A review of mass concentrations of Bramblings Fringilla montifringilla: implications for assessment of large numbers of birds

Abstract

Mass concentrations of birds, or the lack thereof, is a phenomenon of great ecological and domestic significance. Apart from being and indicator of, e.g., food availability, ecological change, and population size, it is also a source of conflict between humans and birds. By attracting the attention of the public—either as a spectacular phenomenon or as an unwelcome pest—they also form the public perception of birds and their abundance. In the context of the mass concentration of Bramblings Fringilla montifringilla in Sweden during the winter 2019–2020, this work reviews the literature on this striking phenomenon. I found that winter roosts amount to about one million birds per hectare of roost area, but the variation between reports is significant. There is support for roosts of up to around 15 million Bramblings, but much larger numbers are frequently cited in the literature. I discuss difficulties related to the assessment of mass concentrations, and argue that reports of very large numbers should always be carefully scrutinized.