Spares the next swath and all its twinèd flowers:
And sometimes like a gleaner thou dost keep
Steady thy laden head across a brook;
Or by a cider-press, with patient look,
Thou watchest the last oozings hours by hours.
Where are the songs of Spring? Ay, where are they?
Think not of them, thou hast thy music too,—
While barred clouds bloom the soft-dying day,
And touch the stubble plains with rosy hue;
Then in a wailful choir the small gnats mourn
Among the river sallows, borne aloft
Or sinking as the light wind lives or dies;
And full-grown lambs loud bleat from hilly bourn;
Hedge-crickets sing; and now with treble soft
The red-breast whistles from a garden-croft;
And gathering swallows twitter in the skies.
To describe lambs as fully grown is an oxymoron, a contradiction in terms. They are sheep when full grown. But perhaps this implies Keats' desire to cling to the idea of summer, denying the lambs have matured or that his own poetry-writing has reached its finality. Or it could be positive, implying the near-completion of the season and life’s maturity. 
 Note also that a lamb is a symbol of innocence, something that Keats may have wanted to cling to. It could imply his instinctive need to deny his impeding death. 
 
 The word ‘bleating’ is poignant, almost a call for help or an expression of fear of death.