64 EGYPTIAN BIRDS 
sides or by the river’s edge, but if any of my readers 
will endeavour to build up a nest with such mud 
against an upright wall, they will attempt an all 
but impossible task, for as the curve begins to grow 
outwards it will with its own weight fall away from 
the wall. What is it, then, that the Swallows and 
Martins do to make their nests adhere? If you 
examine an old last year’s nest and try and break 
the outer shell, you will find it very tough consider- 
ing the material it is made of, and the toughening 
matter is a secretion of saliva. In the case of some 
species of Swallows this secretion is so great that 
the whole of the nest is made of that substance 
alone, with the lining of a few feathers. And it is 
this nest, cleaned of all foreign matter which is the 
base of the much-esteemed delicacy known as bird- 
nest soup. Few who have partaken of this luxury 
are perhaps aware that it is simply solidified saliva. 
Of Martins there are two—the House-Martin 
and the Sand-Martin, both birds common to Great 
Britain. Of the latter, literally thousands and 
thousands will be seen nesting in colonies in the 
mud banks by all who go up and down the river ; 
restless and cheerful, they are one of the welcome 
sights of the Nile trip, and often for miles at a 
stretch the whole banks are honeycombed with 
