For many years I suffered serious bird damage to crops in my small fruit orchard. Finally I started experimenting with bird scare devices and came up with very effective but low tech methods that achieve 90% or better effectiveness. These machines would work equally well for gardens or small vineyard or berry patches.
Videos of my bird scare machines can be seen here:http://sunnyslopeorchard.blogspot.com/2013/05/our-orchard-bird-scare-machines-dont.html
All operate electrically and are controlled by electric timers. For several years I was not aware of a suitable commercial timer so I made my own from surplus parts. However I have recently found the perfect timer available here:http://www.titancontrols.net/products/timing/apollo-12.aspx
The Titan Apollo 12 timer is shown and described in the last of the 5 videos at the link above. Typically I might set the timer to run 5-20 seconds ON and 45-120 seconds OFF. The Titan timer settings can be changed frequently to keep 'em guessing.
The first video shows my first design, a rotating mast with cross arm on top. The mast is scrap steel tubing of two diameters which telescope together allowing height adjustment. It was originally powered by a chain drive garage door opener motor. However that soon burned out since those overheat with too-frequent use. So you may notice in the video that I have mounted a 1/15th HP motor (via flexible coupler) to the original motor shaft. This drives the original gear reduction mechanism and is able to operate all day without overheating.
Wanting to protect larger orchard areas, I next went to the electric leaf blower powered system of PVC pipe and flapping poly bags shown in the last video. This is truly a "shock and awe" system that works great. But for larger areas it requires multiple air outlets and a heavy duty leaf blower. That means lots of electricity and also frequent replacement of the poly bags. Nonetheless it works great and is easy to set up on a smaller scale. I tend to use it for 1-2 trees close together or to alternate with another system to keep birds from getting used to one system. It is somewhat noisy and may bother close neighbors.
Videos 2-4 show the flailing rope system, which uses any scrap or home made gear reduction mechanism to set one or more taught ropes oscillating over the tree canopy. This is best for covering larger areas and is infinitely adaptable to different sites and circumstances.