In Jewett's "A White Heron," there is no doubt in my mind that Sylvia's decision to keep what she knows from the young man is a wise move, and it is believable because this is the kind of person Sylvia is: protecting the bird is something that comes to her naturally.
Sylvia spends a great deal of time out of doors. When she returns home through the woods with the cow...
... their feet were familiar with the path.
Sylvia's grandmother is quite aware of the child's penchant for being outside, far from the manufacturing town in which she had once lived. Mrs. Tilley notes of her granddaughter:
... there never was such a child for straying about out-of-doors since the world was made!
Early on in the story, the author makes it clear that without playmates, Sylvia spends a great deal of time entertained and amazed by nature and its secrets. The child's pleasure is evident (and the author's imagery abounds) as the reader sees just how comfortable Sylvia is in this new world to which her grandmother has brought her—a place of safety from which the child notes she never wants to leave.
Sylvia stood still and waited, letting her bare feet cool themselves in the shoal water, while the great twilight moths struck softly against her. She waded on through the brook as the cow moved away, and listened to the thrushes with a heart that beat fast with pleasure.
Sylvia's delight in the natural world around her is evident. The moths do not annoy her, the sound of the birds quickens her heartbeat, and even Miss Mooley the cow doesn't aggravate her—for in the cow, the little girl imagines the animal is slow to be found because she is playing hide-and-seek.
When the young man shows up, the author has already provided us with characteristics of Sylvia: she loves this world she has discovered living with her grandmother. Jewett also delivers information that places "Sylvy" at the center of an enormous dilemma. The grandmother tells the young man (who is there to hunt animals for his collection) that...
... the wild creaturs counts her one o' themselves. Squer'ls she'll tame to come an' feed right out o' her hands, and all sorts o' birds.
Immediately he knows that if anyone can help, it's Sylvia. There are two reasons, however, that Sylvia struggles with telling the man where the bird can be found. First, he announces that if someone were to help him get the bird, he would give ten dollars—a king's ransom for people such as Sylvia and her grandmother. Sylvia does know about the heron—where it nests. Hearing his words, Sylvia's heart beats wildly.
The second reason that causes Sylvia difficulty is her love of animals and his casual approach to killing them.
She could not understand why he killed the very birds he seemed to like so much.
Sylvia enjoys the man's company. And the family could use the money, there is no doubt, but Sylvia chooses not to tell the young man about the bird. When the time comes to tell her secret, the child cannot:
... Sylvia cannot speak; she cannot tell the heron's secret and give its life away.
It is her loyalty that compels her to remain silent, and I agree with Sylvia's choice—that her loyalty is much more valuable than a collection of dead, stuffed animals that the ornithologist gathers simply to please himself—robbing the natural world of the presence of such beautiful creatures.