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.