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Mass concentrations of birds, or lack of such, is a phenomenon of greatecological and domestic significance. Apart from being and indicator for e.g.food availability, ecological change and population size, it is also a sourceof conflict between humans and birds. Moreover, massive gatherings or coloniesof birds also get the attention of the public -- either as a spectacularphenomenon or as an unwelcome pest -- thereby forming the public perception ofbirds and their abundance. In the context of the mass concentration ofbramblings (Fringilla montifringilla) in Sweden the winter 2019-2020, this workreviews the literature on this striking phenomenon. Winter roosts are found toamount to on the order of one million birds per hectare of roost area, but thespread between reports is significant. Support for roosts of up to around 15million birds was found, but much larger numbers are frequently recited in theliterature. It is argued that these larger numbers are the result ofoverestimation or, in some cases, even completely unfounded (potentiallytypos). While the difficulties related to the count of large numbers of birdscan explain this state, it is unfortunate that "high numbers" sometimesdisplace proper numbers. Since incorrect data, and its persistence, may resultin that incorrect conclusions are drawn from new observations, this matterdeserves attention. As the Brambling is a well-studied species, the matter alsoraises concerns regarding numbers for mass concentrations of other species. Itis recommended that very large numbers of birds should be recited and used withcare: underlying data and methods of the original sources should bescrutinized. Analogously, reporters of large numbers of birds are advised todescribe and document counting methods. In particular, number estimates basedon flock volume and bird density should be avoided.

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