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

Abstract

Mass concentrations of birds, or lack of such, is a phenomenon of great ecological and domestic significance. Apart from being and indicator for e.g. food availability, ecological change and population size, it is also a source of conflict between humans and birds. Moreover, massive gatherings or colonies of birds also get the attention of the public -- either as a spectacular phenomenon or as an unwelcome pest -- thereby forming the public perception of birds and their abundance. In the context of the mass concentration of bramblings (Fringilla montifringilla) in Sweden the winter 2019-2020, this work reviews the literature on this striking phenomenon. Winter roosts are found to amount to on the order of one million birds per hectare of roost area, but the spread between reports is significant. Support for roosts of up to around 15 million birds was found, but much larger numbers are frequently recited in the literature. It is argued that these larger numbers are the result of overestimation or, in some cases, even completely unfounded (potentially typos). While the difficulties related to the count of large numbers of birds can explain this state, it is unfortunate that "high numbers" sometimes displace proper numbers. Since incorrect data, and its persistence, may result in that incorrect conclusions are drawn from new observations, this matter deserves attention. As the Brambling is a well-studied species, the matter also raises concerns regarding numbers for mass concentrations of other species. It is recommended that very large numbers of birds should be recited and used with care: underlying data and methods of the original sources should be scrutinized. Analogously, reporters of large numbers of birds are advised to describe and document counting methods. In particular, number estimates based on flock volume and bird density should be avoided.