BIJRDS— NIDIFICATION. 
295 
unfolded to us, and this from the circumstance of its not hatch- 
ing its own eggs, which, instead of being incubated in the usual 
way, are deposited in mounds of mixed sand and herbage, and 
there left for the heating of the mass to develop the young, 
which, when accomplished, force their way through the sides of 
the mound, and commence an active life from the moment they 
see the light of day . 1 
Sir George Grey measured one of these mounds, and 
found it to be 6 forty-five feet in circumference, and if 
rounded in proportion on the top (it being at the time 
unfinished) would have been full five feet high.’ The heat 
round the eggs was taken to be 89°. 
A curious aberration of the nest-building instinct is 
sometimes shown by certain birds — particularly the com- 
mon wren — which consists in building a supernumerary 
nest. That is to say, after one nest is completed, another 
is begun and finished before the eggs are laid, and the 
first nest is not used, though sometimes it is used in pre- 
ference to the second. 
As showing at once the eccentricity which birds some- 
times display in the choice of a site, and also the deter- 
mination of certain birds to return to the same site in 
successive years, I may allude to the case published by 
Bingley, of a pair of swallows which built their nest upon 
the wings and body of a dead owl, which was hanging 
from the rafters of a barn, and so loosely as to sway about 
with every gust of wind. The owl with the nest upon it 
was placed as a curiosity in the museum of Sir Ashton 
Lever, and he directed that a shell should be hung upon 
the rafters in the place which had been previously oc- 
cupied by the dead owl. Next year the swallows re- 
turned and constructed their new nest in the cavity of the 
shell. 2 * 
The following is quoted from Thompson’s 6 Passions of 
Animals,’ p. 205 : — 
The sociable grosbeak of Africa is one of the few instances 
of birds living in community and uniting in constructing one 
1 Gould, Birds of Australia, vol. ii., p. 155, where see for further de- 
scription. 
% Animal Biography, vol. ii., p. 204. 
