100 HANDBOOK OF BRITISH INLAND BIRDS 
hood of rivers and streams. It can easily be 
distinguished from the two other members of the 
family by its considerably smaller size, feebler, 
fluttering, almost butterfly-like flight, and the 
ashy-brown, mouse-like colour of its upper parts. 
Its wings and tail are somewhat darker ; the under 
parts are white, except for a brownish band across 
the breast. It breeds in strong colonies, often con- 
taining dozens or even hundreds of nests, which 
are situated at the end of long holes or tunnels 
excavated by the birds in the sandy face of a pit, 
quarry, or railway cutting, or on the bank of a lake 
or stream. In exceptional cases they will utilise 
holes in masonry or crevices in old ruins, and have 
even been known to burrow into the decayed 
wood of a pollard willow-tree. The burrow slopes 
slightly upwards for a distance of between two and 
four feet, not always in a straight direction, and at 
the end of it, in a slight enlargement of the 
tunnel, a soft, loose nest is built of straws, lined 
with feathers. The birds do not generally begin 
their excavations much before the middle of May, 
and about the end of that mouth four to six pure 
white eggs are laid, pointed in shape, and with 
shells so thin and clear, when fresh, that the yolk 
gleams through them with a beautiful tinge of 
golden pink. This tint vanishes after a few days' 
incubation, when the eggs grow dull and opaque in 
