308 
BIRD-LIFE. 
saliva.* The Rock Swallow ( Cotyle rupestris ) places its 
nest, which differs but little from that of the last-named 
species, under shelter of over-hanging rocks. The 
House Martin attaches its nest to the outer walls of 
houses or the face of a rock; it is almost semi-spheroidal 
in form, with the small entrance-hole at the upper edge. 
The Alpine Swallow ( Cecropis rufula ), whose nest is, in a 
similar manner, stuck on to the side of a rock, lengthens 
the entrance with a long pipe or tube. The Swifts 
employ the saliva, which the Swallows use to glue their 
materials together, as a chief material in itself in the 
construction of their nests. The Common and Alpine 
Swift carry dry grass, feathers, and plant-down, into 
crannies in walls and rocks, lay them down, and 
cover them with a layer of saliva, which forms the 
whole into a hard mass when dry. The Pigmy Swift 
(Cypshims), by means of the saliva, plasters the cotton¬ 
wool of different plants in clefts of trees, forming a 
shallow mould, the sides and bottom of which are also 
plastered in a similar manner. The Tree Swift (. Dendro- 
chelidon ) chooses thin isolated branches on the tops of 
trees, where to build its disproportionately small nest, 
the walls of which are composed of feathers, lichen, 
pieces of bark, &c., mixed with hardened saliva, the 
whole substance being scarcely thicker than parchment, 
and so very fragile that they can hardly bear the weight 
of the sitting bird, and oblige it to hold on to the branch. 
Lastly, the Edible-nest-Swallow ( Collocalia nidified) uses 
no other material than its own saliva: its nest, so 
highly prized by gourmands, is wholly composed of 
this substance, which resembles a strong solution of 
gum arabic, and is so plentifully and rapidly secreted 
* With us, as is well known, the Swallow often builds in chimneys.— W, J. 
