Point of no return – absence of returning birds in the otherwise philopatric willow warbler Phylloscopus trochilus

Abstract

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.

The first documentation of the Nearctic–Paleotropical migratory route of the Arctic Warbler

“…Our recovery rate of tagged birds was 9.5%, much lower than the 35% resighting rate of color‐banded Arctic Warblers at a different study site, 210 km east of our study area (Lowther & Sharbough, 2020 ). Low recapture rates may have resulted from low breeding site fidelity and low survival rates (DeSante et al, 2015 ; Ralph et al, 1993 ), geolocators reducing survival (Brlík et al, 2020 ; Costantini & Møller, 2013 ), low detectability of tagged birds in the study area, low territoriality of males in the study area, or individual dispersal to new breeding habitat (Hedlund et al, 2017 ; Mizel et al, 2016 ). Furthermore, we noted intra‐season movements that indicated large territories or inter‐territory movements (average = 235 m, range = 85–630; n = 6 banded/tagged birds) that may have contributed to our low recapture rate.…”

Increase in protandry over time in a long‐distance migratory bird

“…In a few rare cases, individual males were too wary to be captured and remained unmarked. Unlike males, females were not regularly color‐marked, and are less philopatric in this species (Hedlund et al, 2017).…”

“…Females may sometimes choose to pair with an already mated male, and in our study population so called “secondary females” were occasionally observed, but were excluded from the analysis of female arrival as the timing of their pair‐formation with the male was difficult to determine. All males were caught with mistnets and color‐banded (Figure 1), and many returned to breed in consecutive years (Hedlund et al, 2017; Jakobsson, 1988). In a few rare cases, individual males were too wary to be captured and remained unmarked.…”