44 The Bird 
Mother Nature about the ground-nesters that she puts 
strength and vigor into the coverts, or upper feathers on 
the little wings; so that these shoot forth with an energy 
far beyond what is usual, for a time lending their aid in 
flight, although they are not true primaries. Later they 
Fic. 27.—Iridescent feather from the breast of a Rufous Humming-bird, showing 
wearing off of the tips of the barbs, caused perhaps by rubbing against the 
petals of flowers. Magnified 25 diameters. 
are far outgrown by the flight primaries, and then func- 
tion only as protectors of these more important feathers. 
The extreme in this precocious development of chicks 
is found in those strange Australian birds, the mound- 
builders, which are left from the first to shift for them- 
selves; even the duties of incubation being shirked by 
the parents. This necessitates a perfect ability on the 
