384 
BIRDS OF PASSAOK. 
migrations on the waters of the Rio Negro. When the 
Orinoco begins to swell* after the vernal equinox, an innu- 
merable quantity of ducks (patos careteros) remove from 
the eighth to the third degree of north latitude, to the first and 
fourth degree of south latitude, towards the south-south-east. 
These animals then abandon the valley of the Orinoco, no 
doubt because the increasing depth of waters, and the inun- 
dations of the shores, prevent them from catching fish, in- 
sects, and aquatic worms. They are killed by thousands in 
their passage across the Rio Negro. When they go towards 
the equator they are very fat and savoury ; hut in the month 
of September, when the Orinoco decreases and returns into 
its bed, the ducks, warned either by the voices of the most 
experienced birds of passage, or by that internal feeling 
which, not knowing how to define, we call instinct, return 
from the Amazon and the Rio Branco towards the north. 
At this period they are too lean to tempt the appetite of the 
Indians of the Rio Negro, and escape pursuit more easily from 
being accompanied by a species of herons (gavaues) which 
are excellent eating. Thus the Indians eat ducks in March, 
and herons in September. W e could not learn what becomes 
of the gavanes during the swellings of the Orinoco, and why 
they do not accompany the patos careteros in their migration 
from the Orinoco to the Rio Branco. These regular mi- 
grations of birds from one part of the tropics towards 
another, in a zone which is during the whole year of the 
same temperature, are very extraordinary phenomena. The 
southern coasts of the West India Islands receive also every 
year, at the period of the inundations of the great rivers of 
Terra Firma, numerous flights of the fishing-birds of the 
Orinoco, and of its tributary streams. We must presume 
that the variations of drought and humidity in the equinoc- 
tial zone have the same influence as the great changes of 
temperature in our climates, on the habits of animaLs. The 
heat of summer, and the pursuit of insects, call the hum- 
ming-birds into the northern parts of the United States, and 
into Canada as far as the parallels of Paris and Berlin : i' !1 
* The swellings of the Nile take place much later than those of the 
Orinoco ; after the summer solstice, below Syene ; and at Cairo in the 
beginning of July. The Nile begins to sink near that city generally 
about the 15th of October, and continues sinking till the 20th of May. 
