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.

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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.

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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.

Warming Temperatures May Cause Birds to Shrink

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The size of adult House Sparrows is predicted by maximum temperatures during development. Photo credit: P. Deviche

Biologists have known for a long time that animals living in colder climates tend to have larger bodies, supposedly as an adaptation to reduce heat loss. However, understanding how temperature affects animals has gained new importance thanks to climate change. A new study from The Auk: Ornithological Advances uses European House Sparrows, which have spread into a variety of climates in Australia and New Zealand since their introduction in the mid-19th century, to show that this trend in birds might actually be due to the effects of high temperatures during development—raising new alarms about how populations might be affected by global warming.

Macquarie University’s Samuel Andrew and his colleagues captured and measured approximately 40 adult House Sparrows at each of 30 locations across Australia and New Zealand. They found that maximum temperatures during the summer, when the birds breed, were a better predictor of adult body size at each location than winter minimum temperatures. This adds support to the idea that excessive heat during development may affect birds’ growth throughout their lives, raising concerns that increasing summer temperatures due to climate change could drive down the average adult body size, with potential effects on the birds’ fitness.

“If variation in body size is linked directly or indirectly to adapting to different climates, then body size could be useful for monitoring the extent to which bird populations are capable of adapting rapidly to changing climates,” says Andrew. “Our work on this common species helps us to understand the adaptive responses of birds to a changing climate and their constraints, and this fundamental knowledge will help future workers and managers focus their work on other species and potentially identify those species most at risk from climate change.”

“This paper is an important addition to a growing body of work that is changing our understanding of the relationships between climate and body size. The big question generated by these results is the extent to which the observed relationship is the outcome of adaptive evolutionary differences among sites as opposed to direct developmental responses to different temperatures. Interestingly, some of these same authors just published experimental evidence for a direct effect of temperature on growth in another bird species,” adds Whitman College’s Tim Parker, an expert who was not involved with the research. “This is not a new idea, but it has been largely ignored by those who have assumed that most morphological variation in birds is due to evolved adaptive variation. We need more work on the direct effects of temperature variation on development in endotherms.”

Clinal variation in avian body size is better explained by summer maximum temperatures during development than by cold winter temperatures is available at http://www.bioone.org/doi/full/10.1642/AUK-17-129.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

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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.

Genetic Drift Caught in Action in Invasive Birds

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Japanese Bush-Warblers have experienced genetic drift as they’ve invaded the Hawaiian islands. Photo credit: S. Price

Studies of island bird populations have taught us a lot about evolution, but it’s hard to catch birds in the act of naturally colonizing new islands. Instead, a new study from The Auk: Ornithological Advances examines what’s happened by looking at the genetics of a species that arrived in Hawaii in the twentieth century through decidedly unnatural means—us.

Japanese Bush-Warblers were introduced to Oahu in 1929 and have since become established on all the main islands of Hawaii, providing a unique opportunity to follow post-invasion evolution on a known, recent timescale. Northern Arizona University’s Jeffrey Foster and his colleagues took blood and muscle samples from 147 bush-warblers living on five islands between 2003 and 2005. Their results indicate genetic drift is occurring—Oahu’s birds have higher genetic diversity than those on other islands, whose populations were founded by smaller groups of individuals, just as population genetic theory predicts. Kauai bush-warblers, however, appear to be on a distinct genetic trajectory from those on other islands. Kauai is three times as far from Oahu as the closest other islands, and appears to have received a unique subset of the overall genetic diversity found elsewhere, but it remains to be seen whether the trend on Kauai will continue in the future or if continued dispersal of birds among islands will blur these differences. “This study nicely showed genetic divergence for a very short period using the artificially introduced Japanese Bush-Warblers,” according to Shoji Hamao of Japan’s National Museum of Nature and Science, an expert on the species.

“I got the idea for bush-warblers as a study system due to the challenges associated with my previous work on native Hawaiian birds,” says Foster. “Most of the native bird species I had worked on were exceedingly rare—several of them were endangered species, in fact—so focusing a new project on species in decline or with low numbers was a risky proposition. However, many of the introduced birds are quite common and one species, the Japanese Bush-Warbler, caught my attention with its loud song.

“Bush-Warblers first arrived on the Big Island when I was living there in the 1990s. The idea that one could study this invasion in progress totally blew my mind. The genetic findings largely followed expectations, such as seeing the most genetic diversity on the island where the birds were introduced and less elsewhere. Birds on Kauai, the island just west of Oahu, appear to be more distinct than those birds on islands east of Oahu, suggesting that over time birds on the respective islands may continue to diverge genetically.” But, Foster adds, many questions remain to be answered. “How much are the birds still flying between islands and potentially mixing any genetic signals of differentiation? Why did it take 50 years for the bush-warblers to colonize other islands after Oahu? How have their vocalizations changed after colonization due to new environments or random chance? We still don’t know.”

Population genetics of an island invasion by Japanese Bush-Warblers in Hawaii, USA is available at http://www.bioone.org/doi/full/10.1642/AUK-17-120.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

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!

Wrens’ Calls Reveal Subtle Differences Between Subspecies

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Biologist Sarah Luttrell records the calls of a Marsh Wren. Photo credit: S. Luttrell

Birds’ songs and the ways they vary between places have been well studied–but what can the simpler vocalizations known as calls tell us about bird biology? A new study from The Auk: Ornithological Advances provides the first detailed description of how Marsh Wren calls vary across eastern North America and hints at the evolutionary processes playing out between wren subspecies.

The University of Maryland Baltimore County’s Sarah Luttrell and Bernard Lohr recorded the calls of five Marsh Wren subspecies at nineteen different sites, encompassing the Gulf Coast, Atlantic Coast, and Great Lakes regions and including migratory, nonmigratory, freshwater marsh, and saltwater marsh populations. Categorizing the recordings into seven different call types, they analyzed how calls differed between subspecies. While some calls were associated with territory patrol, nest building, and courtship, others were used mainly during aggressive encounters with predators or other wrens. Both the acoustic characteristics of some calls and how frequently they were used differed from place to place.

“It was certainly a lot of work to compile data on multiple vocalizations and compare the results, but in the end, it makes for a more nuanced understanding of how various evolutionary processes shape animal behavior as a whole,” says Luttrell. “Twitter” calls differed between migratory and nonmigratory subspecies, for example, while “buzz” and “trill” calls differed between birds that lived in freshwater and saltwater marshes; while differences in habitat can’t directly explain this, all of these call types could be shaped by sexual selection that reinforces the boundaries between subspecies. Atlantic Coast populations produced the “chuck” alarm call more often than others, which suggests they may experience more threats from predators or nest at higher densities that lead to more antagonistic encounters between birds.

Could these differences eventually prompt Marsh Wren populations to diverge into fully separate species? “That’s impossible to say for sure—it all depends on the course that evolution takes!” says Luttrell. “We were excited that the patterns of call variation we observed seemed to coincide with differences ecology and life history, which suggests the possibility that these subspecies are evolving in different directions. In the future, we’re hoping to do some behavioral tests that might help us understand how much the acoustic differences matter to the birds’ behavior in the wild. If we do find that individuals respond less strongly to the vocalizations of another subspecies than to their own, then that would be additional evidence that at least some subspecies are on a trajectory of divergence.”

“This study highlights the diversity of calls that can be found in a single avian repertoire, and nicely illustrates how different elements of those repertoires can evolve independently,” according to the University of Northern Colorado’s Lauryn Benedict, an expert on communication in wrens and other songbirds. “The demonstrated patterns of call use in relation to behavioral context, caller sex, habitat, and migratory behavior raise many future avenues of inquiry. Avian calls are a generally understudied vocalization, and this paper demonstrates how and why we all should pay them more attention.”

Geographic variation in call structure, likelihood, and call-song associations across subspecies boundaries, migratory patterns, and habitat types in the Marsh Wren (Cistothorus palustris) is available at http://americanornithologypubs.org/doi/full/10.1642/AUK-17-110.1.

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

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!