Thanks, Wilbill! That is a good question, that I should have explained before.
We will be using this in the forest of a remote island in the Galapagos archipelago. Once there, we need to lift the device with a telescopic pole between the canopy and dealing with cables to run the pump and a plastic hose to deliver the fluid is not always easy. Besides, having the reservoir at the base requires me to fill up the system (against gravity) every time (or so) I need to reach a nest. I have an older, wired, version of this device that works but wanted to improve this issue.
The activation button should be at the bottom. That's why I'm now thinking to use a 433/866MHz module (suggested by Whandall) powered by 3 3.7 volts batteries (all on the top).
Any ideas are welcomed!
Martín