Study species and feather moult data collection
The study included 21 Palearctic passerine species from the families Motacillidae (5 species), Cinclidae (1 species), Muscicapidae (5 species), Turdidae (2 species), Paridae (2 species), Sittidae (1 species), Laniidae (1 species), Fringillidae (3 species) and Emberizidae (1 species), which are characterized by a wide distribution range across the Palearctic biogeographic zone. For these species, we sampled the extent of feather moult in western and eastern Palearctic populations. The border between these populations was defined as the eastern border of European Russia, the Ural Mountains and the Caspian Sea (Sclater 1858; Tutin et al. 1964; Roselaar 2006). We included species for which we were able to measure at least 10 individuals for each population (Supplementary Table 1).
Data were obtained from bird skins stored in the collections of nine natural history museums: (I) Natural History Museum (Tring; UK), (II) Museum National d’Histoire Naturelle (Paris, France), (III) National History Museum of Denmark (Copenhagen, Denmark), (IV) Museum für Naturkunde (Berlin, Germany), (V) Museo Nacional de Ciencias Naturales (Madrid, Spain), (VI) Naturhistoriska Riksmuseet (Stockholm, Sweden), (VII) Natural History Museum Vienna (Vienna, Austria), (VIII) Steinhardt Museum of Natural History, Tel-Aviv University (Tel-Aviv, Israel) and (IX) the Finnish Museum of Natural History (Helsinki, Finland). Additionally, we used moult data that were collected during bird ringing activities in Israel (several sites, mainly the Jerusalem Bird Observatory, 31.78°N / 35.21°E, and Beit She’an Valley, 32.45°N / 35.54°E) and Mongolia (Khurkh Bird Ringing Station, Khurkh River Valley, Khentii, 48.28°N / 110.49°E). The moult region (Eastern or Western Palearctic) of the individuals that were examined in the study was determined by the collection or sampling location or by plumage and other morphological characteristics (according to published information; (Del Hoyo et al. 2019) used to identify individuals to the subspecies level, which is confined to a certain breeding distribution range (Supplementary Table 1).
The age of each bird was determined using published plumage characteristics for passerine species (Jenni & Winkler, 1994). Juvenile, nest-grown feathers are poorer in texture, duller and characterized by higher abrasion than those grown during the post-juvenile moult (by juveniles) or the post-breeding moult (by adults). Using these plumage characteristics, each individual was aged as either a first-/second-calendar year (post-juvenile) bird or an adult. We used specimens of all first-/second-calendar year individuals that had completed their partial, post-juvenile moult but had not yet begun their first complete moult. Therefore, data from late autumn to early summer was included in the study, with the exception of the Spotted Flycatcher (Muscicapa striata ) that undergoes a complete moult during its first winter; consequently, data for this species was collected from autumn to early winter. Even though the examined specimens were collected at different dates within the sampling period, there is no expected impact of the date of collection on the recorded moult since an additional moult only occurs in the following autumn (or in winter among Spotted Flycatchers). All of the specimens that were included in this work were collected in the Palearctic region in the years 1806-2019. All data were collected by the first author from 2012-2020.
To quantify moult extent, each wing feather of each individual was classified as being either moulted or non-moulted in the post-juvenile moult. In this study, we included only individuals whose moult had ended, confirmed by the absence of signs of moult upon visual inspection of each feather. Indications of active feather moult include the presence of feathers that are short in size and those with protective peripheral sheath remains. Moult extent was determined by documenting the moult of the wing feathers using a score of 0 (non-moulted feather) or 1 (moulted feather). A total of 42 feathers were documented using this method in each individual, as follows: greater-coverts (GC1-10), carpal-covert (CC), alula (Al1-3), primary coverts (PC1-9), primaries (P1-10), secondaries (S1-6) and tertials (T7-9). In addition, we measured the proportion of moulted feathers in two feather tracts, the lesser-coverts (LC) and the median-coverts (MC). These two feather tracts are characterized by numerous small and difficult to distinguish feathers, for which we estimated the proportion of the area moulted.
For each species, we calculated the mean migration distance in each region, Eastern or Western Palearctic, and the mid-breeding (and hence mid-moulting) latitude using published distribution maps (BirdLife International and NatureServe 2014; Del Hoyo et al. 2019). The mid-breeding latitude (and mid-wintering latitude, see below) was calculated separately for each region as the average of the northernmost latitude and the southernmost latitudes of a specie’s distribution range, and the mean migration distance was the distance between the mid-breeding and mid-wintering distribution areas. Due to the inaccuracy of these maps, we calculated the migration distance with an accuracy of 1000 km. We also incorporated published data of the species’ mean body mass (Dunning Jr 2007; Supplementary Table 1).