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Abstract
Estimating how much long-distance migrant populations spread out and mix during the
non-breeding season (migratory connectivity) is essential for understanding and predicting
population dynamics in the face of global change. We quantify variation in population
spread and inter-population mixing in long-distance, terrestrial migrant land-bird
populations (712 individuals from 98 populations of 45 species, from tagging studies
in the Neotropic and Afro-Palearctic flyways). We evaluate the Mantel test as a metric
of migratory connectivity, and explore the extent to which variance in population
spread can be explained simply by geography. The mean distance between two individuals
from the same population during the non-breeding season was 743 km, covering 10-20%
of the maximum width of Africa/South America. Individuals from different breeding
populations tended to mix during the non-breeding season, although spatial segregation
was maintained in species with relatively large non-breeding ranges (and, to a lesser
extent, those with low population-level spread). A substantial amount of between-population
variation in population spread was predicted simply by geography, with populations
using non-breeding zones with limited land availability (e.g. Central America compared
to South America) showing lower population spread. The high levels of population spread
suggest that deterministic migration tactics are not generally adaptive; this makes
sense in the context of the recent evolution of the systems, and the spatial and temporal
unpredictability of non-breeding habitat. The conservation implications of generally
low connectivity are that the loss (or protection) of any non-breeding site will have
a diffuse but widespread effect on many breeding populations. Although low connectivity
should engender population resilience to shifts in habitat (e.g. due to climate change),
we suggest it may increase susceptibility to habitat loss. We hypothesize that, because
a migrant species cannot adapt to both simultaneously, migrants generally may be more
susceptible to population declines in the face of concurrent anthropogenic habitat
and climate change.