  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.
