Late winter is turning toward spring, and the speaker notices it in small things. As the sun climbs higher, shadows get shorter and lose their long, distorted shapes. The speaker compares this to a man in middle age, whose ambitions have shrunk to something more realistic. The comparison is gentle but a little rueful. Growing older, it suggests, means trimming your dreams down to a size you can actually carry, much as the title "Downsizing" hints.
The snow hardening underfoot adds to that sense of settling and firmness. Then the mood shifts. The speaker hears the year's first mourning dove, and its call becomes "desire in a minor key." Desire hasn't vanished, only grown quieter and more melancholy. The poem's feeling is wistful rather than despairing: a calm acceptance of limits, with a faint ache still sounding underneath. The dove's sad song, arriving with the thaw, suggests that wanting something more never quite goes away.