Wildlife deterrence is one part science and one part art.
In order to be effective at a site a deterrent must be selected or designed to address exactly how, when, where, and why wildlife utilize that site.
Deterrents are usually specific to one species or family. In other words, there is no deterrent that will be effective against all different kinds of birds.
An initial site evaluation is essential for any effective deterrent program.
Like most humans, wildlife want life to be comfortable and predictable. The more uncomfortable and unpredictable a deterrent program can make life for wildlife, the more successful the program will be.
The objective of any successful wildlife deterrent program is to always keep wildlife anxious and on edge.
More is not necessarily better as far as employment of only several wildlife deterrents goes. Wildlife, and especially birds, usually get used (become habituated) to a deterrent if continuously presented in one fashion. E.g., move propane cannons around, reposition them, and /or adjust firing time or sequence.
Remember that many forms of wildlife, especially mammals, are active at night.
Habitat modification can be a very powerful deterrent. Care must be taken to not create a significant attractant for other wildlife in the process.
The old adage “An ounce of prevention is worth a pound of cure” applies to wildlife control. It is much easier to prevent wildlife from using a site in the first place, rather than try to get rid of them once they have become established at the site.
Human presence can often be an effective deterrent.
A mated pair of Canada Geese can be very difficult to deter, especially if nesting. Try to slowly position yourself between the two birds. If the pair are together, try positioning one person equidistant on either side of and walking slowly towards the pair. This greatly heightens the anxiety of the birds, often causing them to fly off.
A flock of gulls can usually be made to fly off by slowly raising both arms up and down, as if one were trying to fly.
The more varied deterrent tools in your tool-box, the better!