Effects of Camera Monitoring on Least Tern Nest Success at Cape Lookout National Seashore, North Carolina

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Chapter 2 – Evaluating the Impacts of Military and Civilian Overflights and Human Recreation on Least Terns, Common Terns, Gull-billed Terns, and Black Skimmers at Cape Lookout National Seashore, North Carolina

ABSTRACT

Due to high variability in the types of aircraft overflights and the associated responses by different species of animals, there is a lack of consensus on the effects of overflights on wildlife behavior and demography.  In response to a request by the U.S. Marine Corps (USMC) to the National Park Service (NPS) to conduct low-altitude tactical training operations in the special-use airspace above Cape Lookout National Seashore (CALO), North Carolina, we studied the effects of military air traffic on the nesting behaviors of least terns (Sternula antillarum), common terns (Sterna hirundo), gull-billed terns (Gelochelidon nilotica), and black skimmers (Rynchops niger) at North Core Banks, CALO. We compared the effects of military aircraft to other human activities at CALO, including civilian fixed-and-rotary-wing aircraft, all-terrain vehicles, off-road vehicles, and pedestrians.  We deployed digital audio recorders within colonies and time-lapse cameras at individual nests at 9 waterbird colonies from May-August 2010-2011.  We supplemented USMC-provided military overflight data (aircraft type, altitude, velocity, coordinates of ingress/egress, and time of ingress/egress) with data collected opportunistically for all other types of human events. We post-processed audio recordings and analyzed one-third octave band spectrograms to examine the effects of overflight altitude on received sound levels.  The average altitude (SE) of audible military overflights (n =was 3,291 m (179) and the mean Sound Exposure Level (SEL) and Maximum 1-s Equivalent Average Sound Level (MaxLEQ) were 77.8 dBA (0.77) and 65.5 dBA (0.93), respectively. Controlling for closest point of approach (CPA), lower-altitude overflights—i.e., overflights below the 3,000 m floor for tactical flights speeds (>250 kt, n = 8) —were, on average, > 10 dBA louder in received SEL and > 14 dBA louder in MaxLEQ than overflights at or above the 3,000 m floor (n = 11). We used generalized linear models to analyze how received SEL of fixed-wing military overflights varied with altitude, CPA, colony, and climatic variables. All top-ranked models included altitude and CPA; wind speed and wind direction were also important. There was no evidence that the behavioral responses of colonial waterbirds to the acoustic and/or visual stimuli of military or civilian aircraft impacted demographic rates. The mean proportion of time spent incubating by least terns was less during pedestrian events compared to control periods (0.91 versus 0.79, S = -2.2, p = 0.04). The current patterns of aircraft operations are unlikely to affect colonial waterbird demographics, and NPS beach management policies mitigated adverse impacts to colonial waterbirds associated with human recreation.
KEY WORDS acoustics, beach, black skimmer, common tern, disturbance, gull-billed tern, incubation, least tern, military overflights, North Carolina, recreation Human activity in natural areas is an increasingly pervasive threat to wildlife (Bowles 1995; Carney and Sydeman 1999; Christ et al. 2003). While aircraft or vehicle collisions may have immediate demographic consequences to wildlife populations, behavioral responses to human activities are varied and complex. Whether human activities include visual stimuli such as transient hikers, auditory stimuli such as distant fixed-wing aircraft, or both such as low-flying helicopters, an animal’s behavioral response to these real or perceived threats may indirectly affect its reproduction or survival.
For example, a bird that flushes from its nest in response to a disturbance may increase its probability of being detected by a predator (Skutch 1949; Tewksbury et al. 2002; Weston and Elgar 2007; Chalfoun and Martin 2010), increase intraspecific aggression in colonial species (Burger 1981), expose chicks and eggs to adverse environmental conditions, such as high radiant heat that can cause embryo death (Conway and Martin 2000; Weston et al. 2011), and/or divert energy from feeding or reproduction (Frid and Dill 2002; Pepper et al. 2003). Acoustic stimuli may alter or mask the auditory signals that birds use for information exchange, such as singing or contact/alarm-calling to defend territories or locate mates or offspring, and, particularly for colonial species, to alert neighbors of nearby threats and maintain group cohesion (Lack 1968; Awbrey and Hunsaker 1997; Marler 2004; Caro 2005; Barber et al. 2009; Ortega 2012). Sufficiently loud events may damage hearing (Bowles 1995; Larkin et al. 1996). Birds may also avoid otherwise suitable habitats as a result of aircraft or human presence (Fraser et al. 1985; Andersen et al. 1989; Buehler et al. 1991; Tarr et al. 2010), potentially forcing displaced individuals to use inferior habitats, thereby decreasing productivity or survival. Each of these behavioral responses to human presence may negatively impact wildlife populations.
The existing body of literature on the effects of human activities on avian behavior is varied, and the results are often species and context-dependent. Therefore, it often is not appropriate to extrapolate the results from one study to another species, habitat, season (i.e., breeding vs. non-breeding), or type of activity under investigation. For example, aircraft overflights, or simulated aircraft noise, altered avian behavior in some studies (Andersen et al. 1989; Brown 1990; Conomy et al. 1998a; Conomy et al. 1998b; Delaney et al. 1999; Brown 2001; Goudie and Jones 2004) but had no effects in others (Snyder et al. 1978; Fraser et al. 1985; Trimper et al. 1998; Delaney et al. 2002); the associated effects on reproduction or survival were also mixed. Plumpton (2006) reviewed overflight studies and concluded that birds were more likely to respond to rotary-wing aircraft than to fixed-wing aircraft, and to slow-moving turboprop aircraft than to fast-moving turbofan aircraft; however, the magnitude of response in these studies varied considerably and could not be generalized across guilds or habitats. Moreover, studies on the effects of human activity on wildlife may be confounded by concurrent effects on co-occurring species, including predators. For example, while close proximity (< 100 to off-road vehicle (ORV) trails increased songbird nest abandonment, it also decreased nest predation rates, presumably because both nesting birds and their predators were sensitive to ORV disturbances (Barton and Holmes 2007). Contrary to their expectations, McGowan and Simons (2006) found that reduced time spent incubating by American Oystercatchers resulted in increased daily nest survival; the authors hypothesized that increased time spent away from a nest following a human disturbance decreased the total trips to and from the nest, which reduced the probability that a predator would locate the nest. What is clear from a review of the literature on the effects of human activities on avian behavior is that caution should be taken when extrapolating findings between disparate study species or habitats.
Four state-listed colonial waterbird species nest on North Core Banks, Cape Lookout National Seashore (CALO), North Carolina, including least terns (Sternula antillarum), common terns (Sterna hirundo), and black skimmers (Rynchops niger)—each listed as a species of special concern—and gull-billed terns (Gelochelidon nilotica), listed as threatened (North Carolina Wildlife Resources Commission 2008, Appendix A). All species are regularly exposed to a variety of human activities, including military and civilian air traffic, ORVs (i.e., pickup trucks and sport utility vehicles), all-terrain vehicles (ATVs, defined here as either single-or-multi-rider ATVs or utility terrain vehicles), pedestrians, and, to a lesser extent, motorized and non-motorized watercraft. The beach habitat that these species require is particularly prone to human activity during the summer nesting season.
Colonial waterbirds, along with other species that require open habitats in which to nest and forage, may have a greater sensitivity to audible disturbances than species inhabiting structurally complex systems, due to the characteristics of sound propagation in the absence of physical barriers (Appendix B; Hill et al. 1997; Efroymson and Suter 2001; Pepper et al. 2003). Further, colonially-nesting birds may be more sensitive to disturbance than solitary species due to coordinated flushing and mobbing behaviors, particularly if the response of one individual elicits a colony-wide reaction (Owens 1977; Burger 1981; Erwin 1989; National Park Service 1994; Carney and Sydeman 1999). Therefore, beach-nesting colonial waterbirds may be especially vulnerable to exposure from aircraft or human recreation.
In addition to potential threats from aircraft or human recreation, beach-nesting birds are vulnerable to fragmentation and degradation of coastal habitats, increased competition with and predation from non-native and human commensal species (e.g. great black-backed gulls, Larus marinus, and raccoons, Procyon lotor), sea level rise, altered storm patterns, and changing forage fish populations and harvest (Burger 1984; Thompson et al. 1997; Rodgers & Smith 1995; Najjar et al. 2000, 2010; Erwin et al. 2006; Le V. dit Durell et al. 2006; Brinker et al. 2007). A combination of these threats has resulted in state-wide colonial waterbird declines documented in North Carolina (CALO 2006), the southeast region as a whole (Hunter et al. 2006), and the adjacent Chesapeake Bay region (Brinker et al. 2007). Therefore, it is important to view any potential effects of changes in frequencies or types of human activities in the context of the above-mentioned natural and human threats, many of which are novel, to colonial waterbirds.
In response to a request by the U.S. Marine Corps (USMC) to the National Park Service (NPS) to conduct low-altitude tactical training operations in the special-use Core Military Operations Area (Core MOA) above CALO, we studied the effects of military and civilian fixed- and-rotary-wing aircraft, ORVs, ATVs, and pedestrians on the nesting behavior of least terns, common terns, gull-billed terns, and black skimmers. Before this study, training flights at tactical speeds (> 250 kt) through the Core MOA were conducted at > 3,000 m (i.e., 10,000 ft) above ground level (AGL). Overflights < 3,000 m AGL were permitted down to the 900 m (i.e., 3,000 ft) Core MOA floor but were required to fly below tactical speed. In response to evolving training needs, in 2009, the USMC and the NPS agreed to lower the minimum allowed altitude for tactical flight speeds in the Core MOA from 3,000 m to 900 m AGL with continuation contingent on the findings of this study.
In addition to responding to the specific management needs of the USMC and NPS, our study simultaneously addressed the behavioral, and the potential for demographic, effects of human activities on multiple co-occurring nesting colonial waterbird species. Our first objective was to compare received sounds levels from reduced-altitude (i.e., near 900 m AGL) tactical speed overflights with other audible human events, and to compare each human event type with ambient sound levels. Our second objective was to analyze the proportion of time spent incubating by the focal species before, during, and after military overflights, and to place these findings in the context of recreational activities and concurrently-studied drivers of demographic parameters.

Chapter 1 – Introduction .
Chapter 2 – Evaluating the Impacts of Military and Civilian Overflights and Human Recreation on Least Terns, Common Terns, Gull-billed Terns, and Black Skimmers at Cape Lookout National Seashore, North Carolina 
ABSTRACT
STUDY AREA
METHODS
RESULTS
DISCUSSION
MANAGEMENT IMPLICATIONS
ACKNOWLEDGMENTS
LITERATURE CITED
TABLES AND FIGURES
Chapter 3 – Effects of Camera Monitoring on Least Tern Nest Success at Cape Lookout National Seashore, North Carolina
ABSTRACT
STUDY AREA
METHODS
RESULTS
DISCUSSION
LITERATURE CITED
Chapter 4 – Assessing Bias and Variation in Survey Methods and the Window Census Protocol for Least Terns
ABSTRACT
STUDY AREA
RESULTS
DISCUSSION
ACKNOWLEDGMENTS
TABLES AND FIGURES
Chapter 5 – Conclusion: Recommendations for Management and Future Studies of Colonial Waterbirds at Cape Lookout National Seashore .
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Evaluating the Impacts of Military and Civilian Overflights and Human Recreation on Least Terns, Common Terns, Gull-billed Terns, and Black Skimmers at Cape Lookout National Seashore, North Carolina