Well perhaps these should be called "scarecats" rather than "scarecrows"... The idea is that you get a Nature-friendly alternative to smelly, poisonous, or visually disruptive animal deterrents.
I know of two of these on the market, both manufactured by Contech (who also offer ultrasonic cat repellents). The Contech Scarecrow is a motion activated water sprinkler, whilst the Contech Mini Scarecrow works by emitting a short burst of air and a warning sound.
The Contech Scarecrow
When the Scarecrow sees an intruder, it instantly sprays the trespasser with water under full garden hose pressure.The effect is both startling and immediate! Animals quickly get out of the area and avoid it in the future.
The Scarecrow will protect an area approximately 35 feet wide and 45 feet deep, which makes it an excellent device for keeping cats off lawns, and away from ponds and gardens. It operates off a 9V battery which might typically last up to 6 months if operated day and night (or 3000+ activations). The device needs to be connected to a good quality consumer water hose with a water pressure of around 30 to 80 P.S.I.
The Contech Mini Scarecrow
The Mini Scarecrow is designed to protect an area such as a covered porch or patio, potted plants - or even used indoors to protect countertops. A sensor detects motion up to a yard (one meter) away and activates a warning sound as well as a brief, harmless spray of compressed air to keep the cat away.
The motion sensor and spray are separately adjustable so they can be positioned precisely.
The device uses 4 AAA batteries and replaceable air canisters are available from Contech and approved distributors.