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.

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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.

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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.

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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.

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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.

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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).

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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).

AUTHOR BLOG: How Canada Warblers Keep Up with the Joneses

Anjolene Hunt

Linked paper: Forestry and conspecifics influence Canada Warbler (Cardellina canadensis) habitat use and reproductive activity in boreal Alberta, Canada by A.R. Hunt, E.M. Bayne, and S. Haché, The Condor: Ornithological Applications 119:4, November 2017.

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Photo credit: Torin Heavyside

Could a behavioral phenomenon, like the tendency of birds to live near neighbors, change the way we think about the effects of habitat disturbance? Results from new paper in The Condor: Ornithological Applications suggest that for Canada Warblers, this may be the case.

These iconic, brightly colored boreal songbirds are declining in number. Habitat loss and degradation are likely the main culprits. While it is true that forestry changes the landscape across much of the Canada Warbler breeding range, it remains contentious whether this results in habitat loss or whether they will use and thrive in regenerating postharvest areas. We suspected this discrepancy in our understanding of forestry effects could be explained by social factors influencing how Canada Warblers choose where to live.

Just as you might choose a neighborhood based on criteria like access to green space, birds also assess their physical environment when choosing where to live. These species-habitat relationships help biologists understand which areas to protect.

But the physical environment is far from the only cue birds use to choose a place to live, nor is it the only cue we use to understand why they live where they do. Just as you might rely on friends’ opinions of a neighborhood to snatch up a good place before it’s off the market, birds may choose to live near members of their own species. If another bird picks an area, chances are it has high quality food, nesting areas, and mates. However, if everybody makes the same decision, problems can arise as they pack into the same location. When habitat is disturbed, choosing to live near neighbors can result in overcrowding in the remaining undisturbed areas and potentially force some birds into the outskirts of high-quality neighborhoods.

After returning from a long spring migration, Canada Warblers have limited time to find a breeding site (they have the shortest breeding season of wood warblers in Alberta!), and strangely, researchers always seem to find Canada Warblers clustered together in some areas while absent from others. Based on this information, we suspected that where Canada Warblers choose to breed might depend on where their neighbors chose to settle. If this is the case, it could explain why they will live in postharvest areas, and in turn, how we perceive the effects of forestry.

Taking quads down muddy trails and bushwhacking through dense shrubbery to our destination, we surveyed for Canada Warblers in boreal Alberta, Canada. We used a recorded song to mimic an intruding male and lure territory owners into our nets. Once captured, we attached color bands to their legs to distinguish between individuals and followed them throughout the breeding season. We documented their space use in and around postharvest areas, how close they were to neighbors, and whether they found a female mate and raised young. See what a day in the life of Canada Warbler biologist looks like here.

Our results showed that numbers of male Canada Warblers were much lower in postharvest areas compared to unharvested forest. The few males that did live partially in postharvest were typically at the edge of nearby unharvested forest. Males were also more likely to live nearer to neighbors rather than spacing out. Hence, Canada Warblers in the boreal forest may not prefer to live in postharvest areas, but may live there as a side effect of trying to be near neighbors in unharvested forest.

But are Canada Warblers reaping the benefits of living near neighbors, or are they feeling the pressure to keep up with the Joneses? We found males living in areas with more neighbors were more likely to be “single” (without a female mate) than males living in areas with fewer neighbors. Overcrowding may lead to increased male-male competition for limited females. These rivalries also mean that males have to spend more time and energy defending their territory, leaving less time and energy to court females.

Our results suggest that protecting large stretches of unharvested forest near sites occupied by Canada Warblers will be important to provide enough habitat and prevent crowding effects. It also goes to show that appearances can be deceiving when it comes to the use of disturbed habitat and that the influence of social behavior should not be underestimated.

Follow Anjolene on Twitter: twitter.com/AnjoleneHunt

AUTHOR BLOG: Geolocator Effects May Have More to Do with Marking Method Than Mass

Henry Streby

Linked paper: Comment on ‘‘Mixed effects of geolocators on reproduction and survival of Cerulean Warblers, a canopy-dwelling, long-distance migrant’’ by H.M. Streby and G.R. Kramer, The Condor: Ornithological Applications 119:4, November 2017.

Author Blog picUnprecedented numbers of species and individual birds have been marked with all sorts of tracking devices in recent years, and those numbers will continue to rise in the future. The data ornithologists are gathering by marking birds with tracking devices are providing a wealth of previously elusive knowledge about all stages of birds’ life cycles. The rapidity with which new tracking studies are being initiated places an ever-growing burden on the USGS Bird Banding Lab (BBL) and other agencies charged with assessing auxiliary marker requests and determining permission to mark birds on a case-by-case basis. In many cases, those agencies have little to no species-specific information on which to base their decisions, either because a species has never been marked before or because those who marked them did not study or did not report marker effects during the course of their research. We as research biologists have an ethical responsibility to objectively assess the potential effects of our research activities on the animals we study for the sake of minimizing harm to the animals and potential bias in our data caused by markers impacting animal behavior or survival.

We applaud the authors of Raybuck et al. (2017) for honoring a request from the BBL to band additional birds as control groups during their geolocator study of Cerulean Warblers. We believe Raybuck et al. conducted their study and published their paper with ethical and admirable intentions and it was not our intention to criticize the integrity of their research. However, their conclusion of an overall geolocator effect on annual survival of Cerulean Warblers was overreaching in light of the modest sample-sizes and the confounding factors of year, site, and marking method. We believe our analysis and interpretation of their apparent annual survival data is more appropriate and clarifies some small but critical shortcomings in their assessment. Of primary concern, the negative geolocator effect on annual survival of Cerulean Warblers was driven by a strong negative effect associated with one marking method in one year, and there was no support for a negative effect of geolocators when they used a different marking method in the second year of their study. This difference was not driven by geolocator mass, because the reduced survival occurred in the year when a lighter geolocator was used.

In our opinion, there are two broad take-home messages from our paper. First, when marking a species for the first time, it is critical to use available knowledge and replicate methods known to work well in closely related species. When the smallest details are considered, there are almost as many methods for deploying a geolocator on a songbird as there are species that have been marked with geolocators. It should not be necessary for each research group to reinvent methods and relearn unfortunate lessons when simple, safe, and effective methods have been developed and made widely available. Second, there is only one control group that is relevant for comparison to a marked group of birds during a geolocator study or any other study of marked birds: a control group identified at the same site(s) during the same period of the same year. Parameters of interest, especially survival, can vary widely among years and populations. It is therefore not appropriate to compare parameters like survival of geolocator-marked birds with those of control birds from any other population or year or even long-term averages from the same population. Just as we have a responsibility to assess the potential effects of our research on the animals we mark, so too do we have a responsibility to design and present those assessments in a statistically rigorous and scientifically appropriate manner. It is then that we achieve our goal of providing accurate information that may be useful to other researchers and those in oversight positions who use this type of information to decide whether continued or new research is permissible.