The gathering of Jackdaws in the Kulla Gunnarstorp Nature Reserve

Every winter thousands of Western Jackdaws gather in the Kulla Gunnarstorp Nature Reserve on the south coast of Sweden. In the early winter months you can see large flocks flying above Hittarp, which is the community closest to the nature reserve.

Often the Jackdaws choose trees or rooftops for a few minutes of rest.

The empty bathing pier is another popular gathering place.

This is how Wikipedia describes the Jackdaw gatherings in various areas:
Highly gregarious, Western Jackdaws are generally seen in flocks of varying sizes, though males and females pair-bond for life and pairs stay together within flocks. Flocks increase in size in autumn and birds congregate at dusk for communal roosting, with up to several thousand individuals gathering at one site. At Uppsala, Sweden, 40,000 birds have been recorded at a single winter roost with mated pairs often settling together for the night.

The way Jackdaws interact in a flock is quite interesting:
In his book King Solomon's Ring, Konrad Lorenz described and analysed the complex social interactions in a Western Jackdaw flock that lived around his house in Altenberg, Austria. He ringed them for identification and caged them in the winter to prevent their annual migration. He found that the birds have a linear hierarchical group structure, with higher-ranked individuals dominating lower-ranked birds, and pair-bonded birds sharing the same rank. Young males establish their individual status before pairing with females. Upon pairing, the female assumes the same social position as her partner. Unmated females are the lowest members in the pecking order, and are the last to have access to food and shelter. Lorenz noted one case in which a male, absent during the dominance struggles and pair bondings, returned to the flock, became the dominant male, and chose one of two unpaired females for a mate. This female immediately assumed a dominant position in the social hierarchy and demonstrated this by pecking others. According to Lorenz, the most significant factor in social behaviour was the immediate and intuitive grasp of the new hierarchy by each of the Western Jackdaws in the flock