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.
When it comes to larger populations, counting individuals is an impossible task and other methods come into play. Scarce species such as the Lesser Spotted Woodpecker Dryobates minor often slip between the cracks, being too common for individual breeding occurrences to be logged but spread too thinly to be picked up by general surveys. Data from regional breeding-bird atlases have helped to fill the gap. In this month’s issue, an analysis of data at a tetrad level is presented, allowing for an updated population estimate of Britain’s smallest woodpecker.
436 BB eye: The ethics of optics Lucy McRobert
439 News and comment Adrian Pitches
443 Report on scarce migrant birds in Britain in 2019. Part 2: passerines Steve White and Chris Kehoe
465 Using tetrad-atlas data to estimate the numbers and recent range changes of the Lesser Spotted Woodpecker in Britain Ken Smith, Linda Smith and Rob Clements
480 A review of snake mimicry in the Eurasian Wryneck Gerard Gorman
500 Recent reports