INTRODUCTION. xlvii 
Aquatic birds of several kinds, according to Humboldt, cross 
the line on either side about the time of the periodical rise of 
the rivers. Waterton, likewise, who spent much time in Dem- 
erara and the neighboring countries, observed that the visits of 
many of the tropical birds were periodical. Thus the wonder- 
ful Campanero, whose solemn voice is heard at intervals tolling 
like the convent-bell, was rare to Waterton, but frequent in 
Brazil, where it most probably retires to breed. The failure 
of particular food at any season, in the mildest climate, would 
be a sufficient incentive to a partial and overland migration 
with any species of the feathered race. 
The longevity of birds is various, and, different from the 
case of man and quadrupeds, seems to bear but little propor- 
tion to the age at which they acquire maturity of character. A 
few months seems sufficient to bring the bird into full posses- 
sion of all its native powers ; and there are some, as our Marsh 
Titmouse or Chickadee, which, in fact, as soon as fledged, are 
no longer to be distinguished from their parents. Land ani- 
mals generally live six or seven times as long as the period 
required to attain maturity; but in birds the rate is ten times 
greater. In proportion to their size, they are also far more 
vivacious and long-lived than other animals of the superior 
class. Our knowledge of the longevity of birds is, however, 
necessarily limited to the few examples of domesticated species 
which we have been able to support through life: the result of 
these examples is, that our domestic Fowls have lived twenty 
years; Pigeons have exceeded that period ; Parrots have at- 
tained more than thirty years. Geese live probably more than 
half a century ; a Pelican has lived to eighty years ; and Swans, 
Ravens, and Eagles have exceeded a century. Even Linnets, 
in the unnatural restraints of the cage, have survived for four- 
teen or fifteen years, and Canaries twenty-five. To account for 
this remarkable tenacity of life, nothing very satisfactory has 
been offered; though Buffon is of opinion that the soft and 
porous nature of their bones contributes to this end, as the 
general ossification and rigidity of the system perpetually tends 
to abridge the boundaries of life. 
