Moult of overwintering Wood Warblers Phylloscopus sibilatrix in an annual-cycle perspective

Abstract

Wood Warblers, an Afro-Palearctic migrant species, are declining steadily in Europe likely due to mortality outside their breeding grounds. However, little is known about their overwintering, and records about the sensitive life-cycle stage of moult in Africa are practically absent. To fill this gap, we report on moult of Wood Warblers captured over two winters (January–February) in 2019–2020 in Cameroon. We caught 14 individuals, of which 12 were monitored for flight feather moult. All inspected individuals showed advanced stages of flight feather renewal. Despite low sample sizes, Underhill-Zucchini moult models aptly explained variation in primary and secondary moult (R2 = 0.61). Estimated moult onset date was 26 December, completion date was 25 February, and moult duration was 61 days. These findings fit well with experimental data on the annual cycle and the timing of recently published migration tracks of Wood Warblers. Jointly, the data suggest that moult timing is set by an internal programme, which enables Wood Warblers to organise their multi-stage migration such that they reach suitable moulting habitat in time, and can depart in time with a fresh plumage for the breeding grounds. In our study, moult occurred during the peak of the dry season, which in Cameroon nonetheless shows high relative humidity. During our mist-netting on 28 cocoa plantations of varying shade cover, Wood Warblers were caught on 6 farms whose canopies were comparatively open. These data suggest that the birds encounter in Cameroon relatively stable climatic conditions for moult, and do not measurably prefer closed-canopy forests. Our findings are important, because successful moult increases survival prospects and because moult needs to be safely embedded in a migratory life cycle. Hence, information on moult timing and location is essential for identifying year-round vulnerabilities of Wood Warblers.

The conservation of Afro‐Palaearctic migrants: What we are learning and what we need to know?

“…However, species preferring open woodlands may exhibit some resilience to loss of tree cover, e.g. Wood Warbler (Mallord et al 2016, 2018, Awa II et al 2018, Buchanan et al 2020, Jarret et al 2021) and possibly other migrant species that prefer moderate forest cover, e.g. Willow Warbler (Thorup et al 2017, Willemoes et al 2018.…”

“…2020, Jarret et al . 2021) and possibly other migrant species that prefer moderate forest cover, e.g. Willow Warbler (Thorup et al .…”

Avian migration clocks in a changing world

“…2 b) (Bäckman et al 2017 ; Rime et al 2023 ; Briedis et al 2020 ). Data so far indicate that within species, phases of Zugunruhe indeed set a window in the annual cycle during which wild birds migrate (Jarrett et al 2021 ; Akesson et al 2017 ). However, actograms of free-flying birds also reveal stark differences from those of captive birds (Fig.…”