This kō is the other half of a pair already written into the calendar's own autumn: in 白露, in early September, 玄鳥去 (#45) marks the swallows' departure. Now, seven months later, the wires start filling again. What actually triggers the flight north is day length rather than temperature — swallows read the lengthening daylight itself, which is why some years they arrive into weather that still feels more like winter than spring.
In the original Chinese calendar this slot descended from, the same few days were marked 桐始華, "the paulownia begins to flower." Japan's own 本朝七十二候 revision, worked out in the Edo period once astronomers stopped simply importing the Chinese dates wholesale, swapped it for a bird whose arrival could be pinned to this exact week far more reliably than a tree whose Japanese bloom time runs later and looser than the mainland's.
Fattened up for spawning and tinged pink, at its best right around the cherry blossoms it's named for.
A slender, silver spring fish, prized as much for its delicate white flesh as for the striking black lining inside its belly.
Greenhouse-grown strawberries hit their best balance of sweet and tart around now, ahead of the true field season.
The single defining vegetable of this exact window — dug within hours of sprouting, before the bitterness sets in, and simmered whole the same day if possible.
Thin-skinned and mild enough to eat raw in salads, unlike the cured onions sold the rest of the year.
Yamabuki's simple five-petaled yellow blossoms line riverbanks and shaded paths through this window; the color became common enough in classical Japan that 山吹色 (yamabuki-iro) is still the standard word for a warm, saturated gold, used for everything from autumn leaves to old koban coins.
Swallows build under the eaves of busy entrances, not quiet ones — crows and other predators tend to avoid places with constant human traffic, so a doorway with people passing through it all day is, from a swallow's perspective, one of the safer spots in town. Shopkeepers noticed the correlation and inverted it: a nest over your door came to be read as a sign the business itself was thriving, and in many regions it's still considered bad luck to knock one down, even mid-season when the mess under it is real. A small shelf or tray fixed beneath the nest, catching droppings without disturbing the birds, is the traditional compromise.
"When the swallow flies low, rain is coming" — A folk-weather saying with real mechanics behind it: insects fly closer to the ground as air pressure drops ahead of rain, and swallows hunting on the wing simply follow their food downward — the bird's altitude ends up quietly reporting the coming weather before any cloud does.
玄 (gen) means black, but a deeper, more mysterious black than the everyday 黒 — the word used for the color of night sky, or for something profound and hard to fully see into. Calling the swallow 玄鳥, "the dark bird," rather than using its common name つばめ, lifts an ordinary backyard visitor into more formal, almost occult language, the same register the calendar reaches for at both ends of the swallow's Japanese residence — arrival here in spring, departure in #45.
Most of Japan sees only the migratory barn swallow (家燕, ie-tsubame) arrive and leave on this seasonal schedule. Okinawa additionally hosts the Pacific swallow (リュウキュウツバメ), a related species that breeds locally and doesn't migrate at all — so further south, a version of this kō is arguably happening year-round.
Swallows favor entrances with steady foot traffic over quiet ones — a nest starting now over a busy doorway is a good sign by the old logic, not a mess to clear away.
A strip of cardboard or a small shelf a few inches below the nest keeps droppings off the ground without disturbing the birds — the traditional local solution.
Once mud-and-straw building starts in earnest, structural cleaning has to wait until the young fledge — so handle it now if it's needed.
Even though most birds rebuild elsewhere, a small number do return to precisely the same spot, so a site that worked before is worth checking first.
Only around 15% of swallows that survive the round trip return to the exact nest they used the year before; most rebuild from nothing, sometimes under a different eave in a different town, because the spot that worked last year isn't always available or safe this time. The bird doesn't experience this as a setback — arriving and building again is simply what the season asks of it, wherever it lands. What have you assumed you needed to return to exactly as it was, when starting fresh nearby might serve you just as well?