I used the design from Justin Rhodes at Abundant Permaculture, but I made a number of modifications (one of which is critical to address a security vulnerability and will be addressed in Part III). The basic tractor design is roughly four feet wide, eight feet long, and two feet high. It has a number of perches and two nest boxes made out of milk crates. The front sides are half-inch hardware cloth mesh, and the back sides are galvanized roofing panels. The original plan calls for six feet of the roof to be roofing panels and the other two feet to be mesh. The original design has eye bolts for attaching a rope at either end to pull the tractor across the grass--no wheels (PVC skids on the bottom are suggested to help it move better).
My major modifications were:
1) I changed the frame base from 2x2 to 2x4, to give it increased mass (probably not needed) and increased rigidity (maybe needed, maybe not). The bigger base frame also enabled me to attach wheels (below). I also downsized the vertical supports to 21 1/2" to keep the overall height about the same.
2) I added five-inch lawnmower wheels and extra supports for the eye bolts. This was accomplished through short 2x4 pieces attached to the outsides of the 2x4 frame at each end (this made the tractor wider than four feet at the ends, but it would still fit in a full-size pickup truck bed because the midpoint of the tractor--what would be between the wheel wells--is still four feet). This makes the tractor one-way only; it can no longer be pulled in either direction. If two-way movement is needed, with a little bit of additional modification it could be made to accept wheels at either end (or even both ends simultaneously). More details on this in a subsequent post.3) I put galvanized roof panels along the entire top--no mesh at all. This simplified the cutting process; I only made one long cut in one roofing panel to cover the entire top. It also provides more shelter from the sun and rain. The mesh on the front four feet of the tractor provides plenty of scenery for the birds, and they can get direct sun for part of the day if they want it. The sun shines directly into the tractor in either the morning or afternoon, depending on which direction the tractor is facing.
The picture at the right shows the frame when it was largely complete, before adding the wheel blocks (those came last). I also later added horizontal supports at the top (to hang a feeder from at the front and also to provide additional structural support).
If you look at the plans, you'll see that my setup is reversed in some respects--my door will be on the left in the picture; the plans call for it to be on the right...that wasn't necessarily by design--I reversed the diagonal supports that run along the sides by mistake--but I adapted as I went along. Either way works.
You'll also note something from the picture that might not be immediately apparent at the outset when you're looking at the cut list: all of the 24" (or 21 1/2") pieces have to be more or less exactly the same length. If they aren't, they can't firmly attach to the top and bottom of the frame.
I have problems with precision--partly because of my tools. I can usually get within 1/8", but if I have to be exact, I'm probably not going to get it right. So I had to shim a couple of my vertical supports before screwing them down. Two were off by about 1/4"; I cut plugs for them and glued them in before driving the screws (one is the vertical post at the center front of the picture above). Ditto the perches--I had to shim one. Fortunately, with all of the cuts, I had as much leftover 2x2 small pieces as I needed for any shimming needs.
It definitely helps to have multiple people for some steps, particularly when attaching the vertical pieces to the bottom rail and when attaching the top to the frame with hinges (which is one of the last steps).
More in Part II.
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