On what page does Atticus say, "Shoot all the bluejays you want, if you can hit 'em, but remember it's a sin to kill a mockingbird" in To Kill a...

In Chapter 10, paragraph 7, page 94 of the Warner Brothers edition, Atticus gives his children permission to shoot blue jays if they can hit them, but they must not kill mockingbirds.

Interestingly, Les Line, an avid bird watcher who quotes this passage from To Kill a Mockingbird, blames American ornithologist, naturalist, and painter John James Audubon partly for the notorious reputation that blue jays have.

His stunning plate of three glorious specimens sucking...

In Chapter 10, paragraph 7, page 94 of the Warner Brothers edition, Atticus gives his children permission to shoot blue jays if they can hit them, but they must not kill mockingbirds.

Interestingly, Les Line, an avid bird watcher who quotes this passage from To Kill a Mockingbird, blames American ornithologist, naturalist, and painter John James Audubon partly for the notorious reputation that blue jays have.

His stunning plate of three glorious specimens sucking eggs “pilfered from the nest of some innocent dove or harmless partridge” was widely reproduced on calendars handed out by insurance companies in the mid-20th century, helping to foment blue jay hatred. (Audubon)

While these smart birds know how to avoid the trip on a trap filled with sunflowers after experiencing it once, they are actually helpful to nature because they disperse acorns and beechnuts from North American forests. 

It does seem somewhat out of character for the kind-hearted Atticus, who himself is reluctant to use a gun on even a rabid dog, to condone the killing of any creature.
Of course, Harper Lee wrote her novel before blue jays fell under the protection of the federal Migratory Bird Treaty Act, and killing blue jays was probably acceptable in the culture of Southern Alabama, especially in the 1930s. Nevertheless, Atticus's words about the blue jays do seem to mitigate the sincerity of the concern for mockingbirds, those grey birds who mimic the songs of other birds prettily, but who also often mimic the sounds of insects and amphibians loudly and in rapid succession. So they are not exactly quiet themselves.

But To Kill a Mockingbird is a fictional novel, so the mockingbird makes a convenient symbol, one to which Tom Robinson and Boo Radley can be compared in their innocence. Also, it is one that Atticus can use with the children in order to teach them to be kind to innocent creatures be they bird or man.