Alien species invasions are often characterised by stages: transport, introduction, establishment, and finally invasive spread, but not all established alien species succeed in expanding, and reasons for their differing relative success are not well documented. Although widely introduced, the Java sparrow (Lonchura oryzivora) is endangered in its native range (Java and Bali). Free-ranging Java sparrows were first reported in northern Bangkok, Thailand, 90+ years ago. They have an established breeding population, but have not spread beyond this area. We investigated the present distribution, habitat use, and nest survival of the Java sparrow population in Bangkok to obtain a greater understanding regarding its lack of expansion. Forty-one 400 ha grid cells were randomly selected from northern Bangkok, a primarily urban area interspersed with paddyfields. Field surveys were conducted February 2017–July 2017 and nest observations during August 2016–August 2017. Java sparrows were detected in 37.5% of grid cells, but the detection rate was low (2.8% of surveys). Models suggested that Java sparrows preferred areas with smaller paddyfields closer to roosting sites rather than larger patches of paddyfields further from nesting or roosting sites. All nest cavities were in buildings or other artificial structures; roosts were in trees. Nest survival was 49%. However >50% (41 of 78) of nesting attempts were abandoned; they were also displaced from five of 67 nest sites by native species. Lack of expansion was possibly caused by low reproductive rates, perhaps exacerbated by nest-site competition from native species. Given the low probability of invasive spread and relative similarity of bird communities in Bangkok and in the Java sparrow’s native range, this Bangkok population may have potential conservation value as reintroduction stock for the species.

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