From OMAFRA:

If you're using propane cannons, remember to operate them only when birds are feeding (dawn to dusk) which starts at about 30 minutes before sunrise, and ends about 30 minutes after sunset, since there is enough light for birds to forage. It is considered normal farm practice to operate between this period, but never operate propane-fired-bird-scaring cannons, or electronic bird scarers outside of this time period.  Other points to remember as you try to keep the birds at bay:

• Start bird abatement measures before they get used to feeding in one place; it is harder to scare them away once they have established a feeding pattern. Start 10 days before the crop is attractive to them.
• Vineyards near roosting areas, bush or ponds are at higher risk of bird damage so place more emphasis on disrupting feeding in those areas.
• Random, unexpected noise is more effective than a predictable pattern.
• Vary timing of sounds and vary placement of devices.
• Find novel ways to disrupt bird feeding.
• Use a combination of methods to scare birds; a variety of methods is far more effective at reducing bird damage than any single method alone.
• Netting is extremely effective    

For more information:
Bird Control on Grape and Tender Fruit Farms
Bangers and Birds
Using Propane-Fired Cannons