Red knots of the rufa subspecies winter as far south as Tierra del Fuego and breed in the Canadian Arctic. The journey depends on a small number of stopovers where they can gain mass very fast, and the most important is Delaware Bay, where they arrive as horseshoe crabs come ashore to spawn in late spring.
Horseshoe crab eggs are an unusually good refuelling food: small, soft-shelled, high in fat and available in enormous density in the surface sand. A knot can put on a large share of its body mass in roughly a fortnight, enough to complete the flight north and arrive in condition to breed. Timing is tight; a bird that misses the spawn cannot make it up elsewhere.
Horseshoe crab harvest for bait and for the biomedical lysate industry reduced spawning densities, and rufa red knot numbers fell sharply. The subspecies was listed as threatened under the US Endangered Species Act in 2014. The mismatch risk is the unresolved part: spawning is temperature-cued and knot arrival is largely calendar-cued, so warming may pull the two apart.