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 is 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
arc 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 afield, and at intervals soars up vertically, singing, and, at
the highest ascending point, flight and song end in a kind of aerial
somersault and vocal flourish at the same moment.
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