Migration tracking reveals geographic variation in the vulnerability of a Nearctic-Neotropical migrant bird

Diana L Humple,Kristen E Dybala,Ryan D Burnett,T Will Richardson,Nathaniel E Seavy,Thomas Gardali,Renée L Cormier

doi:10.1038/s41598-020-62132-6

Diana L Humple, Kristen E Dybala + Show 5 more

Open Access

https://doi.org/10.1038/s41598-020-62132-6

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Abstract

We compared the vulnerability of a Nearctic-Neotropical migrant (Swainson’s Thrush, Catharus ustulatus) for three geographically-defined breeding populations in California by linking breeding and wintering regions, estimating migration distances, and quantifying relative forest loss. Using data from light-level geolocator and GPS tags, we found that breeding birds from the relatively robust coastal population in the San Francisco Bay area wintered predominantly in western Mexico (n = 18), whereas the far rarer breeding birds from two inland populations that occur near one another in the Sierra Nevada and southern Cascades mountain ranges migrated to farther wintering destinations, with birds from the Lassen region (n = 5) predominantly going to Central America and birds from the Tahoe region (n = 7) predominantly to South America. Landscape-level relative forest loss was greater in the breeding and wintering regions of the two Cascade-Sierra populations than those of coastal birds. Longer migration distances and greater exposure to recent forest loss suggest greater current vulnerability of Cascade-Sierra birds. Our results demonstrate that for some species, quantifying migration distances and destinations across relatively small distances among breeding populations (in this case, 140–250 km apart) can identify dramatically different vulnerabilities that need to be considered in conservation planning.

Highlights

Quantifying the vulnerability of wildlife populations to environmental change is one step toward identifying and prioritizing management actions designed to preserve biodiversity[1,2,3]
Recent forest loss occurred in the wintering regions of all three breeding populations, and was greater for the two Cascade-Sierra breeding populations (11.2% for Lassen birds; 7.5% for Tahoe birds) than the coastal-breeding population (4.7%)
Our study further revealed that birds from the nearby Lassen and Tahoe regions within the Cascade-Sierra, whose closest study sites were a mere 140 km apart, migrate to fairly distinct regions within Central America (Lassen birds) and South America (Tahoe birds), and should be considered as distinct populations with respect to conservation

Summary

Introduction

Quantifying the vulnerability of wildlife populations to environmental change is one step toward identifying and prioritizing management actions designed to preserve biodiversity[1,2,3]. Vulnerability assessments synthesize information on exposure (extrinsic factors), sensitivity (intrinsic traits), and adaptive capacity (evolutionary potential or plasticity) of species or populations[4]. The loss and degradation of riparian habitat on the breeding grounds in California[12], as well as deforestation of wintering habitat of populations breeding in the Cascade-Sierra[16,21], have been hypothesized as drivers of Swainson’s Thrush extirpations and declines. We assess spatial variation in vulnerability of Swainson’s Thrush breeding in northern California, using data from an earlier tracking study of coastal birds[22] combined with additional tracking of coastal birds from the San Francisco Bay area, and novel tracking of two populations from the Tahoe and Lassen regions within the Cascade-Sierra. Based on the differences in population size, distribution, and trends in these breeding regions, including from the authors’ own extensive observations across all three regions, we predicted different wintering locations for these populations, and that the two Cascade-Sierra populations would have greater sensitivity and exposure than coastal birds, and be more vulnerable

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