Table 1 Proposals of empirical tests to disentangle the relative contribution of vagrancy and migration to the occurrence of the Yellow-browed Warbler (Phylloscopus inornatus) in Europe

Ageing YBWs across Europe

The low survival probability of young individuals (inferred by the small size of the YBW [103]) and the probable costs of reaching a region (northwestern Europe) whose climate is very different from the normal wintering habitats in Southeast Asia imply that most vagrants will probably not survive after the first outbound migration. On the contrary, since migrant passerines generally follow the same migratory route during their life [104, 105], individuals returning to the breeding area will head towards the previous wintering ground in subsequent years

If only juveniles in Europe: only compatible with vagrancy

If adults and juveniles in Europe: compatible with both vagrancy and migration but a large proportion of adults would support the occurrence of regular migration to Europe

1) Ageing with plumage: adult birds undertake a complete post-breeding moult between July and September whereas juveniles undertake a partial post-juvenile moult between July and September involving head, body and probably some wing-coverts and occasionally some tail feathers [106]. Experienced ringers may therefore find a difference in structure and ground colour of greater-, median-, lesser-coverts, primaries and tail-feathers that can allow ageing

2) Proving the return of the same individuals in successive winters by marking individuals in wintering areas

1) Ageing this species remains challenging even for ringers with direct experience of the species, while most European ringers have no experience with adult YBWs

2) Evaluating age-ratio could be challenging because of a differential occurrence of adult and juvenile birds at coastal ringing sites [72, 107]: adults could use a more direct migration route through Central Europe. In addition, further efforts should be made to age birds that are observed early during autumn migration, especially in southern Europe and North Africa as adults might migrate before juveniles [108]

Orientations of YBWs in Europe

Birds that go west will end up in the Atlantic and will not be able to survive or re-orient to find suitable wintering conditions. Conversely, if some birds spend the winter in the Iberian Peninsula, they are expected to orient, at some point, south or southwest. Expectations are different depending on European regions

For localities in Western Europe (e.g., Norway, Shetlands, Western France): if only west, only compatible with vagrancy (in these locations)

In same locations and elsewhere: if west to south, compatible with both vagrancy and migration

1) Orientation in modified Emlen-funnels for innate heading of migration direction (i.e., with access to only the Earth’s magnetic field, with access to the stars or a combination of cues; [109, 110])

2) In free-flight conditions with the Motus network technology, considering external factors and heading direction (i.e., access to stars, wind/rain, ecological barriers ahead; [111])

3) Route simulations based on known preferred orientation and alternative compass mechanisms [112,113,114]

External factors can influence the flight direction in field experiments

Inheritance of the westward orientation

Vagrancy may or may not have a genetic basis, but migration orientation is under genetic control in passerines

If no genetic inheritance of the westward orientation: only compatible with vagrancy

If inherited genetic basis: compatible with both vagrancy and migration

1) Genomic analyses: by comparing the genome of individuals using westward and eastward orientation, we might be able to find genomic regions involved in the determination of orientation (e.g. Delmore et al. 2016, 2020)

2) Cross-breeding experiments: the heritability could be tested by reproducing individuals with known orientation and testing the orientation of their F1 offspring

Breeding small insectivores over several generations is highly challenging

Breeding origin of YBWs reaching Europe

Vagrant YBWs can come from anywhere within the breeding range of the species, but we expect that birds from the western part of the distribution are more likely to reach Europe and therefore will be overrepresented in Europe [27]

If YBWs seen in Europe come from different breeding localities: compatible with both vagrancy and migration

If all YBWs breeding in a distinct section of the breeding range migrate to Europe: only compatible with migration

With a two-step process: 1) isotopic analyses from feathers collected in Europe would determine if birds that migrate west originate from only a “rather small” area or come from the entire breeding range [115]. See also [116] for a combined approach of several methods

2) if this step identifies a particular area where YBWs in Europe come from, field work in this area to catch breeding YBWs and sample their feathers grown during winter to check with isotopes if all birds migrate west in this area

The results of the isotope analyses will depend on the quality of sampling across the entire breeding range to calibrate the models

Tracking of YBWs reaching Europe

In contrast to vagrants, regular migrants are expected to return to the breeding grounds, transmit the information of the westward migration route to their offspring and return to Europe to winter

If some YBWs tracked from Europe return to the breeding ground and migrate back to Europe: compatible with, but not proof of, migration

Catch and deploy tracking device (i.e., light level geolocator: GLS) on YBWs in Europe in winter

GLS small enough for YBW are not yet available. Low site fidelity in winter would reduce retrieval rate

Population dynamics

Investigating changes in breeding distribution as well as understanding the impact of conditions that will influence breeding output of different breeding populations, success of migration from Siberia to Europe and survival of birds attempting to over-winter in Europe will help to determine the cause

If breeding success across a wide part of the range and conditions promoting successful migration to Europe are important: more compatible with vagrancy. If over-winter conditions in Europe are important for number next year: more compatible with migration

Relate numbers occurring in Europe each year to: weather conditions in breeding areas (will determine breeding success), weather conditions during autumn migration through Siberia, weather conditions in wintering areas in Europe. Ideally compare historical and current information on breeding range, breeding habitat, densities and reproduction success

Data availability:

data to directly determine population size and breeding success in Siberia are unlikely to be available so it is necessary to use ecological proxies, which may not accurately capture causes of variation