Methodology & Data Sources
Exactly how Birch Compass builds each part of a state page — the datasets, the calculations, and where the numbers are measured versus modeled. Updated each January.
Data updated: August 2026Sources & method · Corrections
Birds
The ranked bird lists come from the USGS Breeding Bird Survey (BBS). We aggregate the most recent survey window (2014 onward, standard roadside routes, RPID 101) and rank each state's species by how consistently they appear on its routes. Which birds occur in a state is therefore measured. The month-by-month presence curve shown for each species is modeled from its migration status (resident, short-distance, or neotropical migrant), because the BBS is a single early-summer count and does not track month-to-month timing. Modeled timing is labelled as such and is under ongoing review.
Weather & daylight
Temperature and precipitation are the NOAA / NCEI 1991–2020 U.S. Climate Normals, from the statewide nClimDiv series. These are genuine 30-year normals, but they are statewide averages — a single figure for the whole state — so a mountainous or coastal corner will differ from the number shown. Per-region normals are a planned upgrade. Sunrise and sunset are computed with the standard NOAA solar equations for a midstate point on the 15th of each month, in local clock time; they shift by a few minutes across a state and with daylight saving.
Night sky
Moon phases and illumination are computed with Jean Meeus's astronomical algorithms (the standard reference), validated against published ephemerides. Meteor-shower peak dates are the nominal annual peaks (they drift by up to a day year to year); the peak rate (ZHR) is the typical maximum under ideal dark skies at the radiant's zenith — real counts from a backyard are lower. The moon-brightness figure next to each shower is computed for that year's peak, since a bright moon is what usually spoils a shower.
Native plants
Species are matched to a state when their native range (by ecoregion) reaches it and their hardiness range overlaps the state's USDA hardiness zones. Use-case groupings (sun, soil moisture, deer resistance, wildlife value) come from horticultural references. Always confirm a plant is native to your specific county with your state Native Plant Society before planting — native ranges are finer than a state line.
Planting & frost dates
Last-spring and first-fall frost dates are typical averages by USDA zone; they vary year to year and with local microclimate. Treat them as a starting point and check your local Cooperative Extension for your exact area.
Hummingbird arrival
Which hummingbird species occur in a state comes from the Breeding Bird Survey records above. The arrival months are modeled from each species' documented latitude-driven spring migration — they are not measured first-sighting dates, and real arrivals vary year to year and run earlier on the coast and at low elevation than a statewide figure.
Updates
We refresh the whole reference each January: new climate normals as they publish, the current year's meteor and moon calendar, and any corrections. The "Data updated" stamp on a page reflects its last build.
Every source, linked
Each dataset below is public and checkable. We list what we actually take from each one, so you can go to the source and verify any claim on this site rather than taking our word for it.
- USGS Breeding Bird Survey — which birds occur in each state, and how consistently they are recorded
- NPS NPSpecies checklists — park-level species presence used as a cross-check
- USDA PLANTS Database — native status and distribution for every plant named
- GBIF occurrence records — whether a plant is actually recorded growing in a given state
- USDA Plant Hardiness Zone Map — the zone ranges behind planting advice and plant matching
- Lady Bird Johnson Wildflower Center — native-plant horticulture: sun, soil, bloom period, wildlife value
- The Xerces Society — pollinator and butterfly host-plant relationships
- USA National Phenology Network — typical timing of leaf-out, bloom, and seasonal events
- NOAA/NCEI U.S. Climate Normals (1991–2020) — statewide temperature and precipitation normals
- Cooperative Extension (land-grant directory) — regional planting dates and frost guidance
- State fish & wildlife agencies — state-specific wildlife status and viewing guidance
- International Meteor Organization — the annual meteor-shower peaks and typical rates behind every sky page
Review and corrections
Before launch, every regional passage was checked against the datasets above in a structured review pass, and its corrections are already in the text. Since launch, anything we get wrong on a live page is fixed and logged on the corrections page, which also explains how to report one. Spotted something? email us.
Colophon: passages are drafted with AI assistance from the public datasets listed above, then checked by the automated gates and the review pass described on this page before publication. The data, the corrections log, and the limitations stated here are the part we ask you to judge us on.