“‘Swifts have to be on the wing a great part of the time. They never voluntarily alight on the ground, and if by some accident they fall to earth they cannot rise again into the air without extreme difficulty, by dragging themselves up on to a little mound or by climbing with beak and claws upon a stone from which they can spread their long wings. If the ground is quite level they either lie prone on the stomach or sway from side to side with a balancing motion, or they manage to struggle forward a little by beating the earth with their wings. After repeated efforts they sometimes succeed in flying off. The earth, therefore, is to them a great danger that must be avoided with the utmost care. Any state between swift motion and absolute rest is hardly possible with them. Violent exercise in the air and perfect quiet in their place of retreat—these two, as a rule, make up their existence. The only variation known to them is to hang on to some wall near their hole and then drag themselves into the nest by clambering along with the help of beak and any chance support that they can find. Usually they enter their retreat in full flight. After passing and repassing its entrance more than a hundred times, all at once they dart in so quickly that they are lost to sight before you know whither they are gone. You feel almost inclined to believe they have vanished into thin air.’ “Their nest is nearly always placed in a deep hole in the wall and at a great height. It is made of hemp threads, little wisps of tow, bits of straw, feathers, rags, and cotton-like down from poplar and willow catkins. These materials are stuck together with the viscous saliva that constantly oozes from the swift’s throat and serves as glue to entangle captured insects. The bird spreads it over the nest and thoroughly moistens the successive layers. In drying this saliva hardens and takes on the shiny appearance of gum, giving consistency and even elasticity to the whole structure. If you squeeze the nest between your two hands it will shrink up without breaking, and when the pressure is removed it will resume its former shape.