Fred the Red Shirt schreef
If you want to maximize the strength to weight ration then saw the
tree into lumber and construct box beams for your posts. Try sug-
gesting that over on misc.rural. Heck you can drill a little hole
in the side and they can double for birdhouses.

+ + +
Given the typical size of posts they had better be small birds.
Also pretty industrious to fill it all up to just below the hole!

Wouldn't a cross be stronger than a box?
(The birdhouse could be on top)
PvR