DOI: 10.5061/dryad.m905qftzp
title: Data from: A tale of two seasons: the link between seasonal migration and climatic niches in passerine birds.

description: The question of whether migratory birds track a specific climatic niche by
 seasonal movements has important implications for understanding the
 evolution of migration, the factors affecting species’ distributions and
 the responses of migrants to climate change. Despite much research,
 previous studies of bird migration have produced mixed results. However,
 whether migrants track climate is only one half of the question, the other
 being why residents remain in the same geographic range year-round. We
 provide a literature overview and test the hypothesis of seasonal niche
 tracking by evaluating seasonal climatic niche overlap across 437
 migratory and resident species from eight clades of passerine birds.
 Seasonal climatic niches were based on a new global dataset of breeding
 and non-breeding ranges. Overlap between climatic niches was quantified
 using ordination methods. We compared niche overlap of migratory species
 to two null expectations, 1) a scenario in which they do not migrate and
 2) in comparison to the overlap experienced by closely related resident
 species, while controlling for breeding location and range size. Partly in
 accordance with the hypothesis of niche tracking, we found that the
 overlap of breeding vs. non-breeding climatic conditions in migratory
 species was greater than the overlap they would experience if they did not
 migrate. However, this was only true for migrants breeding outside the
 tropics and only relative to the overlap species would experience if they
 stayed in the breeding range year-round. In contrast to the hypothesis of
 niche tracking, migratory species experienced lower seasonal climatic
 niche overlap than resident species, with significant differences between
 tropical and non-tropical species. Our study suggests that in seasonal
 non-tropical environments migration away from the breeding range may serve
 to avoid seasonally harsh climate; however, different factors may drive
 seasonal movements in the climatically more stable tropical regions.
description: <strong>Range maps
 migratory birds:</strong> The GeoMiB database was created
 specifically to produce the best possible range map for each species
 classified as at least partially non-resident (migratory or nomadic) in
 Eyres et al. (2017), by using the best source for seasonal range maps for
 each region and combining each species’ seasonal range map across regions.
 The distribution maps in the GeoMiB database are polygon range maps or
 atlas data (which were summarized into a 1° latitudinal-longitudinal grid
 and turned into polygon range maps), depending on the source. Sources were
 chosen as the best available information on species’ seasonal range maps
 for a region, so maps were compiled for most species from multiple
 sources. Each source was either downloaded from existing online databases
 or species maps were scanned from books, geo-referenced, and digitized
 into a geodatabase (Table 2). As some sources covered the same regions, we
 applied a priority algorithm that overlaid the source maps in a specific
 order and then deleted areas covered by higher-priority sources in the
 lower-priority source maps to derive a final species map (from highest to
 lowest priority: book sources, regional databases, global maps;
 Supplementary Table 2). This ensured that sources with higher priority
 were used for those regions rather than maps from sources with lower
 priority, unless no map from sources with higher priority was available,
 in which case lower-priority sources supplied the map. Therefore, maps for
 each species were pieced together from the best available sources for each
 region of the species’ occurrence. The GeoMiB database was compiled in
 PostgrSQL version 9.1/9.6 with PostGIS version 2.0/2.3.2. As most of the
 underlying data are copyrighted, the database cannot be made fully
 publicly accessible. <strong>Peak breeding and
 wintering months:</strong> For each species, we
 characterised the breeding and non-breeding climatic niches using seasonal
 distribution and climate data. Breeding time is species-specific so we
 determined the three peak breeding months for each species individually
 using information from the literature (del Hoyo et al., 2019, and others;
 see Table S1 for details). Where no information was available on the
 breeding months, these were chosen using information from con-generic
 species breeding in the same geographic region (31 of 437 species in the
 final analyses, for details see Table S1). The three non-breeding months
 for each species were defined as 6 months later than the breeding season,
 a somewhat arbitrary decision given the different degree of climatic
 seasonality and migratory timing in different regions and species, but
 chosen to be globally consistent across all species.
description: This dataset includes the non-breeding distributions of
 migratory species. These are as extent-of-occurrence
 polygons sampled into 1 degree grid. These were combined with
 breeding ranges (not uploaded here) from
 https://macroecology.ku.dk/resources/global_avian_distributional_database/ Peak breeding and non-breeding months were collected from the literature (Handbook of the bird online). Migratory classification comes from Eyres et al 2017 and is already deposited on the Senckenberg data repository: http://dx.doi.org/10.12761/SGN.2017.10058