The single most important thing to know about these numbers is that they measure reporting, not true abundance. A count is how many birds someone reported seeing — shaped by how many observers were out, how hard they looked, the weather and the time of day. More reports near cities and on sunny weekends do not mean more birds; they mean more birders. So a quiet-looking region may simply be under-watched, not empty.
Read trends and timing rather than raw totals. The shape of the season — when a species first appears, when reports surge to a peak, when they tail off — is robust even when the absolute counts are noisy, because the biases tend to stay similar from week to week. Watching that curve rise and fall is how you actually ‘see’ migration in the data.
Counts within a single checklist carry their own caveats. ‘How many’ is often a careful estimate of a moving flock, not a headcount, and a blank count can mean ‘present but not tallied’ rather than ‘one bird’. Big roost or flyover numbers are approximations by nature, so treat very large counts as orders of magnitude, not exact figures.
Used with that care, the data is powerful. Aggregated across millions of checklists, eBird reveals range shifts, arrival dates and population trends that no single observer could see — and broader platforms like GBIF combine these records with other biodiversity data. The counts are a window onto migration, as long as you remember you are watching the observers as much as the birds.