Jip Louwe Kooijmans (2025) Seasonal dynamics on communal night roosts of House Sparrow Passer domesticus. LIMOSA 98 (4): 186-192.

In 2024 – the Year of the House Sparrow – extra attention has been paid to counting House Sparrows on communal night roosts, because relatively little was known about this phenomenon. A total of 112 counts were carried out at 16 different roosts. A total of 3623 overnight stays of House Sparrows were recorded. Communal night roosts had a different maximum size per location, but all showed a similar change in numbers during the different months of the year (Fig. 1a). To show the relative change, August – the peak month – was set at 100 (Fig. 1b). Calculated over all 12 months of the year, a roost consisted of an average of 34.7% of the maximum. From July to December, group size was above average and from January to June, below average. The location where House Sparrows spent the night changed over the course of the year. House Sparrows appeared to sleep solitarily at least part of the year. This is remarkable for a species that lives in groups all year round. The composition of the group changed over the course of the year (Fig. 2). Some of the House Sparrows, mainly females, used the nesting sites for roosting almost the entire year. The locations where House Sparrows spent the night also changed over the course of the year (Fig. 3). The small increase in the size of the communal roost in March coincided with the courtship and pair formation period. At the same time, the percentage of House Sparrows sleeping in pairs increased sharply compared to the previous months (Fig. 2). As soon as pair formation was complete, the communal roost was actually only used as a pre-gathering place. In May, after fledging of the first juveniles, communal roost groups were formed again (Fig. 2). In August, all House Sparrows slept on the communal roost. In this month, communal roosts were at their largest (Fig. 1a). In September, the average size of communal roosts decreased sharply and the percentage of House Sparrows sleeping in buildings or nest boxes increased (Fig. 3). During this period, adult House Sparrows moult their flight feathers. The nest probably offers a safer place to sleep during this vulnerable period. In November, when moult is completed, the adult birds gradually return to the communal roost.

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