Yeah, picture sucks...
After a discussion with a fellow Warriors fan, I adopted her way of shipping; Birchfall and Applefur were meant to be from the beginning, we all know that ^^
Thus I also adopted (or as much as I can remember) the rest of the idea. It goes like this; Whitewing is innocent, and a good friend of Squirrelflight around the time when Ashfur gets his heart broken. The tom can't get his frustrations to end, and with how it seems he can't hurt either Squirrel or Bramble directly, he ambushes Whitewing when she's alone, since she's a dear friend. Seeing as there would only be trouble, and possible bloodshed if she said anything, Whitewing kept silent. The ambush then got her pregnant.
Meanwhile, Birchfall is keeping a secret. He missed the friendship he shared with Toadfoot and Applefur as kits, and after becoming a warrior, he met Applefur again at a gathering. They picked up, and eventually fell in love. Neither could bear to leave the Clan for each other, so they meet in secret. The only other ShadowClan cat who knows, is her brother.
Birchfall is on his way home from such a meeting when he stumbles upon Whitewing, who's greatly disturbed. She bursts out with everything, and he promises to cover her, if she'll do the same for him. Whitewing understand, and later they reveal to the Clan that they are mates.
None of this gets out until after the Dark Forest fight, when Applefur gets pregnant. Dovewing and Ivypool demand a thorough explanation...
My fellow Warriors fan and original(though a little different) story-maker; KalahariMeerkatFan!
Warriors (c) Erin Hunter