Page:Popular Science Monthly Volume 77.djvu/95

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The struggle of conflicting instincts is clearly shown when the normal cycle is brought prematurely to a close by the rise of the instinct of migration, when eggs or young are left in the nest to perish.

 (Dendroica Æstiva), with a "parasitic" cowbird's egg in each compartment, but with proper eggs in the first story only, illustrating the successive breaking of the breeding cycle through fear, the beginning of three new cycles in succession, the new nest being built in each case, and probably through association, on the site of the old, thus admirably "concealing" the successive parasitic eggs. Original in the Field Museum of Natural History, Chicago.

The migratory impulse seems to "overlap" and finally to replace the proper parental instincts. The cycle is scamped near its close.