Randy asked a good question about background in images. It's not always possible to get that nice out-of-focus background that very nicely sets off the subject. Often, the bird or animal is in the shrubs and trees with branches and leaves all around. A pleasing photo can still be obtained with leaves and branches around and I like these showing the birds' natural habitat.
To get that nice out of focus background, smaller aperture lenses helps a great deal. These "fast lenses", usually down to f-stop 2.8 or f-stop 4 are good for getting a background that's out of focus. But this does rely on how close the background is to the subject. Even the fastest lens wont blur out a branch a couple inches behind the subject.
Sometimes composition can make all the difference, even with a 'slower' lens like a f-5.6 and moving a few inches one way or another can make all the difference. In the photo below of the young Barn Swallows I shuffled about a foot to my left and took a second photo. Makes all the difference!