I believe birds are popular symbols because they generally represent positive and optimistic ideas. For example, they are very commonly associated with freedom so when people see them it perhaps gives them hope for something better. As well, they can represent beauty and even confidence, especially in the way that the males of most bird types tend to become more beautiful when trying to show off, so they may give people the incentive to feel the same about themselves. I think overall they represent things people aspire for or to be like which is why they are so commonly used.