I spent some time tonight looking at the Nextstrain H5N1 dairy cattle tree that @SolidEvidence shared. What jumps out at me is not an uncontained national wave, but a dramatic evolutionary bottleneck. Every major 2024 and 2025 lineage went extinct. The massive California wave vanished last fall. So did the Colorado and Midwest lineages, along with the original Texas and Michigan clades. Every single 2026 sequence on that tree traces back to one late-2025 ancestor in Idaho. Bulk milk testing worked. Mandatory bulk-tank screening and farm quarantines in Colorado and California, followed by the USDA National Milk Testing Strategy, cleared the virus from most dairy states. That state-by-state clearance is why the January 2026 USDA guidance added an exemption for certified unaffected states. Rather than circulating across the country, roughly 96% of the 2026 sequences come from two neighboring states, Idaho and Utah, plus one small secondary jump to Texas. Why does the 2026 cluster sit on such a long branch? Sampling cadence explains a lot of the visual gap. Sequencing in Idaho went quiet for five months in late 2025 and then ramped back up this spring with much faster GenBank turnaround. Even with uneven sampling, the fact that every 2026 sequence across cattle, birds, and mink coalesces to a single late-2025 Idaho ancestor points to a real contraction. We ran our selection, clock, and reassortment tools across all eight segments to see what carried that lone Idaho lineage through the bottleneck. Reassortment played no role. All eight segments tracked together clonally, and PB2 did not pick up a single nucleotide change along the entire bottleneck stem. The other seven segments went through a sharp clock acceleration and fixed a tight cluster of co-evolving changes in NS1, HA, and NA. When that lineage spilled into a Utah mink farm, it picked up a set of polymerase changes and immediately reverted an NS1 mutation (A60T) that the virus had acquired in Idaho cattle back in 2024. Inside the surviving Idaho and Utah pocket, peridomestic birds and small mammals sit interleaved with dairy cattle on the tree. Curious what @SolidEvidence and others think: is this last lineage holding on in the Intermountain basin through local farm-to-wildlife cycling, or did those NS1 and surface changes alter within-herd persistence? xitter.cf/SolidEvidence/status/2…