Among the approximately forty most common species for which a trend is identified, 16 species are in decline. Furthermore, at least 6 other indigenous species, previously common in the Brussels-Region, are severely threatened. Common birds showing the most marked long-term decline are species nesting in built structures as well as migratory species. A globally favorable evolution, however, is observed for exotic species and corvids.
Birds, due to their dispersal capacity and presence in most habitats, are excellent indicators of biodiversity status. The annual monitoring of common birds, organized since 1992 using the 'listening points' method, allows for the assessment of these trends. For the period 1992-2024, a statistically significant trend could be identified for 41 species. Among these, 1 species is in sharp decline (the House Sparrow), 15 species are in decline, 15 are stable, 8 are increasing (including 2 exotic), and 2 are in sharp increase (including 1 exotic).
The report highlights that generalist species, which are less demanding, such as pigeons and corvids, are increasing. The Eurasian Collared Dove, for example, has multiplied its numbers by 17 since 1992. Conversely, species linked to built environments and migratory birds are the most affected by the decline.
Several factors contribute to explaining this decline, including the reduction and fragmentation of green spaces, the scarcity of food sources, pollution, climate change, building renovations reducing nesting opportunities, and avian epidemics.
The analysis by species group reveals contrasting evolutions. Indigenous common birds have seen their multi-species index decrease by 30% between 1992 and 2024, while exotic species are experiencing strong growth. Forest birds in the Sonian Forest show a generally stable trend, unlike birds in residential areas which are in decline.
