Bat Echolocation Researc h - Bat Conservation International

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INTRODUCTION Methods to survey and monitor bat populations are facing increased scrutiny by researchers and biological resource managers nationwide (Hayes 2000; Miller et al. 2003; O’Shea and Bogan 2000). There is growing recognition that while advances in bat-detector technology provide an opportunity to expand our knowledge about populations, there is a need for clear guidelines on how to design standardized bat-detector surveys to provide statistically defensible answers to questions with widely varying purpose and scale. Methods that attempt to standardize the way surveys of local species are approached can advance our ability to understand habitat needs, species’ distribution patterns, and population trends at regional and potentially national scales (e.g., Jaberg and Guisan 2001; Macdonald et al. 1998; Walsh and Harris 1996a). The majority of bat-detector surveys are tied directly to conservation management questions. The simplest survey aim is to determine what bat species are present or absent in a given area. Such surveys relate the distribution of species to land use and provide a means of assessing the relative value of landscape areas for bats. The surveillance of species’ distribution over time can be used to monitor temporal changes in the ranges of populations and their use of habitat. A second aim is to determine where bats are feeding and commuting in a given landscape. Surveys to locate key sites and foraging areas for special protection and to quantify bats’ use of different habitats provide important information for the development of habitat-management guidelines. Such surveys can also answer specific questions about the ecology of the study species, such as seasonal activity patterns. A third important aim is to track how bat activity changes at sites over time, allowing researchers to identify potential declines and threats to populations at an early stage. The simplest sampling design is an array of sites at which measurements of activity are made within and across years. Basic design choices include the number of sites sampled, when and how often a site is surveyed within a year, and the duration of each survey and of surveys across years. Standardization and consistency in the application of survey protocols are key elements in designing surveys that are to be extended to longer-term monitoring if results are to be reliably compared over years. In addition, estimating the natural temporal and spatial variation in the bat-activity measure (bat passes) and variation introduced by the measurement process is an important part of evaluating study design. While a strong emphasis should always be placed on the statistical design of surveys and monitoring schemes, there is the need to seek solutions that balance statistical assumptions with the practical and logistical demands of field-based schemes. The selection of survey design is affected by multiple interacting factors. Ideally, the process should involve the following steps: • Define the scope: identify species and populations of interest • Define the purpose and scale: state the questions/hypotheses/aims • List the logistical constraints • Adjust the questions/hypotheses/aims if necessary/permissible • Select field techniques and a suitable sampling design and protocol to minimize bias and maximize accuracy and precision. DEFINING THE SURVEY’S SCOPE: IDENTIFYING SPECIES AND POPULATIONS OF INTEREST Knowledge of the ecology and behavior of the species to be studied is the first step in planning a survey, and will dramatically affect the choice of survey design. This includes any prior knowledge of roosting habits, foraging behavior, seasonal movements, and how environmental factors may affect activity and local abundance. Bats are a diverse group of mammals in terms of ecology and will often require the application of multiple survey approaches. For some species, the use of bat detectors may not be appropriate (e.g., calls are too low in intensity to be detected easily or the animals fly too high to be detected from the ground). A review of the current scientific literature is vital prior to planning a survey. When target species have been selected or information about the species expected to occur in the survey area have been listed, then a consideration of their ecology is warranted. While some species are colonial and gather in huge colonies at well-known underground sites, others are widely dispersed, roosting cryptically in foliage, tree cavities, and rock crevices. Detection probabilities therefore differ between rare and common, restricted and widespread species, and among landscapes. Further, the varied ultrasonic repertoire of bats is related both to the species and the type of environment in which they are flying. Some species can be readily identified from echolocation calls using a bat detector; others can only be identified in favorable situations, with considerable experience or by computer analysis; and for some species, identification success rates are low even when using the most advanced analytical techniques (e.g., some Myotis spp.). Echolocation calls are produced to perceive the environment, and the nature of the environment dictates the type of call that is produced. This contrasts with bird song (Barclay 1999), which is a repetitive series of notes sung irrespective of the environment the bird is found in. For example, bats in open areas produce loud sounds that travel long distances, whereas the same bat in cluttered habitat may produce quieter sounds to minimize listening to non-target echoes (most bats avoid pulse-echo overlap to minimize self deafening, irrespective of call 158 Bat Echolocation Research: tools, techniques & analysis
intensity). Our ability to detect most bat species has not been measured accurately in the field because of the difficulties in doing so, although some attempts have been made (Forbes and Newhook 1990; Waters and Walsh 1994). Recent developments in equipment to broadcast simulated echolocation calls of different species may enable experiments to measure detectability with more precision (G. McCracken, unpubl. data). Differential detection rates among species mean that comparisons of abundance cannot be reliably made among species and can only be made if data are weighted for differential detectability. While it is generally assumed that detection rates remain constant within a species, making possible an evaluation of changes in abundance, differential detection rates among habitats (open, edge, clutter) may confound such comparisons if the habitats sampled differ dramatically in structure (Parsons 1996; Patriquin et al. 2003). For example, if repeated monitoring of a location is carried out over time in which the area of cluttered woodland declines and becomes fragmented, sampling of the same sites may lead to the erroneous conclusion of an increasing population if detection of more bats forced into open/edge situations occurs. Knowledge about the potential magnitude of this bias is essential. Generally the assumption is made that the magnitude is small, since many species appear to show habitat preferences for either open or edge/cluttered habitats and their flight maneuverability may restrict them to these habitats (e.g. Nyctalus noctula, a fast-flying species with narrow wings, is rarely found in cluttered habitats). However, bats can exhibit surprising flexibility in their habitat use in different landscapes. Myotis emarginatus, a gleaning bat generally considered to hunt in dense tree crowns, was recently recorded hunting over open salt marshes in western France (H. Limpens, pers. comm.). DEFINING THE PURPOSE AND SCALE OF SURVEYS: STATE THE QUESTIONS/HYPOTHESES/AIMS An important factor to consider at the outset is the purpose and scale of the intended study. Appropriate methods for a particular study become more obvious if there is a clear goal, specified in advance. Most studies are tied directly to conservation-management needs and often have multiple but interrelated objectives. The most basic survey objectives include the need to detect the presence/absence of bats or the activity level of bats – also referred to as the index of abundance or relative abundance. Key sites and feeding areas may need to be located to provide information pertinent to local site management. Often, the objective of the survey is to provide data about specific species, and thus the need for species’ discrimination is introduced. Frequently, managers are required to inventory (list) the species present in an area, or to identify habitats used/not used by each species or by a species of special interest. A more demanding objective is to determine the relative activity Section 4: Resources, Research and Study or abundance of species and identify the most preferred or avoided habitats. Finally, the objective might be to monitor changes in distribution or relative abundance to ascertain patterns or trends over seasons or years. LOGISTICAL CONSTRAINTS Once the general objectives and scope of the survey have been defined, the next important step is to list the logistical constraints and adjust expectations where necessary. Often, logistical constraints will predetermine the end objectives and scope of the survey. Observers The number of skilled observers available to operate technical bat-detector equipment will constrain the number of sites which can be visited within a time window. A volunteer force of trained observers operating low-cost, easy-to-use equipment is one way that large numbers of sites spread across a large region can be surveyed (e.g., Limpens 1993; Walsh et al. 2001). However, this will introduce higher levels of observer variability. Examples where this approach might be used include: where the aim is to quantify the presence/absence or abundance of bats without a requirement for species identification; in regions where only a single species is present (e.g., Hawaiian hoary bat, Lasiurus cinereus); where more than one species is present, but they are easily distinguished (McCracken et al. 1997; O’Donnell 2000); or where one species has highly specific calls and can be distinguished with confidence from others (e.g., Euderma maculatum in western North America; Fenton et al. 1983). Equipment Whether or not more than one system is available, you can still choose to survey one or more than one site per night. Money and time Costs and the availability of funding and time will always be key limiting constraints. SELECTING FIELD TECHNIQUES Bat Detectors and Quantifying Bat Activity A range of bat detectors are available to conservation biologists and professional bat researchers at reasonably affordable cost. Although these are described in detail in other chapters of this volume, we briefly summarize them in relation to practical field use. The transformation of ultrasound into audible sound or visual depictions can be made with one of three main types of detector: heterodyne, frequency-division, and time-expansion. The choice of a bat-detector system depends upon the planned use for it and budget. Heterodyning and frequency-division are real-time methods, and thus observers can hear the sound from the 159
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Bat Echolocation R esearc h tools,
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TABLE OF CONTENTS CO N T R I B U T
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CONTRIBUTING AUTHORS Ingemar Ahlén
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I N T R O D U C T I O N Bat Echoloc
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BAT NATURAL HISTORY AND ECHOLOCATIO
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species (Thies et al. 1998). Before
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y the African bat Cardioderma cor (
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participants in this symposium. Sin
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INTRODUCTION Bat detectors convert
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less sensitive than a heterodyne de
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14 advanced computer software made
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COMMUTING AND FORAGING BEHAVIOR Fig
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NIGHTLY ACTIVITY BASED ON CAPTURE W
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20 Recordings of echolocation calls
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22 and analyzing echolocation calls
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24 Resource partitioning in rhinolo
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Section 2 ACOUSTIC INVENTORIES ULTR
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f tuned = f bat . As an example, wi
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The chance of detection also depend
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ent species. But in the field, it i
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Section 2: Acoustic Inventories Fig
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Suchflug und Richtungskarakteristik
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Journal N Percentage of total (%) A
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ential power of data. Hayes (1997)
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For instance, we recorded a single
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KALCOUNIS, M. C., K. A. HOBSON, R.
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and McCracken (this volume) discuss
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using other methods (Fig. 4). Ident
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Figure 6: Schematic diagram of diff
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AMPLITUDE-RELATED PARAMETERS Loudne
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tereri. Hand netting on the followi
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flight in bats. Pp. 93-108 in Bat b
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difficult to understand why the aut
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Figure 5: Call sequence showing the
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function analysis and artificial ne
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species identifications. Echolocati
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tially from calls emitted in the op
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LITERATURE CITED BARCLAY, R. M. R.,
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HETERODYNE AND TIME-EXPANSION METHO
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74 QUANTIFYING SOUND FEATURES To qu
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Figure 14: Pulse rhythm diagrams fo
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A FINAL WORD Figure 21: Heterodyne
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80 INTRODUCTION Ultrasonic detector
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82 munity was uncovered when ultras
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84 INTRODUCTION Being nocturnal, ba
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A Figure 2: Surveys of bat activity
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LITERATURE CITED Figure 5A: Pipistr
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90 In order to set a foundation for
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92 diagram, e.g., a spectrogram, fo
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ecording time of the cassette. When
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96 detail is not helpful for specie
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tification, providing the immediate
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spectrum, which is a graph of the e
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since there is no evidence that the
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104 Figure 5: Different modes in se
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Page 113 and 114: 108 INTRODUCTION Much of our knowle
Page 115 and 116: Figure 1: On-axis search call recor
Page 117 and 118: 112 SUMMARY To summarize, flight-ca
Page 119 and 120: 114 SIGNAL PROCESSING TECHNIQUES FO
Page 121 and 122: al a situation as possible. Ideally
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Page 125 and 126: ety of London 69:111-128. JONES, G.
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Page 133 and 134: Section 4 RESOURCES, RESEARCH, AND
Page 135 and 136: A The data extracted from the diffe
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Page 141 and 142: Figure 10: Fast Fourier transform p
Page 143 and 144: ACKNOWLEDGEMENTS I thank the organi
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Page 149 and 150: and van Parijs 1993). Early studies
Page 151 and 152: Figure 5: Variation in the echoloca
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Page 155 and 156: RECORDING TECHNIQUES Section 4: Res
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Page 169 and 170: SHERWIN, R. E., W. L. GANNON, and S
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