The hike in the Fonsbach Valley east of the Rhöndorf Forest Cemetery in the Siebengebirge hills during the winter of 1987 was almost over. Harsh snow crunched underfoot. Dusk was falling. But in the gathering darkness, birds could be seen on the forest floor among the leaves and drying snow. Masses of birds. Countless Bramblings (Fringilla montifringilla) had suddenly appeared. It must have been a gigantic flock that had descended upon this low mountain range.
It wasn’t just in the Rhein-Sieg district in southwestern North Rhine-Westphalia that things were really happening that winter. Anyone who was out and about between Königswinter and Ittenbach back then might still remember that constant buzzing in the air. It felt as if someone had switched on the sky. Suddenly, they were there. Countless bramblings, which in the following days migrated across the hills in gigantic flocks, descended upon the roosting sites each evening like a wandering cloud.
Beyond these personal impressions, the mass influx can be clearly classified from an ornithological perspective. The gatherings in the winter of 1987/88 are among the largest documented winter concentrations of the species in North Rhine-Westphalia. The flocks primarily utilized the extensive Beech stands (Fagus sylvatica) in the Siebengebirge hills, which had a particularly abundant mast year. The high availability of beech mast is considered a crucial factor in the occurrence of large influx waves, as the species is almost entirely dependent on this seed supply during the winter. Accordingly, the birds concentrated mainly in the area of a hill, called Großer Ölberg, and the Tretschbach valley, where observers estimated several hundred thousand individuals daily.
Similar, albeit less intense, events are known from later years. In the winter of 1995/96, there was again a significant increase in local populations. At that time, flocks of up to five figures were recorded, particularly in the area between Löwenburger Hof and Petersberg. The cause was again a good Beech harvest combined with extensive snow cover in Northern Europe, which drove many populations from Scandinavia and Russia to Central Europe.
Another significant influx occurred in the winter of 2012/13. It was less massive than the two previously mentioned incursions but exhibited typical patterns. The bramblings concentrated at altitudes above 200 meters, used the edges of deciduous forests as daytime resting places, and gathered in Spruce stands (Picea abies) in the evenings. Here, too, coordinated counts by the local working group show that the flocks remained stable for several weeks and only began to migrate slowly as the snow cover melted in their regions of origin.
Despite these later examples, the mass influx of 1987/88 remains a remarkable event. It impressively demonstrates how strongly bramblings react to regional resources and how closely their winter behavior is tied to the mast cycles of beech trees. For bird lovers in the Siebengebirge hills, that winter was a rare spectacle; for science, a valuable example of the dynamics of migratory finch species.
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