Skylark(s)

Between Shelly, Clare, and Keats there seems to be a duality of happy and sad with the skylark. Only one Skylark. What I am interested in is why and how did this bird become the symbol of happiness as well as a reminder of despair? The images of the bird flying overhead of these poets knowing that the existence of this bird brings pain and joy in a twisted way seems rather cruel to the poets. Are these poets using the bird as an external reminder of their mortality? Did they pick the skylark because they needed something to connect to others through their emotions such as personality accounts on twitter? meaning that they wanted to say without saying forcing the existence of the bird to say for them. The more I think about it the more I want to say that this bird was chosen because the bird would never know the emotions put on it which is apparent in the poems, but I feel as if I am not in on this culture's idea as much as I want to be.