It is very important to attract a wide variety of birds to your garden and I have written about attracting birds to our gardens before (here). Ideally at some point you should notice a bird of prey if you are going about attracting birds to your garden in the correct manner. At that point you should give yourself a bit of a pat on the back. What you have accomplished is an ecosystem that if maintained properly will keep your insect, bird and rodent population in a sustainable manner.
Birds of prey like the Sharp Shinned Hawk will discourage scavenger birds and destructive parasite birds such as the Brown Headed Cowbird yet my experience has been they don't tend to bother with the smaller song birds. Parasite birds usually arrive in small to large flocks and will clean out your bird feeders in the matter of seconds. At the same time in the case of the Brown Headed Cowbird they will raid other bird nests destroying the eggs, lay an egg in its place for the original birds to hatch. Birds of prey will also keep rodents in check.
Attracting birds into your garden is as easy as making the area attractive to them. Provide the necessities such as food, water, shelter and nesting areas. The kind of bird seed you use as well as the style of bird feeder will determine the bird species you will attract. Water should be fresh, changed often and at various levels including small ponds, bird baths and hanging bowls of water. Empty bird baths and hanging bowls at night and refill early in the morning to prevent mosquito problems. Shelter should include both birdhouses and vegetation. Check birdhouses often to see they have not be overtaken by parasite birds or wasps. For example the House Sparrow is notorious for taking over Purple Martin houses. Before House Sparrows begin nesting cover the holes on Purple Martin houses then remove the covering when you spot the first Purple Martins. Location of birdhouses will also help prevent them being overtaken by an undesirable species.
If you find a nesting bird or nest with eggs do not go near it or touch it regardless of where it is if it is an active nest! An active nest will cause the mother bird to attack which really does stress the bird. Any human scent from touching the nest will cause the mother to abandon the nest. Keep dogs and cats out of your garden during the day! Both will scare off birds and most cats in particular are rather good at catching newly left the nest birds. During the evening hours dogs and cats can be beneficial in controlling rabbits and squirrels. Do not use fishing line to discourage problematic birds! While this method does work it is inhumane to birds that get tangled up or even just fly into the line. It is harmful to beneficial bats as well so please refrain from using this eco-unfriendly method of controlling pest birds.