Introduction
Migratory birds are declining fast (Rosenberg et al. 2019) and
climate change is suspected to be one of the leading causes of this
decline (Both et al. 2010; Saino et al. 2011; Rosenberget al. 2019). Climate constitutes an important constraint on
species distributions (Tingley et al. 2009; Pigot et al.2010) and understanding how it shapes seasonal migration patterns is
crucial to better anticipate how migratory species respond to ongoing
anthropogenic climate change. Previous studies have found that the
migratory avifauna is composed of both species that are tracking
favorable climatic conditions throughout the year (climate niche
trackers) and species that are switching climatic conditions seasonally
(climate niche switchers) (Boucher-Lalonde et al. 2014; Gómezet al. 2016; Laube et al. 2015; Martínez-Meyer et
al. 2004; Nakazawa et al. 2004; Somveille et al. 2019).
However, patterns at the species level could be misleading if the
drivers of bird migration operate at the population level.
Bird migration is a phenomenon that exhibits a great diversity of
individual behaviors and strategies (Newton 2008). Within species,
migratory movements shape patterns of spatiotemporal linkages of
populations between seasons, referred to as migratory connectivity
(Webster et al. 2002), which are being increasingly revealed by
novel technologies documenting differences in migratory behavior
(Faaborg et al. 2010; Finch et al. 2017; Gómez et
al. 2021; Ruegg et al. 2014; DeSaix et al. 2023a). These
migratory patterns have important consequences for the broader ecology
and evolution of a species as it can affect population dynamics via
seasonal carry-over effects (Marra et al. 1998; Sillett & Holmes
2002; Taylor & Norris 2010). Seasonal climate tracking is often assumed
to be an adaptive behavior underlying migration patterns (e.g., Thorupet al. 2021; Gutiérrez Illán et al. 2022), and previous
population-specific studies found evidence of seasonal climate tracking
at population level (Fandos & Telleria 2020; Fandos et al. 2020;
Gutiérrez Illán et al. 2022). If seasonal climate tracking is a
driver of migration patterns, migratory populations are expected to
modify their migration patterns to track changing climate, which has
been suggested in some migratory species (Van Doren et al. 2021;
Dufour et al. 2021; Thorup et al. 2021), and climate
change would thus be a direct driver of the evolution of migration
routes within species. Under this scenario, migratory populations might
be less vulnerable to climate change as they are more likely to track it
by modifying their migratory behavior and routes (Thorup et al.2021).
An alternative hypothesis is that the variation in seasonal climate
tracking among populations might be a consequence of how other processes
shape migration patterns. A recent study found that migratory
connectivity is broadly driven by optimizing the balance between
accessing resources available in the environment and the cost of
movement without directly using information on climate (Somveilleet al. 2021). Migratory connectivity resulting from resource
availability and migration cost could then result in a mixture of
climate-tracking and climate-switching populations whose distribution is
shaped by geographical accident rather than direct causality. The
question of whether migratory connectivity shapes the variation in
seasonal climate tracking in migratory birds has, to our knowledge,
never been investigated despite its important evolutionary and
conservation implications. If the variation in seasonal climate tracking
across populations is a consequence of migratory connectivity, then
populations that by chance track climate could have adapted over time to
a narrow climatic niche and therefore be highly vulnerable to changing
climate.
Previous studies investigating seasonal climate tracking at the
population level (Fandos & Telleria 2020; Fandos et al. 2020;
Gutiérrez Illán et al. 2022) have used tracking and banding data
which makes it difficult and rather arbitrary to define distinct
populations. In contrast, advances in genomics make it possible to
delineate genetically distinct populations across a species and map
their seasonal destinations (Ruegg et al. 2014). A recent study
using genetic markers for Willow Flycatcher, Empidonax traillii ,
for example, found a combination of climate tracking and climate
switching across populations of the species (Ruegg et al. 2021),
but did not explore whether such tracking was a cause or consequence of
migratory connectivity.
Here, we use data on population-level migratory connectivity derived
from genetic markers for genetically distinct populations from 6
different migratory species to investigate the extent to which these
populations track climate throughout the annual cycle. We test whether
population-level seasonal climate tracking is a driver or a consequence
of broad-scale migratory connectivity by first examining if simulation
models of migratory connectivity based on (i) the balance between access
to available energy and the cost of migratory movements (a process
hereafter called energy efficiency ) or (ii) optimizing the
tracking of climatic conditions throughout the year, can explain
empirical variations in seasonal climate tracking across populations. As
migratory connectivity patterns are scale-dependent (González-Prietoet al. 2016) and the underlying drivers might be as well – e.g.
seasonal climate tracking could be affecting migration destinations at
one scale but not at another – we also examine whether migratory
populations track climate conditions at a regional scale better than if
they re-distributed randomly around the most energy efficient
destinations. Overall, this work provides the first explicit test of
whether seasonal climate tracking is a consequence or driver of
migratory connectivity at both broad and regional geographic scales.