Music and Dancing in Nature. 283 
the whole flock mad. There are also species that 
always live in pairs, like the scissors-tails already 
mentioned, that periodically assemble in numbers 
for the purpose of display. The crested screamer, 
a very large bird, may also be mentioned: male and 
female sing somewhat harmoniously together, with 
voices of almost unparalleled power: but these 
birds also congregate in large numbers, and a 
thousand couples, or even several thousands, may 
be assembled together : and, at intervals, both by 
day and night, all sing in concert, their combined 
voices producing a thunderous melody which seems 
to shake the earth. As a rule, however, birds that 
live always in pairs do not assemble for the purpose 
of display, but the joyous instinct 1s expressed by 
duet-like performances between male and female. 
Thus, in the three South American Passerine 
families, the tyrant-birds, wood-hewers, and ant- 
thrushes, numbering together between eight and 
nine hundred species, a very large majority appear 
to have displays of this description. 
In my own experience, in cases where the male 
and female together, or assembled with others, 
take equal parts in the set displays, the sexes are 
similar, or differ little ; but where the female takes 
no part in the displays the superiority of the male 
in brightness of colour is very marked. One or two 
instances bearing on this point may be given. 
A scarlet-breasted troupial of La Plata perches 
conspicuously on a tall plant in a field, and at inter- 
vals soars up vertically, singing, and, at the highest 
ascending point, flight and song end in a kind of 
aérial somersault and vocal flourish at the same 
