This response was provided by Darryl Coates, Illinois Department of Natural Resources District Wildlife Biologist.
Thank you for writing in. Your problem is very common for buildings in some downtown areas. Here are some recommendations for you to try. Plastic owls and inflatable bird chasers can work if used properly. The problem with "scarecrow" techniques is that often people leave the scarecrow in the same place and fail to move the object. Over time the pigeons will no longer be afraid and come right back. An object(s) like an owl or bird chaser must be moved every other day in order for it to be effective at scaring anything. Using mylar tape may also deter the pigeons. Sticky repellents such as Tanglefoot may work, but should not be used if other birds use the site. (The material can impair birds' ability to fly or stay warm if the product comes into contact with their feathers). If you have access to the outside sill in front of your picture window apply a 1/4 bead of Tanglefoot brand bird repellent. Tanglefoot bird repellent is a nontoxic, clear, odorless, non-drying compound which adheres to all surfaces and cleans up with mineral spirit. Apply Tanglefoot bird repellent to any outside sills or sites where pigeons land. Tanglefoot bird repellent is sticky and pigeons will avoid it.
As for the deck, wash off the deck, deck chairs and table with warm, soapy water and rinse with plenty of water prior to using the deck for outdoor activities. Use disinfectant products to clean off working surfaces and eating areas immediately before use.