Data and methods

Every number on this site is the share of complete eBird checklists that reported a bird, for a place and a week, pooled over 2016–2025. This page says exactly how, and what that does and does not tell you.

Source and citation

The eBird Observation Dataset (EOD) from the Cornell Lab of Ornithology, distributed through GBIF under CC BY 4.0. DOI: 10.15468/aomfnb. Data build 2025a, years 2016–2025, United States only for now.

EOD - eBird Observation Dataset. Cornell Lab of Ornithology. Occurrence dataset https://doi.org/10.15468/aomfnb accessed via GBIF.org. CC BY 4.0 WhenToBird is independent and is not affiliated with or endorsed by the Cornell Lab of Ornithology or eBird.

Which checklists count

• Complete checklists only (the birder reported every species they could identify).
• Stationary or Traveling protocol, 5 to 300 minutes, traveling no more than 8 km.
• A checklist shared by several birders counts once per location, date and start time.
• We never publish observer names, checklist ids or raw records.

What frequency means

Frequency = checklists that reported the species ÷ all qualifying checklists, for that place and week. If a bird has 30% in mid-May in a county, three in ten checklists there that week listed it. This is the same definition eBird uses for its bar charts.

What it does not mean

• It is not abundance. A flock of 500 and a single bird both count once.
• It follows birders. Places and hours that people visit more produce more checklists, and a bird that is easy to find or loud is reported more than a skulker that is just as common.
• Observer behavior changes over time. A spring migration spike can include more people going out because it is spring.
• A county with a long list of species is often one with many observers. We show checklist counts next to every ranking so you can judge.

Weeks

The year has 48 weeks, four per month: days 1–7, 8–14, 15–21, and 22 to the end of the month. All years are pooled into one calendar.

Low-data weeks

A week with fewer than 10 checklists is shown in grey and never used to claim anything. Tables show a dash. For arrival, peak and departure we also smooth each week with its two neighbors (3-week centred average of both numerators and denominators), so one odd week does not move a date. Tables and charts show the unsmoothed numbers.

First arrivals, common by, peak, most gone by, year-round

• Peak: the week with the highest smoothed frequency.
• Plateau: the median smoothed frequency of the weeks the bird is clearly present. Dates are measured against it, not against the peak, because a bird that peaks late (young birds, post-breeding flocks) would otherwise seem to arrive weeks after its first birds show up.
• First arrivals: the first week at or above 10% of the plateau (and at least 1% of checklists, capped at half the plateau), in the run of weeks around the peak.
• Common by: the first week at or above 50% of the plateau.
• Most gone by: the last week at or above 50% of the plateau. Last seen: the last week at or above 10%.
• A week needs at least 30 checklists to set any of these dates, and winter birds simply wrap the year end.
• Year-round: at least 36 weeks of data and no week below 40% of the peak.
• A species page exists for a state only when its peak is at least 1% of checklists and the state has 200 or more checklists.

These are typical timings across ten years, not a forecast. Early individuals can show up before the first-arrivals week, and weather moves migration by a week or more either way.

Feeders

Feeder dates are the first-arrivals week minus two weeks (put out) and the last-seen week plus two weeks (take down), using the species in the group that set the earliest and latest dates.

Spots

Spots are eBird locations visited by many different birders. Their rankings use the number of species reported at least three times in the season, and we show checklists so you can see how well sampled they are.

Traits

Size and habitat traits for the ID helper come from AVONET (Tobias et al. 2022, Ecology Letters, doi:10.1111/ele.13898), CC BY 4.0.

Corrections

If a date looks wrong for your area, tell us where and what you see: hello@whentobird.com.