“O little Bird !” Why capital ‘B’ has been used here?

=> Who wrote the poem, Koel?
=> What do you write for “thy” and “art” in prose?
=> What makes thousand memories in heart?
=> What happens with the shades of mangoes?
=> “O little Bird !” Why capital ‘B’ has been used here?
=> What charred the wings of Koel?
=> Why is Koel restless?
=> What fires the Koel?
=> What burns her heart?
=> What does the flaming soul of the Koel ask?