AUTHOR BLOG: Finding the Perfect Spot: Nest-Site Choice and Predator Avoidance in Asian Houbara

João L. Guilherme

Linked paper: Consistent nest-site selection across habitats increases fitness in Asian Houbara by J.L. Guilherme, R.J. Burnside, N.J. Collar, and P.M. Dolman, The Auk: Ornithological Advances 135:2, April 2018.

female_distraction

A female Asian Houbara runs away from the nest area.

For birds that nest on the ground, discretion is everything. As they are especially at risk from predators, choosing where to nest may carry life or death consequences for themselves, their eggs, and their progeny.

We study the ecology of the Asian Houbara (Chlamydotis macqueenii) in the semi-deserts of southern Uzbekistan, as part of a long-term effort to gain insight into the dynamics of this wild population. The landscape has extensive low-density shrub coverage and tends to all look the same on first glance, but a closer look reveals subtly distinct habitats with shrub communities that differ in not just species composition, but also in the size and number of shrubs. The Asian Houbara is a highly cryptic ground-nesting bird inextricably associated with these habitats, breeding throughout. For 23 long days, females have the sole responsibility of laying, incubating, and protecting the eggs, and themselves, from the freezing cold and the strong sun, and from the desert predators such as foxes and monitor lizards.

This behavior of nesting in structurally different habitats made us question if females were choosing similar vegetation structure for nest sites and whether these choices had an impact on their nest success.

Female sitting

A rare glimpse at a female Asian Houbara on her nest.

By following houbara tracks, we succeeded at finding 210 nests. Then we took it upon ourselves to identify and measure the height of the shrubs around all nests and at 194 random locations. Obviously, this was done after the nest was finished and the female and chicks had left the area. In the end, we identified 30 species and measured a total 35,853 shrubs! After running some statistical analysis we found that females were indeed choosing the same nest site features consistently across three structurally different habitats. Their selection was so fine-tuned that the optimal shrub height of about 30 centimeters had the greatest probability of being selected in all habitats. Furthermore, the scrape was consistently in the middle of shrubs that offered some degree of concealment, but enough visibility for the female to anticipate approaching predators.

So, females were choosing similar nest site across habitats, but we wondered if these features were helping them avoid nest predation.

To investigate this, we monitored the nests, placing temperature loggers inside the nest scrape and setting video cameras to collect information through the entire incubation so that we could classify if a nest was successful or if it had failed and, in that case, why (see video here). We found that nests in higher vegetation had a lower probability of being predated, with the likelihood being that the higher vegetation offered more concealment from predators. However, females would not nest in even higher vegetation, as this would eliminate their ability to see around and anticipate approaching predators. In fact, from more than 200 monitored nests, there was not a single time when a female was predated, which normally occurs in other ground nesting birds—females seem to value the old adage “run and hide, live to fight another day.” Nest camera footage showed us that females were eternally vigilant, with their heads extended so they could see just above the vegetation and surreptitiously leave the nest before a predator arrived. In this way, we found the connection between the choice of nest site and the chance of losing the nest to predation.

In a landscape where everything looks the same, there were in reality different habitats where nesting Asian Houbara had to find the “perfect spot” that maximized the chances of hatching while reducing the danger of being depredated. For a species of conservation concern, it is very important to maintain good productivity and minimize changes in vegetation structure away from the optimal choices, as these may lead to abandonment of previously suitable nesting areas, lower nest survival, or increased predation risk for the incubating female.

AUTHOR BLOG: Tracking the Japanese Bush-Warbler Invasion of Hawaii

Jeff Foster

Linked paper: Population genetics of an island invasion by Japanese Bush-Warblers in Hawaii, USA by J.T. Foster, F.M. Walker, B.D. Rannals, and D.E. Sanchez, The Auk: Ornithological Advances 135:2, April 2018.

JABW (c) Jim Denny

A Japanese Bush-Warbler in Hawaii. Photo credit: J. Denny

Over the past several centuries, Hawaii’s native bird populations have been decimated due to an array of factors, including introduced diseases (avian malaria and pox), introduced rats, habitat change, and hunting. As a result, most live near the tops of the mountains and have small populations. Few birds and remote locations make studying many of these native populations incredibly challenging.

In contrast, Hawaii is also home to many introduced birds that can be seen everywhere, from Brazilian Cardinals and Common Mynas on the beaches to Japanese White-eyes and various game birds at the mountaintops. Various organizations in Hawaii introduced these birds from elsewhere in the world to have birdsong fill the air again and occasionally to serve as pest control for crops. Over 170 species have been brought to Hawaii and released into the wild. Of these releases, at least 54 species now have breeding populations, and most seem destined to stay for the long haul. Many species, such as the Japanese White-eye, Northern Cardinal, Zebra Dove, and Common Myna, have robust populations and can be found in a variety of habitats.

One introduced species, the Japanese Bush-Warbler, is perhaps the coolest of them all. However, despite its prominent place as the iconic harbinger of spring in Japan, few people in Hawaii think much of this species—perhaps because it is often heard but rarely seen, or perhaps because when one does finally spy a bush-warbler, it is a drab olive-brown with few prominent markings. Whatever the reason for overlooking it, bush-warblers have successfully colonized most brushy habitats on all of the main Hawaiian Islands. They were released on the island of Oahu in the 1920s, and after decades of population growth on Oahu, they naturally spread to the remaining main Hawaiian Islands by 1997.

Birds on islands have provided some of the best historical examples of the evolutionary process—think Charles Darwin in the Galapagos and Alfred Russel Wallace in the Malay Archipelago. Capturing cases of evolution “in action” is difficult. However, introductions of non-native birds into the Hawaiian Islands provide numerous opportunities for research, particularly in assessing potential evolutionary changes over a relatively short time frame. In this study, we were afforded a unique opportunity to look at the evolution of the Japanese Bush-Warbler within the past several decades by combining population genetic analyses of this species with a detailed invasion timeline on each island. As a result, we were able to see how rapidly genetic changes can occur during an invasion. We found both expected patterns, such as a decline in genetic diversity on the most recently invaded island, and an unexpected pattern, potential assortative mating on each island. These findings suggesting substantial room for future work in a system and setting that is pretty hard to match.

AUTHOR BLOG: Call Variation Suggests Roles for Natural History & Ecology in Marsh Bird Vocal Evolution

Sarah Luttrell

Linked paper: Geographic variation in call structure, likelihood, and call-song associations across subspecies boundaries, migratory patterns, and habitat types in the Marsh Wren (Cistothorus palustris) by S.A.M. Luttrell and B. Lohr, The Auk: Ornithological Advances 135:1, January 2018.

OLYMPUS DIGITAL CAMERA

Marsh Wrens’ calls vary across their geographic range. Photo credit: S. Luttrell

Many bird species have unique geographic signatures in their vocalizations similar to human “accents.” Most of what we know about geographic variation in bird sounds comes from studies of bird song. Song has been a rich subject for studying geographic variation because it is typically learned, allowing song to change more quickly across space and time than a purely genetic trait. Song, however, is only one type of signal in a bird’s vocal repertoire. We wanted to build a broader picture of how vocal behavior evolves and changes among populations by looking at a large repertoire of sounds at once. Most birds have multiple call types in addition to their songs. Each call or song type is an individual trait used under unique circumstances, and that means that each one may be under different selective pressures. As a result, looking at multiple vocalizations may reveal multiple patterns of geographic variation, or, if their geographic patterns are similar, it may suggest a general process of vocal evolution. There are many ways in which vocalizations might change over time or distance. For example, changes could be random—as long as the signal still sends the correct message, some aspects of its acoustic structure could drift among populations. Additionally, vocal signals may be under selection to reduce distortion caused by the habitat in which they are produced and heard. Think about how sounds are distorted differently in an open, bare hallway versus a musician’s sound booth and how the local acoustics might alter a listener’s ability to understand you. Furthermore, if some aspect of the sound is learned, then copy errors or innovations during learning can result in passing down cultural changes over time. These are just a few ways in which sounds might be altered, and no two vocalizations are necessarily influenced in the same way by the same set of selective pressures.

In order to compare vocal repertoires among populations, we looked at several subspecies of the Marsh Wren. Marsh Wrens provide a natural experiment in vocal variation due to ecological and natural history differences among the subspecies. The five eastern North America subspecies we focused on are found in two distinct habitats (freshwater marshes and saltmarshes), and they exhibit three migratory patterns (resident, partially migratory, and fully migratory). Our first challenge was to describe and classify the call repertoire for Marsh Wrens. We identified seven discrete call types. Three of the seven calls varied in acoustic properties that were consistent with differences in either migratory pattern or habitat type. Surprisingly, we also found that four calls were more common in some subspecies than others and that the differences were greatest between habitat types. This variation in call production may indicate differences in behavior or timing of breeding among the subspecies with different ecologies. Our results suggest that while not all vocal signals are changing at the same rate or in the same way, differences in habitat type and migratory behavior may be related to the biggest differences in vocal behavior. Interestingly, the calls that showed the greatest differences were calls used in mate attraction and territory defense, while calls related to alarm or distress were similar across subspecies, natural history, and habitat type. This result suggests that sexual selection could be driving or reinforcing changes between populations with different ecologies.

In the future, we are excited to explore another unusual phenomenon that we report in this paper: the use of calls as embedded elements in song. Like most songbirds, Marsh Wren males sing during the breeding season to attract mates and defend territories. Unlike those of most songbirds, we found that, depending on the subspecies, 73-93% of male Marsh Wren songs contained embedded calls. Do embedded calls confer some additional message to the song? Does this behavior vary across the breeding season? Are there specific structural rules regarding the embedded call pattern within song? Stay tuned as we untangle the structural complexity and geographic variation in songs with embedded call elements in Marsh Wrens across the rest of their range. Does your study species use calls in a song-related context? If so, contact us at luttrell.sa@gmail.com—we would be excited hear about it!

AUTHOR BLOG: Understanding How Management Affects a Flagship Reed Bed Bird Species

Thomas Oliver Mérő

Linked paper: Reed management influences philopatry to reed habitats in the Great Reed-Warbler (Acrocephalus arundinaceus) by T.O. Mérő, A. Žuljević, K. Varga, and S. Lengyel, The Condor: Ornithological Applications 120:1, February 2018.

Photo2

A color-banded singing Great Reed Warbler male, April 2015.

Wetlands are inhabited by disproportionately large number of plants and animals and yet are among the most endangered habitats worldwide due to human-caused habitat loss and fragmentation. Ecologists and conservation biologists work hard on saving wetlands by using various techniques such as vegetation management (e.g. breaking up homogeneous reed beds), water regulation (e.g. maintaining a flood/drought cycle), or reintroduction of extinct species (e.g. cranes in the U.K.). Several recent studies have shown that the management of wetlands such as reed beds by controlling the water level and removing the vegetation by mowing, burning, or grazing can increase species richness and diversity; however, we know less about whether such management provides better conditions for survival and reproduction of single species whose presence is important to other species.

The Great Reed Warbler (Acrocephalus arundinaceus) is an Old World, long-distance migrant bird that breeds in reed habitats of the Western Palearctic and winters in sub-Saharan Africa. In central Europe, the Great Reed Warbler is a widespread breeder inhabiting almost all types of reed habitats (ponds, marshes, canals etc.). Great Reed Warblers arrive in mid-April from their wintering grounds and stay until the end of breeding season in late July.

We have studied the breeding ecology of Great Reed Warblers in northern Serbia for eight years. The region hosts a nice array of wetland habitat types, ranging from oxbows of the Danube to small and large canals, and from sand and clay mining ponds to marshes in natural depressions. For our work, we distinguished six types of reed habitats based on our own observations and information from local water management companies. The six types, which differ in their shape, size, vegetation cover, and water regime, are mining ponds, marshes, large canals, and three classes of small canals.

Photo1

Although large canals are preferred by large-winged, probably high-quality, males for nesting, this habitat type provides suboptimal conditions for breeding due to high brood parasitism by Cuckoos; therefore, this habitat type likely functions as an ecological trap.

These wetlands are managed by reed mowing and burning, which led us to wonder how reed management influences the birds and other wetland animals. Specifically, we were interested in whether and how management influences the survival and reproduction of Great Reed Warblers, a flagship species of lowland wetlands in central Europe. Reed management by burning and mowing offered a good opportunity to study the responses of Great Reed Warblers in each of the reed habitat types. For example, we recognized early on that larger-winged, presumably higher-quality, males tend to occupy reed habitats with little management and deep, stable water, which are typically found along large canals.

We color-banded all individuals (both adults and hatch-year birds) from the beginning of our study and regularly checked all reed beds every year during the nesting season to explore potential differences in survival and reproduction of birds in the six reed habitats. We were also curious to find out how reed management and water availability influence survival and reproduction. We first analyzed data on survival and encounter probability that were collected over seven breeding seasons (2009-2015).

We found that the encounter probability of birds banded as hatch-year birds was higher in reed habitats with shallower water, while that of those banded as adults was higher in reed habitats with deeper water. These opposite relationships between hatch-year birds and adults may indicate that experienced adults occupy qualitatively better habitats, similarly to large-winged males (mentioned above). When data were analyzed separately for the sexes, we found that the encounter probability of males depended on variation in reed management and in water depth. In contrast, for females, encounter probability depended only on water depth, i.e. encounter probability increased with water depth. Furthermore, most of the adults and hatch-year birds returned to the reed habitat that they had been occupying initially, indicating that Great Reed Warblers display unexpectedly high fidelity to the reed habitat type they hatched in or bred in before.

How do these results translate to management recommendations? We all want the best possible management for the birds we admire and study. Evidence found in our study showed that reed management by mowing and/or burning influences return rates of juveniles and adult males and females in different ways. These results suggest that in practice, spatially variable reed management should be applied and water with varying depths should be maintained to maximize the return rates of Great Reed Warblers. This is often easier said than done. However, the multitude of reed habitats in our study and the good working relationships we developed with water management authorities and other stakeholders will allow more detailed, experimental studies of the influence of management and the allocation of optimal combinations of management for the benefit of wetland birds.

AUTHOR BLOG: A New Species of Antbird

Andre Moncrieff

Linked paper: A new species of antbird (Passeriformes: Thamnophilidae) from the Cordillera Azul, San Martín, Peru by A.E. Moncrieff, O. Johnson, D.F. Lane, J.R. Beck, F. Angulo, and J. Fagan, The Auk: Ornithological Advances 135:1, January 2018.

Cordillera Azul Antbird2016-10-10-1_Copyright

A male Cordillera Azul Antbird. Photo credit: A. Spencer

It was July 10, 2016, when Dan Lane, Fernando Angulo, Jesse Fagan, and I rolled into the coffee-growing town of Flor de Café in north-central Peru. This town lies in the Cordillera Azul—a picturesque series of outlying Andean ridges hardly explored by ornithologists. In fact, the first ornithological inventory in the region was only in 1996, when a team of researchers from the Louisiana State University Museum of Natural Science (LSUMNS) bushwhacked into the extremely remote eastern Cordillera Azul. It was on this expedition that Dan, then a beginning graduate student at LSU, discovered the distinctive Scarlet-banded Barbet (Capito wallacei) on “Peak 1538.” Now, twenty years later, we were back to see this iconic species, which graces the cover of the Birds of Peru field guide.

Flor de Café, in the somewhat more accessible western Cordillera Azul, has become the hub for barbet-chasers since LSUMNS associates Todd Mark and Walter Vargas confirmed its presence here in 2011. Thus, we were not surprised to run into another birdwatcher, Josh Beck, as we moved our gear into the single guest house in town. Within moments of meeting, Josh began telling us of a strange, ground-walking antbird he had encountered the previous day and documented with a sound recording. We quickly realized that his bird was a species new to science.

Fast forward a year and a half. This month, December 2017, The Auk is publishing the formal description of the Cordillera Azul Antbird (Myrmoderus eowilsoni). Based on our initial visit and a follow-up expedition led by LSU graduate student Oscar Johnson, we’ve learned a few things about this new species: its closest relative is the Ferruginous-backed Antbird (of which the nearest populations are about 1,500 km to the east in lowland forests of Brazil), it eats insects, the males and females sing different songs, it lives in pristine understory of humid forest, and its future near Flor de Café is very grim.

Chainsaws were an overwhelming component of the soundscape around town. We even asked some locals to delay cutting activities so that we could get better voice recordings of the antbird. Sun-coffee farming, which necessitates clear-cutting, is the main source of income for the residents of Flor de Café. By contrast, birding ecotourism benefits only a few residents, leading to some unfortunate and ongoing tensions within the town. There is clearly a great need for environmental education and conservation work in the region.

What I haven’t yet mentioned is that Flor de Café is located very near the Cordillera Azul National Park, which was created in 2001 and contains over 13,500 km2 of pristine habitat. We are very optimistic that future exploration within the park will produce new localities for the antbird and barbet, both presently facing severe habitat loss around Flor de Café.

From an ornithological perspective, the Cordillera Azul remains mysterious and tantalizing. Perhaps it holds a new hummingbird or tody-tyrant? Regardless of any future discoveries to be made in the Cordillera Azul, I hope that the new antbird brings attention to the incredibly biodiverse and distinctive avifauna of the region. I also hope that this discovery serves as a potent reminder of how far we still have to go in cataloging the diversity of life on this planet!

AUTHOR BLOG: Migrating Birds That Eat Northern Spicebush Berries Are Fat and Healthy Birds

Yushi Oguchi

Linked paper: Fruits and migrant health: Consequences of stopping over in exotic- vs. native-dominated shrublands on immune and antioxidant status of Swainson’s Thrushes and Gray Catbirds by Y. Oguchi, R.J. Smith, and J.C. Owen, The Condor: Ornithological Applications 119:4, November 2017.

banding8

Collecting a small blood sample from a Gray Catbird to assess its health during stopover. Photo credit: Zak Pohlen

“We should not only conserve avian populations; we should conserve ‘healthy’ avian populations.”  – Dr. Jen C. Owen (my M.S. adviser, Michigan State University)

We investigated whether the health status of fall frugivorous Swainson’s Thrushes and the Gray Catbirds differed depending on their use of shrublands dominated by exotic versus native plants. In the process, we came to appreciate the value of one native fruit, northern spicebush (Lindera benzoin), a fruit we couldn’t even identify at first. Our story represents a unique insight that ecophysiology can bring to conservation science and habitat management.

Our prediction was that habitat may influence bird health through their frugivorous diet and that landbird migrants may be able to enhance their immune system by resting and refueling during stopover. In fall, many shrubs are loaded with fruit, and this fruit contains essential nutrients, including antioxidants, which help a bird neutralize reactive oxygen species produced during exercise (flight).

A question of conservation interest is whether birds that forage on exotic fruits and in exotic-dominated shrubland experience a deleterious effect on their health. Many exotic shrubs such as autumn olive and honeysuckle (introduced to the Midwestern U.S.A.) produce fruits that are generally lower in energy than native fruits such as northern spicebush, but some exotic fruits may have high antioxidants, including immunostimulatory carotenoids. We wouldn’t know unless we tested it!

A state-managed land in Michigan had the perfect habitat matrix—exotic-dominated shrubland and native-dominated shrubland occurring side by side, bisected by woodland. The exotic habitat was largely autumn olive, honeysuckle, and multiflora rose, and the native habitat was largely dogwood species, common winterberry, and northern spicebush. During fall migration in 2012 and 2013, we captured more than 800 individual birds, from which we collected blood samples for comprehensive health assessments.

Where Swainson’s Thrushes foraged during stopover had no impact on their health. On the other hand, Gray Catbirds using exotic shrubland experienced poor health; they lost mass and had reduced immune function and lower antioxidant capacity compared to catbirds using native shrubland. We also saw annual variation, with catbirds exhibiting more deleterious effects in 2013 compared to 2012. We further found that the pattern of habitat effect on catbird (but not thrush) health could be at least partly predicted based on the fruits they ate (math on fruit nutrition and bird fecal data).

SPICEBUSH

Fecal sample showing the remnants of northern spicebush. Photo credit: Jen Owen

It is through this nutritional analysis that we saw the value of northern spicebush. Spicebush fruit has very high energy and antioxidant capacity relative to other species. It was a preferred fruit by both bird species and likely contributed to the bulk of the nutrients they acquired in native shrubland. In exotic shrubland, thrushes consumed more exotic fruits like common buckthorn than catbirds did—so they got antioxidants from exotic fruits. Catbirds, apparently more “reluctant” than thrushes to eat exotic fruits, instead consumed other native fruits that were low in antioxidants. Not eating enough exotic fruits in exotic shrubland may be why catbirds experienced a dip in their health there.

To summarize, migrant health may be influenced by human alteration of habitat, and the effects may depends on the diet of the birds. And again, northern spicebush is a great fruit for birds if you are in eastern North America! In a future paper (accepted by The Condor), we will cover more on shrubland habitat use by the birds at our site.

Dr. Jen Owen continues to direct the Burke Lake Banding Station (BULA) where this research was conducted. She and her undergraduate researchers are doing more intensive monitoring of the fruit abundance and phenology at the site, particularly on the interannual variation in spicebush fruiting in relation to climate. BULA is now in its 7th year of operation, and it continues to provide research opportunities for many students as well as inspiring bird enthusiasts of all ages. Visitors are always welcome while they are open during fall and spring migration. Check them out on their website (burkelakebanding.com) or like them on Facebook (facebook.com/BULAbandingstation).

I’ve done a return migration to the University of Wisconsin–Madison for my Ph.D. this fall. Following my renewed interest in nutrient acquisition so key to health, I now study molecular mechanisms of digestive enzyme modulation in the avian gut.

AUTHOR BLOG: Citizen-Science Data and Capture-Mark-Recapture Models to Estimate Numbers of Rare Species

Andrew Dennhardt

Linked paper: Applying citizen-science data and mark–recapture models to estimate numbers of migrant Golden Eagles in an Important Bird Area in eastern North America by A.J. Dennhardt, A.E. Duerr, D. Brandes, and T.E. Katzner, The Condor: Ornithological Applications 119:4, November 2017.

Have you ever looked to the autumn sky above and wondered how many of your favorite feathered friends are out there, as they migrate southward each year? If so, then you are not alone. A common goal in ecology has been to estimate the abundance of wild populations. From basic counts to educated guesses, historically, people have tried it all. During and even before the work of Sir Ronald Fischer, the father of modern statistics, applied mathematics aided in population estimation—for humans and wild animals alike.

F.C. Lincoln (1930) is credited with one of the first modern attempts to approximate population abundance of wild animals based on a sample of marked individuals (Bailey 1952, Le Cren 1965). The premise of his approach, co-credited to C. G. Johannes Petersen (1894) and commonly called capture-mark-recapture (CMR), was simple: trap a random set of animals of the same species in an area, mark those you caught with a unique tag, release them back into the wild for a period of time, return to the same area once more and randomly trap individuals of that same species again, and record which previously tagged individuals, if any, came back to the area. In brief, when you know the proportion of individuals you recaptured the second time, you can assume that you trapped the same proportion of the total population when you trapped the first time. The math is straightforward, too: if you have an initial sample of animals captured, marked, and released back into the wild, M, and multiply that by the total number of animals sampled a second time, n, then you can divide that quantity by the number of marked animals in that second sample, m, and approximate true population size, N.

In our paper, we used modern statistical advancements in CMR to estimate abundance of Golden Eagles (Aquila chrysaetos), a species of conservation concern in the United States, using an unusual approach. In effect, we did not physically capture, mark, or recapture individual animals in the wild; rather, we did so virtually in a computing environment with the help of observational data collected by some savvy citizen-scientists.

Golden Eagles in eastern North America face lethal and sub-lethal threats, many of which are human caused. However, these eagles are rarely seen, broad-ranging, and difficult to capture in the wild because they often avoid areas of human activity. Despite this fact, citizen-scientists observe Golden Eagles frequently and regularly during their annual spring and autumn migrations in Pennsylvania, U.S.A. Moreover, Golden Eagle movements are highly stereotyped, especially in autumn, bringing them within a few hundred meters of hawk counters on ridgetops in the Appalachian Mountains. Better still, because of past telemetry studies, we know how fast Golden Eagles fly when they migrate. Hawk counters collect their data on Golden Eagles and other migrant species and archive their observations in an online database, hawkcount.org. With assistance from managers of the archive, our research team gained permission and access to download historic count data on migrating Golden Eagles observed using the Kittatinny Ridge during their peak migration period, November, over a 10 year period from 2002 to 2011.

Now, here’s where things get really exciting (well, they do for me, at least!). Because (a) historic hawk-count data included information on the timing of Golden Eagles migrating southward past monitoring sites along the Kittatinny, (b) telemetry data gave us an idea of how fast they fly while migrating in autumn, and (c) we could measure the distance between each of the monitoring sites they passed, we could then estimate how long it would take eagles to travel between pairs of sites. Using a customized computer program, we matched records of Golden Eagles together, from site to site, such that we could know when observers at one site counted a bird that had probably been counted at a previous site (i.e. eagles became “captured,” “marked,” and “recaptured”).

Using these virtual data on matched eagle observations, we developed what are commonly called encounter histories. In brief, encounter histories comprise a sequence of 1s and 0s unique to each eagle. In a given sequence, a 1 represents either the event of first “capture and marking” or a subsequent “recapture” after being previously marked and released back into the wild, and a 0 represents an event when an eagle was not “captured or recaptured” at a previous or subsequent migration monitoring site. For modern CMR approaches, extensions of Lincoln (1930) and Petersen’s (1894) early work, encounter histories are the necessary input data for a statistical model. Because we were interested in estimating eagle abundance in a particular area (i.e. the Kittatinny Ridge) over time, we chose the Population Analysis (POPAN) Jolly-Seber model—the right tool for the job.

Most exciting for us, our methodology worked. We produced population abundance estimates for Golden Eagles migrating along the Kittatinny Ridge each autumn. To boot, two sets of our estimates followed the necessary rules (e.g., statistical model goodness-of-fit tests) for us to consider them reliable by modern CMR standards. Together, our best models estimated that approximately 1,350 Golden Eagles migrated along the Kittatinny Ridge each November, 2002–2011.

In the end, we feel that we have produced a useful framework for evaluating other migratory bird populations based on similar data and known movement behaviors. Our proposed methodology not only builds upon the legacy of modern CMR work, but is also far more cost-effective than physical CMR and other costly survey techniques. In the case of fixed-wing aircraft surveys for Golden Eagles in the western United States alone, our work costs far less than the annual ~$320,000 necessary to implement such surveys. Most importantly, such an achievement was only made possible by the tireless work of numerous dedicated citizen-scientists, whose standardized and centrally managed data can provide wildlife researchers and managers with quality information useful in conservation decisions. To our friends at the Hawk Migration Association of North America, for all of their data collection and management year-in and year-out, our team is abundantly grateful—pun intended!

References

Bailey, N. T. J. 1952. Improvements in the interpretation of recapture data. Journal of Animal Ecology 21:120–127.

Le Cren, E. D. 1965. A note on the history of mark-recapture population estimates. Journal of Animal Ecology 34:453–454.

Lincoln, F. C. 1930. Calculating waterfowl abundance on the basis of banding returns. Circulation of the U.S. Department of Agriculture No. 118.

Petersen, C. G. J. 1894. On the biology of our flat-fishes and on the decrease of our flat-fish fisheries: with some observations showing to remedy the latter and promote the flat-fish fisheries in our seas east of the Skaw. Report of the Danish Biological Station No. IV (1893–94).