Birds are often averse to certain strong scents. For instance, the odor of predator urine, such as that from foxes or hawks, can deter them. Similarly, birds may dislike the pungent aromas of garlic, onions, and chili peppers. Essential oils like peppermint, eucalyptus, and citrus can also be effective in keeping birds away. However, it's important to note that individual bird species might react differently to these scents, so it might require some experimentation to find the most effective repellent for your specific situation.

Both spikes and loose nets can be installed over nest sites to prevent the bird from returning, but nets are not a humane option since birds and other wildlife can get tangled in them. 

Spikes are a humane alternative that deter nesting without harming the birds. In small areas, like vents, a very fine mesh net can be installed to prevent the bird from returning. These nets don't generally pose a risk to the birds.

Yes, predator decoys work to keep birds away, at least temporarily. However, if the decoy remains motionless in the same spot for a long enough time, the birds will eventually wise up and realize it's a fake. Keep changing the location of the predator decoy to ensure that the birds keep their guard up and maintain their distance.

Woodpeckers may seek out homes with aluminum siding or tin roofs because these surfaces echo their drumming. Homes with wood siding are more frequently targeted as roosting areas or for food sources. If you have insects living in the exposed wood in your home, the birds are likely looking for a snack, in which case your best option is to hire an exterminator.