re: Bird's Mouth Joint

Duck, if I taper it first, I would have to use a taper jig to cut the bird's mouth because it slides along the fence to make the two cuts.  As soon as the sides are not parallel, I think that the accuracy may be more difficult to achieve too because the distance from the fence is one of the variables to sneak up on the right fit.  If I was making a bunch of these, a dedicated jig might be the way to go though.  I will probably just cut the ends off after I taper it using a miter gauge on the table saw.  The ends should technically also be beveled too to make it flat on the top and bottom after glue up.  

Split, The 8 sided one is super easy because it is based upon a 45° blade angle which is perfect for what I am going to build.  I have not looked to see what blade angle the other number of sides require.  In this case at least, I won't need to get a bird's mouth router bit.