The upright tail feathers signify the backside of a cockerel amongst snow-covered grass on the slope of a hill. It has been suggested that this was Hiroshige’s interpretation of a humorous verse by the poet Hachijintei, included at the upper left. The poem describes a couple awakened early by the crow of a cockerel after a night of passion.
Hiroshige designed a series of 'bird and flower pictures' (kacho-e) for the poet Hachijintei. The poet's name and cursive monogram appears alongside the poems.

Kinuginu no
hanashi no imada
tsumoranu ni
tokekau to naku
yuki ni niwatori

The morning after,

before any conversations have piled up,

crows

a cockerel in snow.

--- Hachijintei

For the poem translation by The Minneapolis Institute of Art, go to: 

https://collections.artsmia.org/art/22456/rooster-on-a-snowy-hillside-utagawa-hiroshige

Other impressions:
The Museum of Fine Arts, Boston, accession no. 06.1458, go to: https://collections.mfa.org/objects/177088
The Art Institute of Chicago, reference no. 1934.278, go to: https://www.artic.edu/artworks/19020/rooster-on-a-hillside-in-winter