LIFE AND BEHAVIOR OF THE CUCKOO 
221 
bears it stealthily away, and probably drops it at some distance 
from the nest, but owing to the wooded character of their sur- 
roundings this could not be determined. Toward the close of 
nest-life, the timidity of the parents seem to rise, and the cleaning 
instinct certainly wanes. Sacs are then allowed to remain on the 
nest, which at an earlier period would not have been tolerated for 
a moment (see fig. 11). This characteristic has been often no- 
ticed in other species. 
The rapidity with which the process of digestion proceeds in 
the young bird is remarkable. The sac of a nestling cuckoo which 
muted four hours after birth (figs, h and c), already contained the 
residue of digested insect-food. This was a small elastic, trans- 
parent bag, with tenaceous wall closed on all sides, and filled with 
fluid and solid substances. It was odorless, and like a rubber 
water-bottle could be picked up and rolled about without break- 
ing or soiling the fingers. When punctured a clear liquid came 
out, leaving a milk-white residue, with streaks of dark masses, 
more or less involved in the mucous, and composed of the hairs, 
scales and chitinous fragments of insects. The milk-white sub- 
stance, which dries to an impalpable powder, is not affected by 
acids or chloroform, but has the appearance of an emulsion being 
composed of very fine globules. 
The removal of egg-shells or of addled or broken eggs is probably 
to be referred to the cleaning instinct, and in some species it is 
promptly and almost invariably performed. When an egg 
hatches in the nest of the blackbill, the empty shell is not as a rule, 
immediately removed (table 6) , the instinct to brood being ap- 
parently stronger than that to clean the nest. They are seldom 
allowed to remain very long, but the shells of the last egg to hatch 
are apt to be entirely neglected. 
E. Rate of feeding young 
When behavior is normal, in those species whose young remain 
in the nest until all are ready to leave, the rate of feeding tends 
to increase from the time of hatching until it reaches a certain 
average, to which it may hold for many days with surprising 
