Pigeon, the word varies depending on who your talking to.
To some they are 'athletes' who get raced during the pigeon racing season, to others they are friends at the pond, or train station who come around when your throwing bits of stale bread around.
Pigeons were even used as a method of communication during the two world wars.
Sending coded messages over Europe carrying important information, and saving lives.
But to most of us, these days, pigeons are generally unwanted, dirty and noisy.
And to add insult to injury its not usually just one or two that you are contending with, its a group of them.
So which is the best way to deter pigeons from your property, humanely? Over the last 20 years bird scarers have gone from home made, word of mouth techniques to high tech scientific like, commercialised products with a sales line to boot.
Unfortunately over those 20 years people have gotten suspicious due to unfulfilled promises.
So here are some tips that I have found.

• Pigeons love lawn seed, if you have an issue and you want to displace them, you can throw some lawn seed in an area you don't mind them being.
  This can work as a short term measure until you find something else a little more permanent.
  
• Some houses, but mostly shops and commercial buildings use spikes which are installed on the tops of buildings and where pigeons usually frequent.
  Spikes do displace pigeons, the only issue is that you can go through quite a lot of them in order to achieve your goal.
  
• Some buildings try to cut down on the amount of spikes by using some wire which runs from a point in the middle of the area to all of the outside sections, this like the spikes can grow to be quite expensive.
  
• There are qutie a few plastic models, snakes hawks and owls which are designed to keep the birds away, but because pigeons don't have anything else to do except work out if your plastic owl is real or not, they will probably work it out, or get hungry enough to risk it.
  
• Some bird scarers are audable but being so loud they will most likely result in an angry neighbour or two.
  

Unfortunately we can't be out there chasing after pigeons all day, and bird scarers aren't always going to work.
Pigeons will test the boundaries.
With plastic inanimate bird scarers you will have to move them around ever so often to give them an effect, once every few days, a week is generally too long.
I'd also consider using two or more, either two of the same thing, or two methods that complement each other.