What does Darzee's wife do that is sensible in "Rikki-tikki-tavi"?
The text of "Riki-tiki-tavi" accredits Darzee's wife with being "sensible" in relation to her understanding that cobra eggs today means deadly cobras later on. Because of her "sensible" understanding, Darzee's wife has determination that Nag and Nagaina's eggs should not survive, and she "flew off from the nest" to be able to do something about it. Her sensible understanding is also important later when Nagaina is escaping to her lair.
Darzee was a feather-brained little fellow .... But his wife was a sensible bird, and she knew that cobra's eggs meant young cobras later on;
The text also accredits Darzee's wife, who is never given a name, with doing something that is "wiser." It is certainly "sensible" to do something that is "wiser."
But Darzee's wife was wiser [than Darzee]. She flew off her nest as Nagaina [the she-cobra] came along, and flapped her wings about Nagaina's head. If Darzee had helped they might have turned [the cobra]; but Nagaina only lowered her hood and went on. Still, the instant's delay [provided by Darzee's wife] brought Rikki-tikki up to her, and as she plunged into the rat-hole where she and Nag used to live, his little white teeth were clenched on her tail, ....
Riki-tiki and Nagaina are engaged in life-or-death battle on the verandah because he challenged her to a battle, saying, "Fight, widow! The big man has gone for his gun! Fight!" after he saved the boy from Nagaina's death blow. Riki-tiki exerts his energy and strength to stay away from her dangerous, poisonous head. In the heat of battle, he forgot about the one remaining egg, and she sees it. In one enormous lunge, while Riki-tiki catches his breath, she grabs her egg and flees like a "whip-lash flicked across as horse's neck."
Darzee is still on the nest, singing his "foolish little song of triumph" over Nag's death, but his wife is wiser and understands the great meaning of Nagaina's escape with one sound egg. Because of her sensible understanding, she attacks Nagaina with her wings, delaying the cobra's escape just long enough for Riki-tiki to catch up and grab ahold of the cobra's tail with his teeth.
The sensible thing Darzee's wife does is help Rikki-tikki destroy Nagaina's eggs.
Darzee isn't entirely sure about the idea of destroying all of Nagaina's eggs. He has some sympathy for the eggs because his species uses eggs to reproduce, too.
On the other hand, Darzee's wife knows each of the cobra's eggs has a chance to grow up and be a dangerous predator against man and animal alike.
Darzee's wife then flew within earshot of Nagaina and began faking an injured wing. Nagaina can't resist the easy target, and she begins slithering after Darzee's wife. Darzee's wife keeps the act up while moving away from Nagaina. Nagaina is now completely focused on Darzee's wife and never once considers that she is being tricked. The distraction technique allows Rikki-tikki time to find the eggs and destroy all of them except one.