Animal Migration

C H A P T E R

1 Animal Migration: A Context for Using New Techniques and Approaches Keith A. Hobson1, D. Ryan Norris2, Kevin J. Kardynal1 and Elizabeth Yohannes3 1

Environment and Climate Change Canada, Saskatoon, SK, Canada, 2University of Guelph, Guelph, ON, Canada, 3University of Konstanz, Konstanz, Germany

1.1 INTRODUCTION

Some key proximate questions related to animal migratory movement include: How do these individuals know where they are going? How do they cope with the tremendous physical demands of travel? How do they adjust to unfamiliar or changing environments along the way to their destinations? What are the costs and benefits of migration? How do migratory patterns influence long-term population dynamics? Ultimately, the evolution of migratory movements remains among the most fascinating and challenging to understand. Finding answers to these fundamental questions has proven to be an immense scientific challenge. A large part of this challenge is related to the limited tools available to researchers to infer or determine large- and small-scale animal movements. This volume explores recent developments in stable isotope techniques which have contributed tremendously to the field of understanding animal movement, dispersal and long-distance migration in terrestrial

The spatiotemporal movement of organisms determines their interaction with their environments and, therefore, comprises a fundamental aspect of their ecology and evolutionary life history. The degree to which organisms move also characterizes the range of resources they encounter, the array of hazards they experience from predators to hurricanes, and the number of organisms with which they may interact. For animals, movement is very much a question of geospatial scale. While some species may wander nomadically over a landscape of a few square meters for their entire lives, others travel across thousands of kilometers in regular movements that constitute some of the most spectacular natural phenomena on the planet. These migrations are the movements that have captured the interest of researchers, citizen scientists, and laypersons alike and leave us with a true sense of wonderment.

Tracking Animal Migration with Stable Isotopes DOI: https://doi.org/10.1016/B978-0-12-814723-8.00001-5

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© 2019 Elsevier Inc. All rights reserved.

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1. ANIMAL MIGRATION: A CONTEXT FOR USING NEW TECHNIQUES AND APPROACHES

and marine ecosystems. As we will demonstrate, stable isotope tools are not only a critical component of the larger toolbox but can typically augment other sources and forms of information. The term migration often evokes images of spectacular seasonal movements of animals,

(A)

especially birds, over vast distances but involves many taxa (Fig. 1.1). The Arctic Tern (Sterna paradisaea) migrates from breeding grounds in the Arctic to wintering grounds in the Antarctic, an annual round trip of a staggering 40,000 km (Egevang et al., 2010), equivalent to 2 years of day-to-day driving by the

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FIGURE 1.1 Examples of migration patterns in different taxa in North America: (A) the Porcupine caribou herd (Rangifer tarandus), (B) songbirds (American Redstart Setophaga ruticilla), (C) insects (Monarch Butterfly Danaus plexippus), and (D) fish (Pacific salmon). Migratory pathways depicted by arrows are only generalizations.

TRACKING ANIMAL MIGRATION WITH STABLE ISOTOPES

1.1 INTRODUCTION

average North American motorist. Salmon return by the millions to natal streams at the end of their lives after spending years moving thousands of kilometers in the open ocean (Quinn, 2005). In the first year of their lives, eastern populations of Monarch Butterflies (Danaus plexippus) in North America travel to overwintering roosts in the Transvolcanic Mountain range of central Mexico, a trip that can be over 4000 km for an insect that weighs only 0.5 grams (Flockhart et al., 2013). For those of us living in temperate (and in subtropical) environments, the annual spring and autumn movements of billions of migratory birds, from warblers to waterfowl, likely provide the most familiar examples of migration (Dingle, 2014; Greenberg & Marra, 2005). Indeed these movements are spectacular, yet much confusion still exists as to what exactly we mean by migration. Clearly, migration can include both to-and-fro and one-way movements. A to-and-fro or round-trip migration can be characterized by animals either returning on the same path or by individuals following a loop migration pattern. Various other patterns of long- and short-distance movement have been described between origin and destination, especially for birds (Berthold, 1999). For example, leap-frog migration involves individuals at the northern limits of their breeding range traveling the farthest south (in northern hemispheric animals) to most distant wintering grounds, whereas those from more southern breeding regions migrate the least distance to more northern wintering grounds (e.g., Bell, 1997; Kelly, Atudorei, Sharp, & Finch, 2002; Ramos et al., 2015). Longitudinal migration involves all individuals migrating the same approximate distance in a chain pattern or in parallel (Norris et al., 2006; Salomonsen, 1955). Animal migration can be obligate whereby all members of a population move or facultative whereby resource availability may act to determine if migration occurs. Partial migration refers to a situation

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when only part of the population migrates (Boyle, Norris, & Guglielmo, 2010; Chapman, Bro¨nmark, Nilsson, & Hansson, 2011; Jahn, Levey, Hostetler, & Mamani, 2010). Differential migration describes those situations when migration patterns differ between sexes, age groups, or morphs within a population ˚ kesson, 2016; Ketterson & Nolan, 1976; (A Woodworth et al., 2016). Some insect species move from the location they were produced to another location where they reproduce the next generation and die. That generation may then move on, repeating the process. For Monarch butterflies, this occurs in a series of one-way steps involving multiple generations before a final cohort returns to their starting point, the discrete overwinter roosts in the mountains of central Mexico (Flockhart et al., 2013). Many salmon species also undertake spectacular migrations from the sea to freshwater rivers where they spawn and die. Although this is often considered one-way movement, it may be more accurately thought of as a “fatal round-trip migration” because offspring first have to migrate downriver where they spend several years at sea and then return to freshwater to spawn and then die (Quinn, 2005). Altitudinal migration is a short-distance movement between lower and higher elevation habitat. It is commonly thought to happen in response to resource availability (Levey & Stiles, 1992) and weather or climate conditions (Boyle et al., 2010). These movements are more common in the tropics where frugivory or nectarivory is well developed and in more temperate areas of high relief (Boyle, 2017). Migration within the tropics over large distances, known as intratropical migration, occurs in many tropical-breeding bird species and has more recently been observed in several temperate-breeding long-distance migrants (Heckscher, Halley, & Stampul, 2015; Levey & Stiles, 1992; Morton, 1977). Such movements are potentially in response to changes in

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1. ANIMAL MIGRATION: A CONTEXT FOR USING NEW TECHNIQUES AND APPROACHES

resource availability and abundance, competition habitat change, or weather events (e.g., flooding; Boyle et al., 2010; Kardynal & Hobson, 2017; Stutchbury et al., 2016). Although it may not seem like a form of migration, nomadism, invasion, irruption, where animals move in irregular patterns and breeding sites are established opportunistically where conditions are favorable, is a strategy employed by some animals. Several species of temperate forest seed-eating birds like crossbills, grosbeaks, and siskins are nomadic because of their reliance on cone crops or mast, which varies considerably in time and space (Newton, 2006; Strong, Zuckerberg, Betancourt, & Koenig, 2015). However, as Dingle (2014) noted, defining migration based solely on the outcomes (or patterns) such as to-and-fro, one-way, altitudinal, and nomadic movements can be somewhat problematic. For example, many animals also show to-and-fro or nomadic movements at a local scale within their home range. Regular movements within a home range can involve commuting between resource patches such as hummingbirds moving among sources of nectar or between resource patches and reproductive sites as seen in wolves during the denning period and in many species of colonial nesting seabirds. These types of movements are not considered migration because they occur within the home range of the species and are irregular with no consistent or strict annual or seasonal basis (Dingle, 2014). Migration also requires special physiological and behavioral adaptations during the period of movement, whereas movements within a home range usually require few such demanding changes. Migration can be considered an adaptation specific to arenas in which changes in habitat quality in different regions occur asynchronously so that movement allows a succession of temporary resources to be exploited as they arise. In this sense, migration involves escape and colonization (Dingle & Drake, 2007). In

their holistic view of migration, Dingle and coworkers (Dingle, 2014; Dingle & Drake, 2007) see the arena as the environment to which migrants are adapted and the migration syndrome is the suite of traits enabling migratory activity that involves both locomotory capabilities and a set of responses to environmental cues that schedule and steer the locomotory activity. In this respect, migratory movements can be differentiated from movements within a home range or dispersal. Their conceptual model also includes a genetic complex that underlies the syndrome and a population trajectory comprising the route followed by the migrants, the timing of travel and stops along it, and the periods and locations corresponding to breeding and other key lifehistory events. The migration system involves a species’ specific migration syndrome that determines how the animal responds to external cues so that preemptive movements can occur prior to resource collapse. A fascinating area of study involves understanding those physiological adaptations to migration, including the incorporation of reproduction and other tasks (Ramenofsky & Wingfield, 2007) and the underlying genetic processes that result in selection for specific migratory traits (Bazzi et al., 2016; Pulido, 2007; Roff & Fairbairn, 2007). Although the overall control of migration, its timing and response are known to be genetically determined, strikingly this “unique” trait appears to be a primitive characteristic also exhibited even in strict resident and nonmigratory avian species (Helm & Gwinner, 2006). However, the ability to navigate and orient during migration is a much more complex phenomenon that may require both endogenous programing and acquired by learning. As Dingle (2014) points out, the definition of migration must be based on the behavioral and physiological processes involved with the movement rather than the spatial pattern or outcome. Thus, we consider migration as the

TRACKING ANIMAL MIGRATION WITH STABLE ISOTOPES

1.2 MIGRATORY POPULATIONS, CONNECTIVITY, AND CONSERVATION

movement away from the home range that does not cease (at least not initially when suitable resources are encountered) and which requires a set of behavioral and physiological adaptations for sustained movement that are unique from day-to-day adaptations related to self-maintenance and breeding.

1.2 MIGRATORY POPULATIONS, CONNECTIVITY, AND CONSERVATION Although selection for adaptive traits related to migration is thought to occur at the individual level, migration systems and patterns are more often described at the population level, typically as the migration of populations or subpopulations within a species. We usually deal with population units when engaging conservation efforts and the conservation of populations that move over large distances presents considerable challenges (Marra, Norris, Haig, Webster, & Royle, 2006; Webster, Marra, Haig, Bensch, & Holmes, 2002). Populations of animals might be well protected and managed at one location, but then suffer no protection once they leave that area. For long-distance migrants, their travels may take them to new geopolitical jurisdictions where conservation measures are absent or inadequate. It makes little sense to expect migratory populations to respond positively to local conservation measures if key aspects of individual fitness or overall population health are being influenced at another location. The problem of conserving species whose life cycles cross geopolitical borders has become a profound issue in the 21st century as habitats everywhere are being reduced in size or quality, and migrants are forced to move among a myriad of patches, each with its own level of quality, safety, and prospects for future existence (Moore, Smith, & Sandberg, 2005; Robinson, Thompson, Donovan, Whitehead, &

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Faaborg, 1995). In this sense, migratory organisms are best considered as a network that is made up of a series of populations (that may or may not mix between different periods of the year) rather than as a single species per se (Taylor & Norris, 2010). For some animals, populations breeding in one area may follow similar migration routes and winter in the same general wintering region. In this case, we would consider the subpopulations to show strong connectivity. Other breeding populations may disperse widely on the wintering grounds (or vice versa), which can be considered a case of weak connectivity. The concept of migratory connectivity is important to conservation. Many forms of connectivity are possible (Boulet & Norris, 2006; Webster & Marra, 2005) ranging from the extreme cases where individuals of breeding populations show high or low philopatry to the same wintering location. Among birds, the Kirtland’s Warbler (Setophaga kirtlandii) is a good example because the small breeding population in Michigan, USA, winters exclusively in the Bahamas. Weak connectivity may involve individuals from many breeding populations mixing among many wintering populations or vice versa. However, such concepts have generally lacked quantification. The first attempt at quantifying connectivity was by Ambrosini, Moller, and Saino (2009) who applied Mantel correlation tests on distances between pairs of birds on breeding and wintering grounds based on ring recovery data of Barn Swallow (Hirundo rustica) breeding in Europe and wintering in Africa. More current approaches to measuring connectivity consider both spatial and temporal dynamics (Bauer, Lisovski, & Hahn, 2016) and sampling bias (Cohen, Hostetler, Royle, & Marra, 2014; Cohen et al., 2017). A recent metaanalysis by Finch, Butler, Franco, and Cresswell (2017) showed that weak connectivity was the norm for longdistance migratory birds in the Old and New World.

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1. ANIMAL MIGRATION: A CONTEXT FOR USING NEW TECHNIQUES AND APPROACHES

Despite a general lack of quantification, the concept of migratory connectivity can be useful for conservation priority setting. Populations with strong connectivity are hypothesized to be those that are most vulnerable to declines because they have little chance to be “rescued” by peripherally connected populations (Marra et al., 2006; Taylor & Norris, 2010). On the other hand, it is the strongly connected populations that may benefit the most from conservation efforts because the connections between two or more areas are well established. Although populations of weakly connected individuals may be inherently “safer,” in the sense that risk is spread among more locations, these populations will understandably be harder to manage unless measures are enacted over large regions that encompass a significant percentage of the subpopulation of interest. It may also be more difficult to identify the causes of population declines in subpopulations that are weakly connected. For example, when migratory populations are mixed between two periods, habitat loss in one subpopulation may produce synchronous declines in subpopulations in the following season (Hewson, Thorup, PearceHiggins, & Atkinson, 2016; Marra et al., 2006; Taylor & Norris, 2010). Knowledge of migratory connectivity is also important for being able to predict how conservation measures in one season will influence populations the following season. For example, Martin et al. (2007) demonstrated that the protection of nonbreeding habitat for American Redstarts (Setophaga ruticilla) based on rate of habitat loss, density, and land cost resulted in the almost complete extinction of one breeding region. This was because the criteria for deciding where and when to conserve nonbreeding habitat did not consider where individuals were spending the remainder of the annual cycle. Consideration of how populations are connected can, therefore, result in radically different decisions on how to allocate resources for conserving species.

In terms of conservation, climatic and environmental changes, and land use progression, the timing of breeding has important implications for avian productivity and is one of the traits most noticeably affected by spring weather conditions (Dunn, 2004; Dunn & Winkler, 1999; Winkler, Dunn, & McCulloch, 2002). Birds that nest earlier generally produce more offspring that are ultimately recruited into breeding populations (Dunn, 2004; Perrins, 1970). It remains uncertain whether migratory birds and other animals will be able to continue to synchronize the timing of their breeding with optimal conditions for reproduction on breeding areas in light of environmental changes (Both & Visser, 2001; Charmantier & Gienapp, 2014; Mayor et al., 2017; Visser, van Noordwijk, Tinbergen, & Lessells, 1998). In their long-distance travels that cross international borders, migratory animals can also act as vectors of parasites and pathogens. There are a number of ornithophilic mosquitoes that can infect migratory birds that in turn act as introductory or amplifying hosts (Rappole, Derrickson, & Hubalek, 2000; Reed, Meece, Henkel, & Shukla, 2003; ˙ Valcu, Bensch, & Yohannes, Kriˇzanauskiene, Kempenaers, 2009). Recent interest in the spread of avian influenza viruses around the globe has increased the interest in determining the migratory origins of infected and noninfected wild birds (Bodewes & Kuiken, 2018; Liu et al., 2005). This level of interest will only increase as the distributions of suitable habitats available to these pathogens change over time. Effective management of species also requires identifying where populations are most productive. We are all familiar with field guides that show the absolute spatial ranges occupied by a given species, but where within that vast space are the most young being produced and recruited into the autumn migratory population and where are individuals experiencing the highest survival rates during

TRACKING ANIMAL MIGRATION WITH STABLE ISOTOPES

1.2 MIGRATORY POPULATIONS, CONNECTIVITY, AND CONSERVATION

the nonbreeding period? Understanding where hot spots of productivity and survivorship occur helps us protect those places and better understand factors that may be limiting populations. Nor are species necessarily more productive where they are most abundant, and numerous examples exist of how animals can be lured into ecological traps where they suffer reduced success. For migratory game birds, understanding how zones of productivity relate to protected areas and the timing of harvest is particularly crucial to their conservation (Asante, Jardine, Van Wilgenburg, & Hobson, 2017; Hobson, Van Wilgenburg, Ferrand, Gossman, & Bastat, 2013). For many years, our understanding of the ecology of migrant organisms was influenced by their activities during the breeding season in locations where they were accessible to scientists working in the temperate regions. Such interest was also driven by an assumption that decisions made by individual animals related to breeding were of paramount importance to an individual’s fitness. Aspects of breeding success related to habitat choice, mate choice, timing of breeding, and the quality of individuals were topics that were considered largely as they played out on the breeding grounds, with little regard to events that preceded and followed them on the wintering grounds and during migration. This situation changed in the 1980s when waterfowl researchers began to discover that female body condition upon arrival on the breeding grounds in spring was linked to conditions she experienced in winter, and that these could have profound influence on the number of young recruited into the autumn population (Heitmeyer & Fredrickson, 1981; Kaminski & Gleusing, 1987). In the late 1990s, it became more widely accepted that events occurring outside the breeding area can profoundly affect the fitness of migratory individuals. Much of this recent interest has been generated from the work of

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Marra, Hobson, and Holmes (1998) who, using stable isotopes, first demonstrated in American Redstarts a clear link between spring arrival times of males and the quality of the habitat they occupied in winter. The quality of a habitat where an animal winters will determine the level and speed with which the necessary body condition for migration can be reached, and so can determine when and in what condition that individual arrives on the breeding grounds. Several studies of birds have shown that earlier arriving birds can obtain better territories, initiate clutches sooner, and generally fledge more young than those arriving later (e.g., Beˆty, Gauthier, & Giroux, 2003; Gill et al., 2001; Norris, Marra, Kyser, Sherry, & Ratcliffe, 2004). How costs associated with reproduction might influence an individual’s ability to compete during the nonbreeding season remains a mystery. Nonetheless, seasonal interactions between the wintering grounds and the breeding grounds, as well as during migration itself, should ultimately play a large role in determining lifetime fitness and population abundance (Harrison, Blount, Inger, Norris, & Bearhop, 2011; Norris & Marra, 2007; Rushing et al., 2017; Sillett & Holmes, 2002; Yohannes, Mihai Valcu, Lee, & Kempenaers, 2010). Establishing details of migratory programs in animals is fundamental to understanding key aspects of their evolution, life history, and conservation. The field has witnessed a renaissance in recent years primarily because the development of new analytical techniques has provided some true breakthroughs. There is also a more urgent sense that we need to more quickly understand key migratory linkages for animal populations in order to help conserve them in a changing world. It is in this spirit that we developed this volume on stable isotope techniques because that approach has and will continue to contribute immensely to the study of animal migration. We also recognize that, as with all techniques and approaches, there is a real need to

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1. ANIMAL MIGRATION: A CONTEXT FOR USING NEW TECHNIQUES AND APPROACHES

provide a firm foundation for practitioners and to point to ways in which the field might best progress.

1.3 SCIENTIFIC TOOLS USED TO STUDY MIGRATION Tracking migratory terrestrial animals has involved numerous techniques over the years (Table 1.1). Until very recently, all approaches involved the use of extrinsic markers applied to individuals with the hope of relocating those same individual elsewhere, or on the use of recognized phenotypic or morphological traits that showed known geographic variation. For birds, there is a rich literature on geographic variation in plumage and morphology and these traits have been used to describe migratory connectivity. A good example is the Yellow-rumped Warbler (Setophaga coronata) that shows plumage variation between eastern (coronata) and western (audubon) races in North America and that also segregates largely on the wintering grounds. Similarly, the Swainson’s Thrush (Catharus ustulatus) occurs as a reddish backed phenotype on the Pacific coast of North America and a more common olive-backed form elsewhere. Birds captured during migration can readily be placed into these types without the need for markrecapture techniques. Most current field guides provide numerous examples of geographic variation in plumages (Sibley, 2014). An excellent example of how plumage and morphometrics have been used to describe migration patterns in Nearctic Neotropical migrant birds that moved through Mexico was that of Ramos and Warner (1980) and Ramos (1983). However, typically the geographic resolution provided by this approach is both poor and highly variable among taxonomic groups. For example, as noted by Boulet and Norris (2006), within the 50 species of migratory wood warblers of the New World, 65 % are monotypic,

30 % include 2 4 subspecies, and only 6 % include 5 or more subspecies. By far the most widespread approach to track migrant animals is through the application of passive extrinsic markers. For birds, these have overwhelmingly involved leg bands or rings carrying a unique number combination and some instruction on where to report the band if it is recovered. Other markers such as patagial tags, numbered neck collars, streamers, or color dyes have also been used. Insects have proven to be more of a challenge due to their small size, but numbered labels have been successfully affixed to the wings of butterflies and later recovered (Urquhart & Urquhart, 1978; www.monarchwatch.org). In the relatively short historical period (i.e., 100 years) of banding birds, many millions of individuals have been individually tagged. For a number of species with small global populations and restricted ranges, some very impressive recovery rates have been achieved (e.g., Owen & Black, 1989) and some key insights into migratory connectivity established (Cohen et al., 2014; Gill et al., 2001; Moore & Kremenetz, 2017). However, for the vast majority of species, extremely low recovery rates (i.e., ,0.01 %) are the norm (Hobson, 2003).

1.3.1 Transmitters, Radar, and Satellites Active extrinsic markers are those that send out signals that can be intercepted with a suitable receiver device. Advances in transmitter technology have allowed the placement of devices on migratory animals. If a receiver is within the range of the transmitter, then the location of the animal can be inferred either by tracking down the individual or by triangulation. Radio frequency transmitters can be made small enough (B0.4 g) to place on small passerines and bats. However, with miniaturization comes a loss of range and battery life so that

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1.3 SCIENTIFIC TOOLS USED TO STUDY MIGRATION

TABLE 1.1 Summary of Various Techniques Used to Track Migration in Animals Technique

Advantages

Disadvantages

Extrinsic

1. Can apply to a broad range of animals.

1. Requires initial capture and then recapture.

2. High spatial resolution.

2. Biased toward initial capture population.

Phenotypic variation (e.g., morphology and plumage)

1. Inexpensive.

1. Low resolution.

2. Can be applied to historical specimens with high degree of confidence.

2. Not applicable to all species.

Banding/marking

1. Inexpensive.

1. Many species have low recovery rates (,0.5%).

2. Provides exact information on start and end of movements.

3. Provides estimate of natal origin only.

2. Can take many years to get adequate data.

References

Ramos (1983), Ramos and Warner (1980), Bell (1997), Boulet and Norris (2006), Conklin, Battley, Potter, and Ruthrauff (2011)

Brewer, Diamond, Woodsworth, Collins, and Dunn (2000), Bairlein (2001), Cohen et al. (2014), Moore and Kremenetz (2017)

3. Still only a small number of major banding stations across globe. 4. Both marking and recovery (start and end points) are biased toward locations of major banding stations. Radio transmitters

1. Produces precise 1. Low range. locations (relative to extrinsic markers through 2. Can obtain precise trajectory if within range of triangulation). transmission. 2. Can obtain precise trajectory if within range 3. Expensive relative to banding or morphological methods. of transmission. 4. Some evidence suggests transmitters can have adverse effect on behavior.

Cochran et al. (2004), Barron et al. (2010), Taylor et al. (2017)

Satellite tags

1. Precise animal trajectory.

Hays, Akesson, Godley, Luschi, and Santidrian (2001), Hallworth and Marra (2015), Kays et al. (2015)

1. Expensive (severely constrains sample size). 2. Transmitters constrained to large animals only (B100 g). 3. Archival tags provide relatively few positions for smaller models, and animals have to be recaptured. 4. Transmitters can have adverse effect on behavior.

(Continued)

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TABLE 1.1 (Continued) Technique

Advantages

Disadvantages

References

ICARUS Initiative

1. Small and light transmitters allow many types of species to be tracked.

1. Huge start-up investment.

Wikelski et al. (2007)

2. Similar to satellite tags, can track individuals over large distances (global).

2. Not yet proven technology. 3. Like above, some evidence suggests that transmitters can have adverse effect on behavior.

3. Relatively inexpensive following start-up investment. Remote sensing/ passive radar

1. Coverage over large geographic area. 2. Inexpensive. 3. Individuals do not have to be captured.

Transponders

1. Small transponder size.

1. Coverage only from existing stations or portable instruments. 2. Poor ability to determine species- and individualspecific movements. 1. Requires external (radar/ microwave) activation.

Gauthreaux and Belser (2003), Chilson et al. (2012), Zaugg, Saporta, van Loon, Schmaljohann, and Liechti (2008), Kirkeby et al. (2016)

Riley et al. (1996), Chapman, Reynolds, and Smith (2004)

2. Low range. 3. Coverage only from existing stations or portable instruments. Archival geolocation tags

1. Produces an animal trajectory; interval between recorded locations can be customized.

1. Individuals must be recaptured to download data.

Shaffer et al. (2005), Stutchbury et al. (2009)

2. Accuracy relative to satellite tags still low ( 6 200 km).

2. Light weight (,1 g). 3. Inexpensive relative to satellite tags (but more expensive than some radio transmitters). Intrinsic

Because only requires one capture, it is: 1. Not biased to initial capture population.

1. Biased to final captured population (can be overcome by comprehensive sampling coverage).

2. Less labor intensive than most extrinsic methods.

2. Typically lower resolution than extrinsic markers.

Hobson (1999), Webster et al. (2002), Rubenstein and Hobson (2004)

(Continued)

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1.3 SCIENTIFIC TOOLS USED TO STUDY MIGRATION

TABLE 1.1 (Continued) Technique

Advantages

Disadvantages

References

Contaminants

1. Potentially high spatial resolution (e.g., Mirex).

1. Lack of a priori maps of distribution and relative abundance.

Ochoas-Acuna et al. (2002), Braune and Simon (2003), Sanpera, Ruiz, Moreno, Jover, and Waldron (2007)

2. Distribution of contaminants may not vary predictably over geographic areas. 3. Potential transport of contaminants may dampen or provide unreliable geographic signal. Parasites

1. Potentially high spatial resolution.

1. Potentially expensive.

Fallon, Fleisher, and Graves (2006), Ricklefs et al. (2005), von Ro¨nn et al. (2015)

Pollen

1. Potentially high spatial resolution

1. Potentially expensive.

Mikkola (1971), Hendrix, William, and Showers (1992)

1. Several markers possible.

1. Species specific

2. Longitudinal resolution of migratory fauna in North America.

2. Typically low resolution

1. Simultaneous measurement of a large number of elements. 2. Potentially high spatial resolution.

1. Can be expensive.

1. Inexpensive.

1. Relatively low resolution.

Genetics

Trace elements

Stable isotopes

2. Generally provides information of recent plant use only.

Kelly, Ruegg, and Smith (2005), Smith et al. (2005), Boulet et al. (2006), Ruegg et al. (2014)

3. Provides estimate of natal origin only

2. Cannot extrapolate to large scales (i.e. site specific).

2. Not species or taxon specific.

Parrish, Rogers, and Ward (1983), Kelsall (1984), Szep et al. (2003), Norris et al. (2007), Kaimal, Johmson, and Hannigan (2009) Chamberlain et al. (1997), Hobson and Wassenaar (1997), Hobson (1999, 2005), Norris et al. (2006)

3. Multiple isotopes can be combined to increase spatial resolution.

optimally these devices provide location information up to a few kilometers. Nonetheless, adventurous researchers have attempted to follow migrating birds, bats, and dragonflies equipped with transmitters over portions of their flight paths (Cochran, Mouritsen, &

Wikelski, 2004; Holland, Thorup, Vonhof, Cochran, & Wikelski, 2006; Wikelski et al., 2003). Using a combination of aerial and ground tracking receivers, Wikelski et al. (2006) followed the southward migration of dragonflies (Common Green Darner Anax junius)

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equipped with 300 mg radio transmitters. Cell phone technology has also provided new possibilities for tracking animals but the miniaturization and development required for animal tracking remains a hurdle. As well, cell phone network coverage is limited internationally and locational accuracy is low (Stokely, 2005). A final concern is that fastening of markers or transmitters may alter flight or movement behavior (Barron, Brawn, & Weatherhead, 2010). A major advance in the way migratory wildlife, especially birds and bats, can be tracked using Very High Frequency (VHF) technology has been the MOTUS system in North America (https://motus.org). This involves a broad-scale automated telemetry array (Taylor, Crewe, Mackenzie, Lepage, & Aubry, 2017) whereby collaborators employ receivers tuned to a single radio frequency across receiving stations over a broad geographic scale, allowing individuals to be detected at distal sites maintained by others. Each individual VHF transmitter can be coded to a specific signal burst rate. Motus also coordinates, disseminates, and archives detections and associated metadata in a central repository. Combined with the ability to track many individuals simultaneously, Motus has expanded the scope and spatial scale of research questions that can be addressed using radio telemetry from local to regional and even hemispheric scales. Since its inception in 2012, more than 9000 individuals of over 87 species of birds, bats, and insects have been tracked, resulting in more than 250 million detections. This rich and comprehensive dataset includes detection of individuals during all phases of the annual cycle (breeding, migration, and nonbreeding), and at a variety of spatial scales, resulting in novel insights into the movement behavior of small flying animals. The value of the Motus network will grow as spatial coverage of stations and number of partners and collaborators increases. Current drawbacks to this technology are cost and limitations of coverage. Thus it is possible to define

a recovery point of individuals if they pass near a tower but no inferences can be made for those individuals not detected. Nonetheless, the system to date has produced some impressive results documenting timing of migration over continental scales (Go´mez et al., 2017). Remote sensing technologies such as radar have made great contributions to our understanding of migration because they can provide information of flying animal movements over considerable distance (Chilson, Bridge, Frick, Chapman, & Kelly, 2012; Gauthreaux & Belser, 2003). However, like automated receivers, radar installations are usually at fixed locations, and mobile radar systems are generally impractical to follow animal movements over long migratory distances. Crossband transponders placed on migratory organisms can be used to elicit a detectable radio frequency signal after the transponder is intercepted by radar. The transponder is dormant until such an event and so can last for a much longer period than conventional “always transmitting” radio frequency tags. However, such active radar systems face a number of limitations, again related to size and weight of instrumentation and the need to intercept the organism of interest within range of the radar system (see www.earthspan.org). Further advances in remote sensing like light in detection and ranging will undoubtedly play a larger role in advancing the study of animal migration in the future (Kirkeby, Wellenreuther, & Brydegaard, 2016). Satellite transmitters have provided a major advance in methods to track migratory animals because they provide extremely accurate positions of individuals remotely. Precise trajectories of individuals are possible over vast portions of the globe that have suitable satellite coverage. Satellite tracking systems such as ARGOS (www.argosinc.com) have been placed on different satellites from the US National Oceanic and Atmospheric Administration, The Japanese Space Agency, and the European Meteorological Satellite Organization. The

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ARGOS system collects data from platform terminal transmitters and delivers telemetry data back to the user. Unlike other extrinsic techniques, this approach does not require the physical capture of individuals once they are marked (although many researchers attempt this to recover the expensive transmitters). One of the most spectacular examples of this approach was provided by Jouventin and Weimerskirch (1990) who tracked Wandering Albatrosses (Diomedea exulans) on foraging flights up to 15,000 km. Future prospects for this technology are encouraging and there is strong interest in diminishing the size of transmitters and batteries so that smaller species can be monitored. Currently, the smallest satellite tags available are about 3.5 g which excludes many of the smaller migratory birds, bats, and almost all insects. The use of satellites may potentially provide a major breakthrough in tracking migratory animals down to the size of large insects. Wikelski et al. (2007) proposed that a new satellite equipped with radio receivers could track radio tags with radiated power as low as 1 mW with an accuracy of a few kilometers under favorable conditions. This power can be achieved from existing radio frequency tags as small as B1 g. This approach is similar to solutions derived by space researchers who need to detect very weak signals from distant galaxies and so the technology exists to modify the solution to detecting weak radio signals from Earth amidst a background of much stronger radio frequency noise. A group known as International Cooperation for Animal Research Using Space (ICARUS, www.icarusinitiative. org) is promoting this idea as a means of tracking small animals around the globe. It is estimated that it will take about US$50 100 million in order to build and launch a satellite. Thus far, the central computer unit of ICARUS has been installed in the cabin of the ISS international space station’s Russian module. In the near future, it is expected that the onboard computer will be responsible for decoding the

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signals obtained by the ICARUS receiver and data streaming. ICARUS foresee its onboard computer to be active and readily operational to user interface on Earth in 2018. A relatively new application for tracking migratory animals is the use of archival geolocation tags (Delong, Stewart, & Hill, 1992; Hill, 1994). These tags estimate longitude and latitude based on light levels (and in some cases, sea-surface temperatures), and long-distance movement data have been reported on many large migratory species such as elephant seals (Delong et al., 1992) and seabirds (Shaffer et al., 2005; Tuck et al., 1999). However, recent miniaturization has allowed their use in the study of smaller species (Stutchbury et al., 2009). The advantage of these tags is that they are significantly lighter (e.g. , 1g)than satellite transmitters. However, the disadvantage is that their accuracy is only 6 200 km that restricts application of these tags to questions pertaining only to large-scale movements although recent mathematical developments have increased the reliability of light-level geolocation data (Bindoff, Wotherspoon, Guinet, & Hindell, 2017; Rakhimberdiev et al., 2016). With the exception of satellite transmitters, all extrinsic markers require that the individual be recaptured, resighted, or at least move within detection distance at a later time. The probability of this recapture will clearly be the product of a number of individual probabilities related to the number of observers, the likelihood of an observer reporting the information, the regions and habitats used by the animal, and so on. As we have seen, combining these probabilities can result in vanishingly small chances of obtaining information on any given individual. Apart from the burden of recovering a marked individual, the use of passive extrinsic markers suffers another fundamental flaw—they provide information only on the movement of marked individuals. The possibility of extrapolating the findings based on a small marked cohort to the population or

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species level depends on how representative the marked individuals are among the population. Our confidence in this approach increases with the number of independent recaptures and some band recovery data show unequivocal patterns that are clearly representative of populations. A single band recovery or satellite track while interesting and ultimately useful may tell us very little about what populations are doing.

1.3.2 Intrinsic Markers The primary advantage of intrinsic markers is that initial marking of individuals is not necessary and that every capture provides information on origin. In this sense, every capture becomes a recapture. The sampling scheme is biased then, only by the limitations of where animals are ultimately located and this typically represents a much less serious form of bias compared to where individuals can be marked initially using extrinsic markers. 1.3.2.1 Contaminants, Parasites, and Pathogens There are several forms of potential intrinsic markers that can be used to infer origins of migratory animals. If the spatial distribution of a suite of contaminants were known, then presumably the occurrence of contaminants in animals could provide some information of the past distribution of the animals being sampled. Although this approach has not yet been used to any significant degree, there is some interesting potential here. For example, Braune and Simon (2003) noted that in contrast to Thickbilled Murres (Uria lomvia) and Black-legged Kittiwakes (Rissa tridactyla), the pattern and concentration of dioxin/furan congeners as well as the ratio of total PBDEs (polybromenated diphenyl ethers) to HBCD (hexabromocyclododecane) in Northern Fulmars (Fulmarus glacialis) breeding in the Canadian high Arctic was suggestive of them wintering in the

northeast Atlantic. In the Great Lakes region of North America, Mirex could be used as a marker to distinguish among those waterbirds breeding and wintering in different regions because the upper lakes have much lower concentrations of this contaminant than the lower lakes. The ratio of DDE (dichlorodiphenyldichloroethylene) to PCB (polychlorinated biphenyl) can also be used to illustrate which birds have likely overwintered in agricultural areas of South/Central America versus coastal areas (Braune, unpublished data). Brominated flame retardants are another material that occurs in greater concentrations in food webs used by animals exposed to European air masses than those exposed to North American air masses (de Wit, 2002). Finally, methyl mercury and other heavy metals are another potential marker in migratory animals because exposure to these varies considerably throughout the world (Hario & Uuksulainen, 1993; Janssens, Dauwe, Bervoets, & Eens, 2002; OchoasAcuna, Sepulveda, & Gross, 2002). Similar to contaminants, parasite and pathogen exposure experienced by migratory animals varies geographically and there is interest in using these markers to examine movements of individuals (Ricklefs, Fallon, Latta, Swanson, & Bermingham, 2005; von Ro¨nn, Harrod, Bensch, & Wolf, 2015). Very little research has been conducted on these sorts of tools to assist with deciphering animal movements, perhaps because we have little information to begin with on the spatial distributions of the potential markers themselves. On the other hand, the use of genetics has generated more interest because it is possible to assay and describe genetic variation across breeding populations and then to use this information to assign a probability that a given individual came from a given (known) subpopulation (Ruegg et al., 2014; Smith et al., 2005). The identification of population structure using genetic markers has included the use of allozymes, mitochondrial DNA sequences, and DNA fragment analyses such as microsatellites

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1.3 SCIENTIFIC TOOLS USED TO STUDY MIGRATION

and amplified fragment length polymorphism. These markers can yield different scales of population structure because they evolve at different rates. In North America, genetic markers have been particularly useful in differentiating between eastern and western breeding origins of wintering Neotropical migrant songbirds (Boulet, Gibbs, & Hobson, 2006; Ruegg et al., 2014; Smith et al., 2005), reflecting patterns of rapid demographic expansions following glaciation events on that continent. The combination of marker techniques that can provide information on north south origins in North America would clearly augment the resolution of genetic approaches. Genetic analyses also hold great potential for developing parasite markers in migrant organisms because PCR-based assays of blood parasites can now identify pathogens to species and haplotype (Ricklefs et al., 2005). 1.3.2.2 Trace Elements Trace elements are another set of intrinsic markers that can potentially be used to track individual movements over both small and large geographic distances. Similar to stable isotopes, the idea with trace elements is that individuals acquire distinctive chemical profiles at one geographic location and then carry that profile with them to another area where they can be sampled to estimate their geographic origin. In the past, one of the limitations of using trace elements was the amount of sample required for analysis. However, the use of inductively coupled plasma mass spectrometers has allowed smaller quantities of samples (Donavan, Buzas, Jones, & Gibbs, 2006; Norris et al., 2007) to be measured with relatively high precision. This technological advance has allowed researchers to focus on nondestructive tissues, such as metabolically inactive keratin, that are grown during specific periods of the migratory cycle. Trace element profiles have been used to infer whether individuals sampled in the same breeding population originate from different

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places the previous winter (Szep, Moller, Vallner, Kovacs, & Norman, 2003). However, because we have little idea of how trace elements vary over the landscape, the ability of this technique to estimate the precise origin of individuals seems, at the moment, limited. To determine how trace elements vary across the landscape, Norris et al. (2007) measured 42 trace elements in feathers of western sandpipers (Calidris mauri) at 5 different locations on their tropical wintering grounds. Feathers were grown during the winter periods and were, therefore, assumed to provide a signature of known-origin. Elemental profiles successfully distinguished between birds wintering at all five locations. However, two locations were less than 3 km apart suggesting that trace elements are likely very specific to the location or origin in which they were sampled. Trace element approaches are probably best suited to species that are aggregated over only a few breeding or wintering sites so the majority of populations can be sampled over the entire range (Donavan et al., 2006). Studies several years ago also show that some trace elements in feathers may be acquired after growth (Bortolotti & Barlow, 1988; Bortolotti, Szuba, Naylor, & Bendell, 1988), implying that this may not be a reliable approach for tracking long-distance movements. Further studies are needed to test the generality of these results before trace elements can be widely used as a marker of geographic location. 1.3.2.3 Stable Isotope Approaches The intent of this volume is to provide the reader with a comprehensive background needed to understand the application of stable isotope tools to the study of animal migration. However, animal ecologists have really only recently become aware of using stable isotope measurements in ecological studies in general. The world of stable isotopes is a multidisciplinary one that has a very rich history involving physics, earth sciences,

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biogeochemistry, animal and plant physiology, and even anthropology and archeology. The 1988 publication of the book Stable Isotopes in Ecological Research edited by Rundel, Ehleringer, and Nagy (1988) marked a significant pivot point that informed ecologists of the immense potential of stable isotope measurements in natural ecosystems. This was quickly followed by Stable Isotopes in Ecology and Environmental Science edited by Lajtha and Michener (1994), which is now updated to Michener and Lajtha (2007). Most recently, Fry (2006) has produced a useful text that provides some of the necessary background to understanding fundamental concepts in stable isotope ecology and Karasov and Martı´nez del Rio (2007) provide a very useful overview of key linkages between animal physiology and isotopic ecology. Compound-specific isotope analysis refers to measurement of the isotopic signatures (typically, carbon, hydrogen, oxygen, nitrogen, or sulfur stable isotopes) of individual compounds (such as amino acids, fatty acids, and sterols) from a complex environmental or biological mixture. The methodology offers information on dietary and trophic source information even in the absence of food collected from the study subject (e.g., birds on migration route). It also provides data on source differentiation, and can be used as quantitative means to delineate reaction pathways. Moreover, it can provide evidence for contaminant monitoring of breeding, migration, or wintering sites with pollutants such as hydrocarbons.

1.4 TECHNICAL ADVANCES AND OUTLOOK The last decade has shown tremendous advances in animal tracking technology heralding the golden age of animal movement ecology. Recent construction of high-resolution Global Positioning System and small designed

sensors integrated into tracking tags has striking effect on animal movement ecology. It has created opportunities to advance science most rapidly but has also created challenges with the generation of massive datasets applicable in several fields (Kays, Crofoot, Jetz, & Wikelski, 2015). Elsewhere, analytical chemistry laboratories have improved instrumentation establishing both the identity of newly synthesized physiological material, or probing the properties of bioactive compounds. Today’s analytical chemistry offers a broad array of equipment with a an alphabet soup of options that range from gas chromatography mass spectrometry, gas chromatography isotope ratio mass spectrometry and nuclear magnetic resonance (NMR) to ICP (inductively coupled plasma) and matrix-assisted laser desorption electrospray ionization. Indeed, the type of hardware is growing, with new developments appearing rapidly. Many of these devices are increasingly used in the biological world to create tools capable of detecting molecular signatures in biological tissues. Smaller molecular signatures may thus be applied not only to track movement but also to understand physiological processes during migration, movement, breeding, and molt. And of course, researchers are constantly discovering ever more creative ways to apply NMR and mass spectrometry to identify complexes of physiological/endogenous molecules that can be applicable to migration ecology and easily coupled to tracking sciences (Mitchell, Guglielmo, & Hobson, 2015). New analytical methods in isotopic and molecular ecology, DNA barcoding, approaches, integrated with remote sensing and theoretical ecological modeling has and will open up an abundance of new questions on the biological, environmental, and physiological control of movement and its consequences for life-history patterns of organisms, populations, communities, and ecosystems. The current approaches of

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REFERENCES

simultaneous tracking of multiple animals (both prey and predator) is opening new insights into food web interactions and combining tracking approaches with analytical tools will scale up our understanding in monitoring migration in a dynamic, changing environment. In summary, webelieve that a golden age of movement and migration tracking has indeed arrived and the coming years will be a time of more discoveries. As tracking technology and analytical instrumentation continue to advance, integrating these approaches will open a whole new paradigm of research on migration. For instance, real-time acquisition of data on the movement and behavior of tagged birds coupled to data from stable isotope, molecular approaches, DNA barcoding, and remote sensing will provide transformational points whereby new insight could be achieved to understand the intrinsic and extrinsic controls of animal migration, movement, and dispersal and finally to help us develop conservation approaches in entirely new ways.

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FURTHER READING

Webster, M. S., & Marra, P. P. (2005). The importance of understanding migratory connectivity. In R. Greenberg, & P. P. Marra (Eds.), Birds of two worlds: the ecology and evolution of temperate-tropical migration systems (pp. 199 209). Baltimore, MD: Johns Hopkins University Press. Webster, M. S., Marra, P. P., Haig, S. M., Bensch, S., & Holmes, R. T. (2002). Links between worlds: Unraveling migratory connectivity. Trends in Ecology and Evolution, 17, 76 83. Available from https://doi. org/10.1016/S0169-5347(01)02380-1. Wikelski, M., Kays, R. W., Jeremy Kasdin, N., Thorup, K., Swenson, J. A., & Smith, G. W., Jr (2007). Going wild: What a global small-animal tracking system could do for experimental biologists. Journal of Experimental Biology, 210, 181 186. Available from https://doi.org/ 10.1242/jeb.02629. Wikelski, M., Moskowitz, D., Adelman, J. S., Cochran, J., Wilcove, D. S., & May, M. L. (2006). Simple rules guide dragonfly migration. Biology Letters, 2, 325 329. Available from https://doi.org/10.1098/rsbl.2006.0487. Wikelski, M., Tarlow, E. M., Raim, A., Diehl, R. H., Larkin, R. P., & Visser, G. H. (2003). Costs of migration in freeflying songbirds. Nature, 423, 704. Available from https://doi.org/10.1038/423704a. Winkler, D. W., Dunn, P. O., & McCulloch, C. E. (2002). Predicting the effects of climate change on avian lifehistory traits. Proceedings of the National Academy of Sciences of the United States of America, 99, 13595 13599. Available from https://doi.org/10.1073/pnas.212251999. de Witt, C. (2002). An overview of brominated flame retardants in the environment. Chemosphere, 46, 583 624. Available from https://doi.org/10.1016/S0045-6535(01) 00225-9. Woodworth, B. K., Newman, A. E. M., Turbek, S. P., Dossman, B. C., Hobson, K. A., Wassenaar, L. I., . . . Norris, D. R. (2016). Differential migration and the link between winter latitude, timing of migration and

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breeding in a songbird. Oecologia, 181, 413 422. Available from https://doi.org/10.1007/s00442-0153527-8. ˙ A., Valcu, M., Bensch, S., & Yohannes, E., Kriˇzanauskiene, Kempenaers, B. (2009). Prevalence of malaria and related haemosporidian parasites in two shorebird species with different winter habitat distribution. Journal of Ornithology, 150, 287 291. Available from https://doi. org/10.1007/s10336-008-0349-z. Yohannes, E., Mihai Valcu, R., Lee, W., & Kempenaers, B. (2010). Resource use for reproduction depends on spring arrival time and wintering area in an arctic breeding shorebird. Journal of Avian Biology, 41, 580 590. Available from https://doi.org/10.1111/ j.1600-048X.2010.04965.x. Zaugg, S., Saporta, G., van Loon, E., Schmaljohann, H., & Liechti, F. (2008). Automatic identification of bird targets with radar via patterns produced by wing flapping. Journal of the Royal Society Interface, 5, 1041 1053. Available from https://doi.org/10.1098/rsif.2007. 1349.

Further Reading Murray, M. R., & Fuller, D. L. (2000). A critical review of the effects of marking on the biology of vertebrates. In L. Boitani, & T. K. Fuller (Eds.), Research techniques in animal ecology: Controversies and consequences. Methods and cases in conservation science (pp. 15 64). New York: Columbia University Press. Weisshaupt, N., Lehmann, V., Arizaga, J., & Maruri, M. (2017). Radar wind profilers and avian migration: A qualitative and quantitative assessment verified by thermal imaging and moonwatching. Methods in Ecology and Evolution, 8, 1133 1145. Available from https:// doi.org/10.1111/2041-210X.12763.

TRACKING ANIMAL MIGRATION WITH STABLE ISOTOPES