This is a two-stage field puzzle. You'll begin by walking to the posted coordinates and collecting some information:
A - the number of different bird species that use Willamette Valley oak cavities
B - the number of birds pictured
You can then use one of three methods to find the final cache location.
Method 1 - Calculate A-(Bx4) and subtract that number from the last three digits of the North coordinates. Then find A-(Bx5) and subtract that number from the last three digits of the West coordinates. Use the new coordinates to find the cache.
Method 2 - Multiply A by 3.75, then walk that many feet south along the trail and look left.
Method 3 - Multiply A by 2. Then walk that many paces south along the trail and look left. (If you have a long stride, you may want to subtract a couple paces.)
Enjoy!