Attaching 2 4x4 pieces of lumber end-to-end.

Probably, but I haven't actually seen such a device. Consider a lap joint which can be made with a router, and probably with a table saw.

Yes, it is called a 16 foot 2x4.

You could also probably crab together a shorter pair of 2x4s, one on either side to fix the connection. Recommend using screws rather than nails... the more and the bigger, the better. At least five per side.

You can buy flat pieces of steel with holes in them for the galvanized nails. I think they are made by Simpson - Strong Tie Connectors. The galvanized nails (also by Simpson, N10 "Joist Hannger Nails") are used with many of the Simpson type connectors; I assume you could use screws too. However, there are quite a few nail holes in these items. With a 4 x 4 doing the spanning, you are going to need posts to hold that beam up. I would put one every four feet myself. There are other simple steel ties that can make the final duck house
attachments more secure. A good old fashioned lumber yard could help with this.

Reply to Wolfi
Wolfi, I don't know if you've started your project, since I only read your post today 5/7.

However, I wonder why you are struggling to unite two 8 foot posts to create one 4" x 4" x 16 feet high? You can buy 16 foot posts, as noted in the following site example.

-----BumbleBeeBoogie

http://www.prairieforest.com/cedar.htm


WIDE ASSORTMENT OF SIZES AND LENGTHS

Cedartone Classic pressure treated wood is available in a wide assortment of popular sizes and lengths of lumber for any outdoor project.


DIMENSIONS: PIECES PER LIFT:
2x4xR/L-8' through 16'
2x6xR/L-8' through 16'
2x8xR/L-8' through 16'
2x10xR/L-8' through 16'
4x4xR/L-7' through 16' (except 9')
4x4x9'
1x6x5' & 6' S4S
1x6xR/L-8' through 16' S4S

yeh a lap joint or lap-side dado or some japanese joint would be strongest, but you will need one to be 10ft cuz youll lose about 6 inches to the joint. glue and screw.

They can deliver 16 footers you know. why go through the bother?

just re read your post. Forget what we said. Theres a joint made for purple martin boxes, in which you lay the wood end to end, then you get 2 more short pieces of wood and lay them across the ends of the other boards. You drill 3 holes through ALL the boards and put bolts or thick plugs clean through and (if bolts, you add washers and nuts, if pegs, you drill holes in the peg ends and put a cotter pin through. This is the way the Amish put up their MArtin boxes, except they use a 4X4 set of posts , a 2X4 may be a bit too thin with the duck box and the weight of ducks and nest , 16 ft in the air. What if a mighty wind blows? duck boxes can be a big sail and can crash , killimg all ducks aboard. How about double plying your 2X4s ??