid, they 
must waste much time here unnecessarily, and indeed arrive 
at too early a period to find a supply of food. Very little time 
is lost after their arrival, before they form their connubial 
alliances.* The business of nesting then begins ; and as a 
convincing proof that nesting is the chief cause of their errand 
here, this, and its natural consequences, occupy their attention 
from the time of their coming to the day of their departure. 
This is illustrated by the dispatch which some of them make 
in performing the object of their mission. The cuckoo 
its digestive powers. While the swift is feeding on small beetles that have hard 
cmstaceous wings, and whose habitations are the air, its nestlings are fed in their 
early state with gnats. The sparrow, a granivorous bird, feeds its young for several 
days after they are hatched, with the softest insects only, now and then introducing 
a little coarse sand, smooth on the surface, to inure the stomach, as I suppose, to 
bear the same kind of substances in a more rugged state, which will shortly be 
required. 
* Should a fatal accident befal either the male or female bird after this alliance is 
newly formed, no time is lost in unavailing sorrow, nor any great nicety shown in 
forming a new connection, as the following little history will evince. A pair of 
magpies began to build their nest in a gentleman’s garden at Burbage, in Wiltshire. 
Disliking their familiarity, he shot one of them from an ambush made for the pur- 
pose. The next day there were again a pair going on with the work. One of these 
was also shot. The loss was not long in repairing, for the day following the pair 
were again complete, when another fell a victim to the gun. Thus the gentleman 
went on destroying one of them daily until he had killed seven ; but all to no pur- 
pose — the remaining magpie soon found another mate. The nest was finished, and 
young ones were produced, which were suffered to fly. This is an extraordinary 
fact.— It seems to show that nature has a reserve of birds in an unconnected state, 
immediately ready to repair losses. Were the whole to pair at once, the circum- 
jacent country might be insufficient to furnish food for the immense number of 
young ones that must burst forth at the same time. 
