When you take apart your Plant Pal moisture meter you'll find a little circuit board like Image 2 above. There are two large prongs which you stick into the dirt that act as an on/off switch. When the soil is moist the circuit is turned off and no chirping, when the soil is dry the circuit is on and it chirps. Unfortunately mine never worked properly, no matter how much water I added to the soil it was always chirping. I ended up taking out the battery, stuck it in drawer and forgot about it -until now.
When I decided to use it for this project I was hoping I could use the prongs somehow as a switch to turn on and off the chirping in my chickadee, but the only way the circuit would stop chirping is if I dunked the prongs in salt water. I really didn't want to dunk my chickadee into salt water to stop the singing so I had to explore other options. The circuit board also has a LDR resister so that the plant moisture meter doesn't chirp at night. This seemed like a good way to control the circuit.
So what I did was to trim off the LDR resister and solder a wire to each side where it connects to the circuit board. Now if the two wires come together the circuit is on and there is chirping. I would wrapped each end of the wire around a magnet and when they are connected by a piece of metal the circuit is turned on. So when the chickadee sits on the table there is no chirping but when I move it to the perch where it sits on a small piece of metal it starts to sing.