Good puzzle! Crows are smart, harder to outwit than children. But I read your objective as not outwitting it, instead to cleanup afterwards.
Lateral approach might be to present a better offer to the crow … a more convenient bowl of water, or something.
Tell us more about the task. If the crow is hopping on the cover, I guess you’ll want area coverage, what shape/size area are we talking? When cover is off, I guess he does his dunking off the side, more suited for perimeter-style detection? Maybe he always starts on the side, so we can cheat a bit and only cover the edge? How much of a problem is it when a cat saunters along the edge and gives a false alarm? Is there much windy wavy vegetation and/or shadows to deal with?
The most effective solution is likely to be video based detection. A dedicated camera mounted high to look down would probably give best effects. Risk of problems with reflections off wavelets, maybe a polaroid lens filter would help.
Considering physical motion detection instead, for outdoors I would recommend to forget stuff targeted at home automation e.g. zwave PIRs.
There is a specialist outdoor “intruder detection” market which offers a range of detectors, mostly wired, which can be linked to systems of your choice.
Because they’re specialised, they’re better able to deal with outdoor environment e.g.below freezing, long distances, sun shining in lens, etc. And some are priced accordingly.
Available tech - IR break-beam sensors are very reliable and precise, long ranged (100m), good for perimeters or choke points e.g. alley or gateway. Best ones are multi-beamed so as to detect human size and ignore small creatures - you’re better off with the cheapest chinese made twin-beam type which a crow or cat would set off, but a sparrow usually not.
Choice of mounting site crucial, two devices to site and wire which is a pain
Outdoor PIR sensors can give you both long range and wide area coverage, but designs trade one against the other. Dual tech sensors combine microwave radar with PIR sensing to improve reliable detection. Many are ‘pet proof’ to ignore cats, obviously not wanted here.
Lens design allows for traditional “area” coverage of a fan-shaped sector, best results are always from targets moving across the area, rather than to and from the sensor.
‘Curtain’ lensed style sensors cover a narrow angle, acting like a single-ended beam and detecting passage through the curtain. Consider one laid on its side would provide a horizontal fan detecting things going up and down, could be effective at covering a pool surface. Not sure about sky reflections etc. from water wavelets might be a nuisance, but careful siting should cover the area above without any view of actual surface.
Beware, “serious” equipment in this category (sold into prisons etc.) can get very expensive, but cheap stuff is available too. As said, mostly wired, 12V powered (but very low power consumption). Could be linked to openHAB using any “dry contact” sensing device in your chosen technology e.g. zwave or WiFi.
