[2 CIRCULAR 363, U. S. DEPARTMENT OF AGRICULTURE 
students of migration. Observations made with telescopes focused 
on the full moon have shown processions of birds. The estimate of 
one observer that birds passed his point of observation at the rate 
of 9,000 an hour gives some indication of the numbers of birds that 
are in the air during some of the nights when migration is at its 
height. While the passage of migratory birds has thus been recorded 
throughout the night, the bulk of the flocks pass during the earlier 
hours of the evening and toward daylight in the morning, the periods 
from 8 o’clock to midnight and from 4 to 6 a. m. seeming to be 
favorite times for nocturnal flight. 
It has been claimed, with some reason, that small birds migrate 
by night the better to avoid their enemies, and that most of the 
nocturnal travelers are those that are naturally timid, sedentary, or 
feeble-winged. Included in this group are not only small song and 
insectivorous birds, but also such weak fliers as the rails, as well as 
the wrens, the small woodland flycatchers, and other species, which, 
living habitually more or less in concealment, are probably much 
safer. making their long flights under the protecting cloak of dark- 
ness. This cannot fully account for the nocturnal habit, however, 
since among the night migrants are the snipe, sandpipers, and 
plovers, birds that are generally found in the open and are among 
the more powerful fliers, some of them making flights of more than 
2,000 miles across the ocean. Such exceptionally long flights, of 
course, require both day and night flying. 
Night travel is probably best for the majority of birds, chiefly 
from the standpoint of feeding. Digestion is rapid in birds, and 
yet the stomach of a bird killed during the day almost always con- 
tains food. To supply the energy required for long flight, it is 
essential that food be obtained at comparatively short intervals, the 
longest of which in most species is during the hours of darkness. 
If the smaller migrants were to make protracted flights by day they 
would be likely ‘to arrive at their destination at nightfall almost 
exhausted, but unable to obtain food until the following morning 
since they are entirely daylight feeders. This would delay resump- 
tion of flight and result in great exhaustion or possibly even death 
were they so unfortunate as to have their evening arrival coincident 
with unusually cold or stormy weather. Traveling at night, they 
pause at daybreak and devote the entire period of daylight to alter- 
nate feeding and resting. This permits complete recuperation and 
resumption of the journey at nightfall. 
Many species of wading and swimming birds migrate indifferently 
by day or by night, as they are able to feed at all hours and are 
not accustomed to seek safety in concealment. Some diving birds, 
including ducks that submerge when in danger, sometimes travel 
over water by day and overland at night. The day migrants include, 
in addition to some of the ducks and geese, the loons, cranes, gulls, 
pelicans, hawks (fig. 7), swallows, nighthawks, and the swifts (fig. 6), 
all strong-winged birds. The swifts, swallows, and nighthawks 
(sometimes called bullbats) feed entirely on flying insects, and use 
their short, weak feet and legs only for grasping a perch during 
periods of rest or sleep. Thus they feed as they travel, the circling 
flocks being frequently seen late in summer working gradually south- 
ward. Years ago, before birds of prey were so. thoughtlessly 
slaughtered, great flocks of red-tailed hawks (Buteo borealis), Swain- 
