I'm the opposite of Wizardgoat. I split large yew logs as soon as they're cut, and let the splits follow the grain. Then I seal the ends of each stave, and seal any areas where the bark has been removed by accident, and let them sit in a nice tight pile somewhere airy for about 12 months.
That's when I rough them out and get them down to almost bow dimensions, before leaving them another 12 months. Never had them warp or twist on me doing this, but part of me thinks that splitting avoids this from happening. By sawing, you're chopping right through the natural movement of the log and it's gonna want to move around as it dries.
Still, either way you do it works fine, and once you've got your stave dried out nicely after a year or more you can start really going at it.
The woodpecker holes would worry me if they're going through sap into heartwood. Yew can take a hell of a lot of abuse, rookie mistakes and clumsiness, rot, twist, knots, holes and so on, but that many deep violations through the back of the bow would definitely be a cause for concern in my book. The only thing you can do however is make a bow and see what happens. No point worrying about it until you know what it means.
