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Résumé :
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The use of open cup nests by birds for roosting during the non-breeding season is a poorly documented yet intriguing phenomenon. Here, I describe the use of old Barn Swallow (Hirundo rustica) nests as winter roost sites by Dark-eyed Juncos (Junco hyemalis) as well as agonistic interactions at these structures. In 2016–2023, I observed juncos roosting in four Barn Swallow nests in New Jersey, USA, and used digital video to closely monitor activities at one nest. A junco roosted at the monitored nest on 56 of 71 nights, entering the roost 71 ± 26 min before sunset (mean ± SD; range: 20–168; n = 54) and exiting 27 ± 5 min before sunrise (range: 7–32; n = 43). In spring, a pair of House Finches (Haemorhous mexicanus) began preparing the swallow nest for egg laying during the day while a junco still slept there nightly. The House Finches increasingly came into conflict with the roosting junco, evicting it at least temporarily on nine evenings despite efforts by the junco to defend the site. Similar agonistic encounters with three other juncos, three unidentified birds, and a Carolina Wren (Thryothorus ludovicianus), which also roosted in the Barn Swallow nests, were documented but did not result in eviction. Winter roosting in open cup nests appears to be rare in birds despite potential thermal benefits. The occupancy rate and tenacity observed at the monitored Barn Swallow nest suggest that open cup nests may be desirable roost sites, and that the phenomenon deserves further study.
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