Selection of benthic habitat by yellow-phase American eels (Anguilla rostrata)

This thesis examines habitat preference of yellow-phase American eels (Anguilla rostrata) and relationships between age and length with that preference. The thesis is comprised of two chapters: (1) an introduction and literature review on American eel life history, their habitat selection, and the study of resource selection, and (2) an experimental study of yellow-phase American eel habitat preference and relationships between preference and age and length. Given widespread habitat alteration of North American rivers, an understanding of the use and selection of habitat is important to conservation and management of the American eel. Yellow-phase American eels are often considered as habitat generalists, in part, because of their occurrence across a wide range of habitat types, but few experimental studies have examined microhabitat selection. In a laboratory experiment, I quantified microhabitat use of small yellow-phase American eels (n = 130, 224--338 mm TL) conditional on five benthic substrate types common to many rivers within the geographic range of the American eel. During nine, 4-day trials replicated with three aquaria, American eels were given a choice to burrow into five equally available benthic substrates: cobble (90--256 mm), gravel (4--16 mm), sand (0.125--1 mm), silt/clay (\u3c 0.0625 mm), and leaf pack. Five American eels were used per aquaria for each trial, and individuals were used one time only. All eels were injected with PIT tags prior to the study, which allowed for determination of lengths and otolith-based ages of each individual following each trial. Leaf pack was selected with a significantly higher probability than other substrates (63 of 130 individuals). However, other substrates were also used (cobble, 21 of 130; silt/clay, 18 of 130; gravel, 16 of 130; and sand, 12 of 130). Length and age covariates were not associated with substrate selection. Selection of leaf pack habitat supports the importance of terrestrial organic material and riparian zones to yellow-phase American eels in riverine systems

Influence of Anthropogenic Subsidies on Movements of Common Ravens

Anthropogenic subsidies can benefit populations of generalist predators such as common ravens (ravens; Corvus corax), which in turn may depress populations of many types of species at lower-trophic levels, including desert tortoises (Gopherus agassizii) or greater sage-grouse (Centrocercus urophasianus). Management of subsidized ravens often has targeted local breeding populations that are presumed to affect species of concern and ignored “urban” populations of ravens. However, little is known about how ravens move, especially in response to the presence of anthropogenic subsidies. Therefore, subsidized ravens from distant populations that are not managed may influence local prey. To better understand this issue, we deployed global positioning system – global system for mobile communications transmitters to track movements of 19 ravens from September to December 2020 relative to 2 land cover types that provide subsidies: developed areas and cultivated crops. On average, ravens moved 41.5 km (±30.5) per day, although daily movement distances ranged from 0.13–206.1 km. Raven movement among cover types during the non-breeding season varied widely, with 100% of individuals each using land cover types that provide subsidy and other types at least once in the season. On 100% of days ravens used areas that did not provide subsidy, on 86.7% of days they used developed areas, and on 20.5% of days they used cultivated crops. Although on some days a raven would stay exclusively in areas that did not provide subsidy, there were no days in which a single raven ever stayed exclusively in developed or cultivated crops. Ravens moved shorter distances on days when they used subsidies more frequently. Further, time spent in developed areas and cultivated crops increased when ravens roosted closer to them, although this effect was greater for developed areas than for cultivated crops. Individual ravens were not associated exclusively with either of the subsidy-providing landscapes we considered, but instead all birds used both subsidized and other landscapes. Our research suggests that management of ravens during the non-breeding season and possibly during the breeding season, intended to reduce risk of predation on desert tortoises, will be most effective if conducted on a broad scale because of distances the birds travel and the lack of separation between putative “urban” and “natural” populations of ravens

State-space modelling of the flight behaviour of a soaring bird provides new insights to migratory strategies

Characterising the spatiotemporal variation of animal behaviour can elucidate the way individuals interact with their environment and allocate energy. Increasing sophistication of tracking technologies paired with novel analytical approaches allows the characterisation of movement dynamics even when an individual is not directly observable. In this study, high-resolution movement data collected via global positioning system (GPS) tracking in three dimensions were paired with topographical information and used in a Bayesian state-space model to describe the flight modes of migrating golden eagles (Aquila chrysaetos) in eastern North America. Our model identified five functional behavioural states, two of which were previously undescribed variations on thermal soaring. The other states comprised gliding, perching and orographic soaring. States were discriminated by movement features in the horizontal (step length and turning angle) and vertical (change in altitude) planes and by the association with ridgelines promoting wind deflection. Tracked eagles spent 2%, 31%, 38%, 9% and 20% of their daytime in directed thermal soaring, gliding, convoluted thermal soaring, perching and orographic soaring, respectively. The analysis of the relative occurrence of these flight modes highlighted yearly, seasonal, age, individual and sex differences in flight strategy and performance. Particularly, less energy-efficient orographic soaring was more frequent in autumn, when thermals were less available. Adult birds were also better at optimising energy efficiency than subadults. Our approach represents the first example of a state-space model for bird flight mode using altitude data in conjunction with horizontal locations and is applicable to other flying organisms where similar data are available. The ability to describe animal movements in a three-dimensional habitat is critical to advance our understanding of the functional processes driving animalsâ decisions. A plain language summary is available for this article

Synergistic and singular effects of river discharge and lunar illumination on dam passage of upstream migrant yellow-phase American eels

Abstract Monitoring of dam passage can be useful for management and conservation assessments of American eel, particularly if passage counts can be examined over multiple years. During a 7-year study (2007–2013) of upstream migration of American eels within the lower Shenandoah River (Potomac River drainage), we counted and measured American eels at the Millville Dam eel pass, where annual study periods were determined by the timing of the eel pass installation during spring or summer and removal during fall. Daily American eel counts were analysed with negative binomial regression models, with and without a year (YR) effect, and with the following time-varying environmental covariates: river discharge of the Shenandoah River at Millville (RDM) and of the Potomac River at Point of Rocks, lunar illumination (LI), water temperature, and cloud cover. A total of 17 161 yellow-phase American eels used the pass during the seven annual periods, and length measurements were obtained from 9213 individuals (mean = 294 mm TL, s.e. = 0.49, range 183–594 mm). Data on passage counts of American eels supported an additive-effects model (YR + LI + RDM) where parameter estimates were positive for river discharge (β = 7.3, s.e. = 0.01) and negative for LI (β = −1.9, s.e. = 0.34). Interestingly, RDM and LI acted synergistically and singularly as correlates of upstream migration of American eels, but the highest daily counts and multiple-day passage events were associated with increased RDM. Annual installation of the eel pass during late spring or summer prevented an early spring assessment, a period with higher RDM relative to those values obtained during sampling periods. Because increases in river discharge are climatically controlled events, upstream migration events of American eels within the Potomac River drainage are likely linked to the influence of climate variability on flow regime.</jats:p

Relative Importance of Experience and Meteorological Factors in Migratory Performance of Golden Eagles (Aquila chrysaetos)

Migratory performance of large soaring raptors primarily is influenced by experience and by weather conditions. We evaluated the relative importance of these factors on the migratory performance by eastern North America migratory Golden Eagles (Aquila chrysaetos). We analyzed GPS data from 65 eagles that spanned three age classes (juvenile, sub-adult, and adult). We defined migratory performance as the distance an eagle travelled at hourly and daily scales and we only considered movements we defined as migratory (\u3e10 km over 1 hour). We paired each GPS point with NCEP/NARR meteorological covariates linked to wind speed and direction, solar radiation and updraft potential. Our results show that on average, Golden Eagles traveled from approximately 10 to 470 km per day and had strong influences of both weather (birds travelled farthest on days when potential for thermal updraft was highest) and experience (there is strong seasonality in migratory movements). Next steps in the analysis include model selection on a set of candidate linear mixed models to evaluate the importance of each of these factors. Then, we will evaluate these models in the context of potential evolutionary pressures the birds face when making migratory movements

Golden eagle movement data

The file includes the time series of the four variables required to fit the state-space model for the classification of eagle behaviour. Altitude (in meters), step length (in meters) and turning angle (in radians) were derived from GPS data, while hierarchical slope position was computed using package spatialEco in R and based on ground elevation data obtained from the Global Multi-resolution Terrain Elevation Data 2010 at 30-arc-second spatial resolution (data available from the U.S. Geological Survey: https://earthexplorer.usgs.gov/). Measurements are reported for each minute, together with the ID of the corresponding track segment

Data from: State-space modelling of the flight behaviour of a soaring bird provides new insights to migratory strategies

1. Characterizing the spatiotemporal variation of animal behaviour can elucidate the way individuals interact with their environment and allocate energy. Increasing sophistication of tracking technologies paired with novel analytical approaches allows the characterisation of movement dynamics even when an individual is not directly observable. 2. In this study, high-resolution movement data collected via global positioning system (GPS) tracking in three dimensions were paired with topographical information and used in a Bayesian state-space model to describe the flight modes of migrating golden eagles (Aquila chrysaetos) in eastern North America. 3. Our model identified five functional behavioural states, two of which were previously undescribed variations on thermal soaring. The other states comprised gliding, perching and orographic soaring. States were discriminated by movement features in the horizontal (step length and turning angle) and vertical (change in altitude) planes, and by the association with ridgelines promoting wind deflection. Tracked eagles spent 2%, 31%, 38%, 9% and 20% of their day time in directed thermal soaring, gliding, convoluted thermal soaring, perching and orographic soaring, respectively. The analysis of the relative occurrence of these flight modes highlighted yearly, seasonal, age, individual and sex differences in flight strategy and performance. Particularly, less energy-efficient orographic soaring was more frequent in autumn, when thermals were less available. Adult birds were also better at optimising energy efficiency than sub-adults. 4. Our approach represents the first example of a state-space model for bird flight mode using altitude data in conjunction with horizontal locations, and is applicable to other flying organisms where similar data are available. The ability to describe animal movements in a three-dimensional habitat is critical to advance our understanding of the functional processes driving animals’ decisions

Eagle Flight Behavior and Risk from Wind Energy

Wind power is a fast-growing industry with broad potential to impact flying wildlife. Understanding these impacts is critical to developing effective strategies and recommendations for siting turbines and for mitigating impacts to animals. We tracked ~80 Golden Eagles Aquila chrysaetos in eastern North America with GPS-GSM telemetry systems. The fast data collection frequency we used allowed us to understand details of flight behavior and thus risk to birds from wind turbines along migration routes and on wintering grounds

Demographic Risk to Eagles from Anthropogenic Causes of Death

Human actions effect wildlife in both indirect and direct ways. Indirect processes – habitat loss, climate change, reductions in reproductive rates – are often difficult to quantify. However, direct processes that cause death or nest failure are empirically measurable. A study of ~400 tracked Golden Eagles (Aquila chrysaetos) from 1997–2013 assessed causes of death of golden eagles in North America. From 34–63% of recorded fatalities were caused by humans. The most common anthropogenic causes of death of golden eagles were secondary poisoning, shooting, electrocution, and collision with vehicles or wind turbines. However, other sources of death – especially trapping and lead poisoning – were also regionally important. In total, survival rates of golden eagles in North America would be ~10% higher without human-caused deaths and this population likely is held below the carrying capacity by human-caused fatalities