Using this setup, I'm limited to one camera per cable run:
Solution: The Wireless Point access bird box or 'WAP-Box'
I've taken an existing raspberry Pi (Raspberry Pi 2, model B)-based bird box, added a wifi dongle and set it up as a wifi access point (as per these instructions). This is the 'WAP-Box', which helpfully can still be used to run its camera as well as being a wireless access point for other nearby devices.
In this setup, 48v PoE ethernet cable goes to the base of the tree. At the tree base is a junction box where the cable is split out to ethernet-only and power, dropped to 5v using a TP-Link PoE splitter. The 5V output is then used to power a Mini USB hub using a female USB to female barrel socket adapter, and the barrel connector supplied with the TP-link PoE splitter. The usb hub can be used to power up to 4 devices. (note that this PoE kit also includes the power injector used in the house).
One of these boxes needs to be the 'WAP-box', which is a based around a wired ethernet Raspberry pi 2B with wifi dongle. A PoE injector is used in the junction box to recombine 5V from the USB hub with the ethernet-only cable coming out of the TP-link PoE Splitter. I hacked this cable from a pack of micro USB cables (keep the micro end to be used later) and a DC barrel connector. I couldnt find a link for the PoE injector I used, but it came with a print server ages ago and had been in the 'may come in useful' drawer. The 5v PoE cable then supplies power & data to the WAP-box up the tree. I opted to separate the usb hub from the WAP-box as I didnt fancy climbing a tree each time I wanted to connect/disconnect a device.
At the WAP-box, the 5v PoE cable is split into ethenet-only and 5V using a conventional PoE splitter and the otther half of the hacked micro usb spliced into a DC barrel connector to power, and supply wired ethernet to the the Raspberry Pi
So... we now have a wireless access point up a tree.
The additional 'Satellite NestBoxes' take a 5v feed via cables made from a length of black 2-core bell wire and the same microusb and DC barrel connectors linked to above. The cables power a Raspberry Pi Zero-W + Raspberry Pi v2 IR camera module combo in each box which will also have a couple of IR LEDs to for illumination, powered direct from the 5V feed (not via the Pi ZeroW). The satellite boxes are pre-configured to connect to the WAP-box, and thus can save any captured video direct to my home network, so I dont need to worry about the little Raspi ZeroW's running out of space on their SD cards.
This all currently works on the bench with 4 concurrently running camera setups using PikrellCam software, and will be going up in the next couple of weeks once their various boxes are completed. In testing, the Pi Zero-W can 'see' the WAP-box up to about 30m away - my main concern is that if the 5v cable runs are too long the voltage drop may lead to instability of the Pi Zero-W's, so that is likely to limit the cable run distance rather than the strength of the WiFi signal. I also need to have a think about the current load on the 48v power injector at the house end...
More to follow once some boxes are up...
Of note is that the Raspbery Pi foundation recently released a new version of the highest performing version, called the Model 3B+. This is interesting this context as it could potentially make a better WAP-box core since it has a faster ethernet connection. Currently, simultaneously live streaming 4 Raspi-zeros via the existing Pi2 Model B is a bit choppy, which may be due to a bottleneck at the WAP-box's ethernet connection... or this could just be me using this as an excuse to play with one...