I heard a thousand blended notes,
While in a grove I sate reclined,
In that sweet mood when pleasant thoughts
Bring sad thoughts to the mind.
To her fair works did Nature link
The human soul that through me ran;
And much it grieved my heart to think
What man has made of man.
Through primrose tufts, in that green bower,
The periwinkle trailed its wreaths;
And 'tis my faith that every flower
Enjoys the air it breathes.
The birds around me hopped and played,
Their thoughts I cannot measure:—
But the least motion which they made
It seemed a thrill of pleasure.
The birds are  antropomorphised , that is behave like humans. 
 Note the rhythm of this line, which imitates the movement of the birds. The speaker imputes human characteristics to the birds; the likelihood is that they were seeking food.
Lines Written in Early Spring