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).