Indexed on: 25 Jan '17Published on: 23 Jan '17Published in: Journal of Avian Biology
The return of individual birds to a specific area in successional years, i.e. philopatry, is a remarkable behavioural trait. Here we report on the remarkably reversed: the complete absence of returning individuals of a migratory passerine with otherwise pronounced philopatry. At a high latitude study site in Abisko (68°32ʹN, 18°80ʹE) in northern Sweden none of the banded adult willow warblers Phylloscopus trochilus returned to breed 2011–2014. This is in stark contrast to all other reports in the literature and also to our two southern study sites (at 56°56ʹN, 18°10ʹE and at 58°94ʹN, 17°14ʹE) where 18–38% of adults returned. We investigated this aberrant pattern found in Abisko by analysing three parameters known to influence philopatry; nest predation, breeding success and breeding density, and predicted that absence of philopatry should co-occur with low breeding success, low breeding density and/or high nest predation. The results did not corroborate this, except that breeding density was lower at Abisko (49–71 pairs km–2) than at the southern sites (106 pairs km–2, 101 pairs km–2). Instead, we suggest the hypothesis that the absence of philopatry is caused by an influx of southern, dispersal-prone individuals deploying another breeding strategy and that this intra-specific range expansion is enabled by milder climate and low population density.