On the face of it, producing good estimates for how many individuals there are of a given species doesn’t seem that hard a task. In reality, the estimates can vary pretty wildly depending on the methods used to calculate the numbers and the amount of data available. The Scarce Migrants report gives a solid minimum number of a given species present in Britain. Its statistics are based on sightings: a sighting of nine different birds equates to nine individuals in the dataset. That might not be the true number – some birds might remain undetected – but it gives comparable year-on-year data. Indeed, on occasions, some records are omitted to keep a level playing field across all of the reports. The most notable case is the Ortolan Bunting Emberiza hortulana, where a recent uptick in records is due to increased detection of nocturnal migrants rather than, we assume, an increase in the number of birds reaching Britain.