In addition to the modifications described in the first two installments, I made some other changes to the original design. One is that I added a couple of cross-braces at the top. These increase the strength of the frame somewhat and also provide places to hang small feeders or waterers if desired. For a while, I had a three-pound feeder hanging from one of the crossbars.
For reasons I haven't figured out, I have a lot of movement of the top in relation to the frame--this is noticeable with the latching hasp in the middle of the tractor body. Sometimes it doesn't line up. It can shift to either side. I put some corner braces inside the top hoping it would help, but it largely hasn't (shown below behind the late black widow, which was discussed in an
earlier post).
One very important modification addresses one of the structural vulnerabilities in the design.
Shown below is the back of the tractor. The milk crates serve as nest boxes (you'll also note that the hasp is reversed from the plans; I think the original plan is better but I just assembled it wrong).
The milk crates themselves are part of the back wall. The openings at the bottom of the crate are bigger than with the half-inch hardware cloth, but not too bad. The holes for the handholds on the crates, however, are pretty big. I was aware that this was a potential problem at the time I was building the tractor, but thought it probably wouldn't be a problem, because the chickens would be roosting on the bars at the front of the tractor.

As it turns out, the chickens do like to roost back there. As the picture above shows, the nest boxes are at the top of the frame--this simplifies construction because they're right up against the top bar of the frame, but also puts them relatively high in the tractor. I think the next time I make one of these I'm going to lower the nest boxes down a little bit. That will complicate the design but may make them less attractive. There is a flip-up perch in front of the nest boxes that is intended to be used to block the nest boxes if the chickens start roosting in there, but my chickens just flipped it back down when I tried to block the nest boxes. Also, depending on ones' schedule, that might not be viable. If you leave for work early in the day (before the chickens are moving around), it's not feasible to flip the bar up in the evening and down in the morning before they start laying.
So the chickens who roosted in the nest boxes essentially are within reach of the handholds. And eventually, I did lose some birds that way (two in one night).
It was a fox that got them (the above picture is not from the night of the attack, but does indicate that there are predators in the vicinity).
Mitigation is pretty simple (I think). I simply attached some hardware cloth to the board that serves as the gate holding the nest boxes in.

For now, the hardware cloth is overly large. To actually open the gate and pull the boxes out, some of the hardware cloth on the left edge of the picture will have to be trimmed. However, the cutaway around the hasp should allow this to open and close. The two pieces of firewood at the bottom hold it in place, but may not even be necessary. This should solve the nest box vulnerability (this was done the day after the attack, by the way, which is why blood is still visible on the back of the tractor).
It would also be pretty easy to just service the tractor from the top, collecting eggs that way. My chickens aren't laying yet and I don't know if they will even lay in the milk crates. Time will tell.
One other thing I've done that isn't really a modification of the tractor: As seen in the picture at the top, I cut a 6x8 white tarp to drape over the top. This is so that the sun isn't hitting the galvanized metal directly. I don't know if it helps keep the tractor any cooler when the sun hits it, but the chickens seem to be doing okay.