Animal migration as a moving target for conservation: intra-species variation and responses to environmental change, as illustrated in a sometimes migratory songbird.

• Author: Atwell, Jonathan W.
• Position: Symposium

1. Introduction II. "Sometimes Migratory Songbird": The Dark-Eyed Junco A. Diversity Within the Genus Junco B. The Junco as a Model in Science C. Conservation Issues and Status D. Migration in the Junco III. Geographic and Subspecific Variation in Migration A. Migratory Diversity Among Junco Groups B. The Generality of Intra-Species Variation in Migrations C. Geographic Variation in Migrations: Implications for Conservation IV. Intra-Population Variation: Partial and Differential Migration A. Partial Migration B. Differential Migration C. The Junco, a Differential Migrant D. Differential Migrations: ImpUcations for Conservation V. Migrations as Dynamic Phenomena: Responses to Environmental Change A. Shifts in Junco Winter Distribution and Sex Ratio Associated with Climatic Warming B. Rapid Loss of Junco Migration Following Establishment in a Novel Urban Environment C. Migration as Dynamic in Response to Changing Environments: Implications for Conservation VI. Conclusion. I. INTRODUCTION

   Animal migrations are dynamic phenomena that vary over space and time, even among closely related species, populations, and individuals. For example, in many animals there is substantial geographic variation in the migratory tendencies of different subspecies, races, or populations-birds that breed in the north may migrate long distances south to spend the winter, whereas members of the same species that breed at lower latitudes may be entirely sedentary (i.e., non-migratory). (1) Further, even within a discrete population, there can be systematic differences in the distance, routes, endpoints, or seasonal timing of migrations among male versus female or younger versus older individuals. (2) Such variation in migratory behaviors can emerge rapidly over "evolutionary time scales" (e.g., thousands of years)-including over contemporary times (e.g., years or decades) in response to human activities such as habitat alteration and climate change. (3) Therefore, effective conservation agendas for animal migrations must consider the implications of both spatial and temporal variation in migratory behavior, even within a single "migratory species" or a single local population.

   Our primary goal in this paper is to introduce the following three biological topics to nonspecialists, and to discuss their potential implications for legal, policy, management, and research agendas related to the conservation of migrations: 1) geographic variation in migratory behavior within-species (i.e., inter-population variation in migration), 2) variation in migration of different individuals within a single population (i.e., intra-population variation in migratory behavior among individuals), and 3) the sensitivity of migratory behavior to environmental change-with dramatic changes observed even over relatively short time scales.

   To illustrate these topics, we use a common "backyard" North American songbird species, the Dark-eyed Junco (Junco hyemalis). (4) We chose the "sometimes migratory" junco, not because this species's migration is of immediate conservation concern (it is not), (5) but because past scientific research has revealed the complexity of its migration, (6) allowing it to serve as a model to convey why intra-species variation in migration-and the sensitivity of migratory behaviors to environmental change-provide important challenges and opportunities for policy efforts to protect migrations. Principles derived from the junco almost certainly apply to many other migratory species, including those of immediate conservation concern, and we provide selected examples. (7) However, for most species, intra-specific variation in migration or the potential impacts of changing environments on migration have not been well characterized. Even for the junco, which has received much research attention from biologists studying migration, there remain many unanswered questions about migratory variation within and among junco subspecies and populations. These types of information gaps have the potential to confound or frustrate conservation initiatives and should be dealt with by future research efforts.

   The rate at which migratory processes can be altered by environmental changes-habitat destruction or alteration, climate change, construction of barriers to migration, pollution, or anthropogenic food or water supplementation-underscores the need for immediate conservation action and the articulation of ongoing research agendas-both of which must be drafted to accommodate intra-species variation and rapidly changing biological systems. Although both intra-species variation in migratory biology and the sensitivity of migrations to environmental change challenge the desire to generalize in the context of conservation law, policy, management, and research, these topics must be considered if the most effective migration conservation strategies are to be developed.

   In Part II, we introduce the Dark-eyed Junco, providing relevant background information for this species, which we subsequently use as an example to illustrate our key points throughout the following three Parts. In Part III, we explain the extent of geographic (inter-population) variation in migration as a general phenomenon, and we consider the implications of this type of variation for conservation agendas. Part IV introduces the topics of "differential migration" and "partial migration" (both types of intrapopulation variation in migratory behavior among individuals), and we discuss the implications of such intra-population variation for conservation. In Part V, we highlight two recent landmark studies which demonstrate how contemporary environmental changes have rapidly altered migratory biology in the junco, and we consider how these types of studies can inform approaches to conservation. In Part VI, we conclude by summarizing our key points, emphasizing that future research, along with improved communication and collaboration among scientists, policymakers, and managers, could proceed to more effectively consider intra-species variation and response to environmental change in the context of conservation of animal migrations.

2. "SOMETIMES MIGRATORY SONGBIRD": THE DARK-EYED JUNCO

   The Dark-eyed Junco (Junco hyemalis) is a medium-sized (18-22 gram average) sparrow that breeds primarily in montane or higher latitude coniferous and mixed forest habitat throughout the northern part of North America and south through the Western United States. (8) The winter range of the junco includes lower latitudes and lower altitudes throughout North America. (9) The breeding and wintering ranges of Dark-eyed Juncos are illustrated in Figures 2a through 2d, and discussed in more detail below. Juncos forage and nest primarily on the ground, they form conspicuous wintering flocks of ten to fifteen individuals, and they flash their white outer tail feathers when they fly. (10)

   Wintering flocks of juncos are common at birdfeeders and juncos actively feed during winter conditions, earning them the colloquial nickname "snowbirds." (11) Where it occurs, the Dark-eyed Junco is typically one of the most common and abundant songbirds in both its breeding and wintering range, which visualized together, cover most of the United States and much of Canada (Figure 2a). (12) Thus, for millions of North Americans, the junco is arguably the easiest songbird to observe in their backyards and local habitats, including its seasonal arrival and departure on breeding and wintering grounds. The Dark-eyed Junco is technically considered a single species by current taxonomic criteria, (13) but as we discuss below, there are many subspecies and races that differ in feather plumage coloration, body size, life-history and social behavior, and importantly, in their migratory tendencies. (14)

   We refer to the Dark-eyed Junco as "sometimes migratory" because some junco populations are long-distance migrants while others are non-migratory (i.e., sedentary), and yet others are regional "short-range" or "altitudinal" migrants. (15) This geographic variation among junco groups is introduced at the end of this Part and expanded upon in Part III. Further, even within particular junco populations, some individuals (e.g., females) migrate farther than others (e.g., males), (16) and we discuss this intra-population variation below in Part IV. Recent studies, highlighted below in Part V, reveal how climate change and habitat alteration are associated with rapid changes in migratory behavior in some junco populations. As described above, we chose the junco to illustrate pur main points regarding the ubiquity and relevance of intra-species variation and sensitivity of migrations to environmental change not because junco migrations are of immediate conservation concern, but because prior research in this species allows it to serve as a helpful model to convey the importance of understanding the variable and dynamic nature of migratory behaviors over space and time.

   1. Diversity Within the Genus Junco

      The genus Junco has historically been considered to include three species: Volcano Junco (Junco vulcani), found in Costa Rica; Yellow-eyed Junco (Junco phaeonotus), found in Mexico and Guatemala; and Dark-eyed Junco (Junco hyemalis), found in North America north of Mexico. (17) The Dark-eyed Junco, which is the primary focus of this Article, is divided into at least fifteen subspecies, (18) including a group of eight western subspecies collectively known as the "Oregon" Junco. (19) The range of the Oregon Junco complex extends along the west coast of North America from Alaska into Martir Mountains in Baja California. (20) Each junco subspecies has distinctive markings and can be distinguished from the others based on size and coloration. (21) Juncos are conspicuously patterned, and their plumage varies significantly with geography. (22) Some of this variation is illustrated in Figure L For example, in the Oregon complex of the west, juncos have a...