Nominees for BioOne Ambassador Award

The Auk and The Condor have each nominated a recent author for BioOne’s inaugural Ambassador Award, which will recognize “early career authors working to communicate the importance and impact of their focused research to communities beyond their fields of expertise.” Each of up to five winners will receive a $1000 cash award. More information can be found on BioOne’s website.

The Auk has nominated Emily Williams, lead author of the recent paper Patterns and correlates of within-season breeding dispersal: A common strategy in a declining grassland songbird (press release).

The Condor has nominated Andrew Dennhardt, lead author of the 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 (press release).

We wish Emily and Andrew luck as the selection process for the awards continues!

Terns Face Challenges When They Fly South for Winter

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A Common Tern wearing a geolocator. Photo credit: C. Henderson

The Common Tern is most widespread tern species in North America, but its breeding colonies in interior North America have been on the decline for decades despite conservation efforts. The problem, at least in part, must lie elsewhere—and a new study from The Auk: Ornithological Advances presents some of the best information to date on where these birds go when they leave their nesting lakes each fall.

The University of Minnesota’s Annie Bracey and her colleagues attached geolocators—small, harmless devices that record a bird’s location over time based on day length—to 106 terns from breeding colonies in Manitoba, Ontario, Minnesota, Wisconsin, and New York. When the birds returned to their breeding grounds in the following years, the researchers were able to recapture and retrieve data from 46 birds. The results show important migratory staging areas in the inland U.S. and along the Gulf of Mexico—a surprise, since it was previously thought that most Common Terns head for the Atlantic coast before continuing south. Birds from different colonies intermingled freely in the winter, but most ended up on the coast of Peru, suggesting that the population could be especially vulnerable to environmental change in that region.

For long-lived birds such as Common Terns, adult survival likely drives population trends more than breeding productivity, so identifying causes of mortality is crucial for effective conservation. Coastal Peru is vulnerable to multiple effects of climate change, including increasingly frequent and severe storms, changes in the availability of terns’ preferred foods, and rising sea levels. “Because survival is lowest during the non-breeding season, identifying coastal Peru as a potentially important wintering location was significant, as it will help us target studies aimed at identifying potential causes of adult mortality in this region,” says Bracey.

“This paper is both important and interesting, because it takes a species we consider ‘common’ and examines the reasons for its decline,” adds Rutgers University Distinguished Professor of Biology Joanna Burger, a tern conservation expert who was not involved in the research. “In short, this is one of the first studies that examines the entire complex of terns breeding in inland US locations, along with migratory routes, stopover areas, and wintering sites. It vastly increases our knowledge of the causes of declines and the locations and times at which terns are at risk, and more importantly, provides a model for future studies of declining populations.”

Migratory routes and wintering locations of declining inland North American Common Terns is available at http://www.bioone.org/doi/full/10.1642/AUK-17-210.1.

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.

Long-Term Study Reveals Fluctuations in Birds’ Nesting Success

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Long-term data on Song Sparrows reveals factors that affect their nesting success. Photo credit: D. Janus

Understanding the factors that affect a bird species’ nesting success can be crucial for planning effective conservation efforts. However, many studies of nesting birds last only a few years—and that means they can miss the effects of long-term variation and rare events. A new study from The Auk: Ornithological Advances demonstrates this with nearly four decades of data from Song Sparrows in British Columbia.

The University of British Columbia’s Merle Crombie and Peter Arcese used 39 years of data from an island population of Song Sparrows to examine how the factors influencing their nesting success changed over long periods of time. Over almost 3,000 nesting attempts, 64% of which were successful, a number of patterns emerged. Some, such as the fact that older female birds were less successful, remained consistent over time. However, others, such as the effects of rainfall, population density, and nest parasitism, interacted with each other in complex ways that caused their importance to wax and wane over the decades, and inbreeding only became a significant negative factor when it increased sharply during the middle portion of the study. Unpredictable, rare fluctuations such as this can have large effects that shorter-term studies rarely capture.

“Researchers have been learning about the Song Sparrow population on Mandarte Island since 1960, and monitoring the population continuously since 1975,” says Arcese. “Because the population is semi-isolated, small, and resident year-round, we band all birds in the nest and have genotyped all nestlings since 1991.” A close focus on individuals, fitness, and relatedness in the Mandarte Song Sparrow population has allowed researchers to report the most precise demographic and population genetic parameters yet estimated in wild populations.

“Most studies of plant and animal populations in nature last three to five years, but ecological processes are often dramatically affected by climate and community change, which plays out over decades,” he continues. “Long-term studies like ours provide an invaluable record of change in population processes, which can help interpret the results of short-term studies of species not as easily studied as Song Sparrows.”

Temporal variation in the effects of individual and environmental factors on nest success is available at http://www.bioone.org/doi/full/10.1642/AUK-17-189.1.

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: Are All Eggs Created Equal? Saltmarsh Sparrows Support Gender Equality

Bri Benvenuti and Adrienne Kovach

Linked paper: Annual variation in the offspring sex ratio of Saltmarsh Sparrows supports Fisher’s hypothesis by B. Benvenuti, J. Walsh, K.M. O’Brien, M.J. Ducey, and A.I. Kovach, The Auk: Ornithological Advances 135:2, April 2018.

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Three Saltmarsh Sparrow chicks banded as part of a study on offspring sex ratios. Photo: B. Benvenuti

In birds, females have the ability to control the sex of individual eggs; therefore, a mother may be able to choose whether she prefers each egg laid to be a male or female. This means that offspring sex ratios are not usually left to chance. From an evolutionary standpoint, this can be very beneficial, as different circumstances may favor the success of sons versus daughters.

But how does one know if it would be better to have more sons or daughters? Evolutionary theory suggests that if the potential benefits of raising one sex over the other vary in relation to environmental or maternal conditions, then females should favor the production of that sex. Typically, high quality sons are more beneficial to mothers, because they have the potential to produce far more grandchildren than daughters can (males can mate many times, but females are limited by how many eggs they can produce, incubate, and raise to fledging). More grandchildren = greater lifelong success.  Still, there is a risk to biasing offspring production toward sons; if the son is low quality (competitively inferior), he may not reproduce at all.  On the other hand, daughters tend to be “cheaper” to raise than sons. They need less resources to reach maturity, and if they survive, they almost always reproduce. With this in mind, one could logically say that producing daughters represents the “safe bet”—you might get a smaller payout (in terms of numbers of grandchildren), but you know you’ll get something.

Armed with this information, we chose to investigate whether Saltmarsh Sparrows manipulate the sex of their offspring based on environmental or maternal conditions as we would expect based on evolutionary theory. These tidal marsh specialist birds live a stressful life—they build nests in the marsh grasses just inches above the marsh surface that is regularly subjected to tidal flooding. Nests are more likely to escape these flooding events and successfully fledge offspring if they are timed to fledge within the 28 day lunar tidal cycle. Saltmarsh Sparrows are also one of the world’s most promiscuous birds, with almost every egg in a nest having a different father thanks to the scramble of competition among males for access to females. These characteristics provide interesting hypotheses in the context of evolutionary theory, so we asked, would Saltmarsh Sparrow mothers produce more sons, who would be larger and a) more likely to survive a nest flooding event and b) have the ability to produce more offspring through multiple matings? Or would they take the “safe bet” and produce more females?

To test our hypotheses, we collected nesting data from Saltmarsh Sparrow breeding locations in New England marshes over five years. We used DNA analysis to determine the sex of Saltmarsh Sparrow chicks and calculated the offspring sex ratio for our four study sites and across the whole study population. We then used a modeling approach to determine if there was an influence of environmental conditions (year, tidal flooding, precipitation), temporal effects (nest initiation in relation to flood tides, timing within the breeding season), or maternal condition on offspring sex ratios.

Surprisingly, we found an even offspring sex ratio of 1.03:1 (males to females) across all years and sites, and offspring sex ratios did not vary as a function of the environment, tidal flooding risk, or female condition. What we did find was an interesting pattern of annual variation between male and female bias that mirrored the adult sex ratio in the preceding year.

While numerous studies have provided evidence that female birds may have the ability to adjust offspring sex ratios in an adaptive way, we found no evidence for adaptive sex ratio manipulation in Saltmarsh Sparrows in relation to our hypotheses. Instead, the observed time-lagged relationship between offspring and adult sex ratio meets expectations of frequency-dependent selection, whereby females respond to higher frequencies of one sex by increasing production of the rarer sex, which would have a temporary fitness advantage. Our findings overall show support for balanced offspring sex ratios at a population level over time.

Scientists Remind Their Peers: Female Birds Sing, Too

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Northern Cardinals are among the familiar North American bird species in which females sing in addition to males.

When North American ornithologists hear a bird singing, they’re likely to assume it’s a male. But in many species, the females sing too—and a new commentary in The Auk: Ornithological Advances argues that a better understanding of these unappreciated female songs could lead to advances in many aspects of bird biology.

Authors Karan Odom of Cornell University and Lauryn Benedict of the University of Northern Colorado both discovered the world of female birdsong through their own research. “I started studying California towhees 17 years ago, and I was fascinated by the duet vocalization given by females and males,” says Benedict. “That led me to start looking for female song in other North American bird species, and I was surprised to learn that it was much more common than I expected. The reports of female song are buried in odd corners of the literature, but when you put them all together, you start to see some interesting patterns.”

Though singing females were likely the norm among the ancestors of today’s songbirds, female song today is understudied and is underrepresented in collections of bird sound recordings. This, say Odom and Benedict, may be result of bias toward the world’s temperate regions—though more widespread in temperate species than many ornithologists appreciate, female song is most common among tropical birds. They argue that better documentation of which species female song is present in and more detailed descriptions of female song structure and output could improve our understanding of birds’ comparative physiology, neurobiology, behavioral ecology, evolution, and even conservation. Birds of conservation concern are often located and identified by song during surveys, and assumptions that all singing birds are male could mislead wildlife managers about the state of populations.

Odom and Benedict urge their fellow ornithologists to spread the word that female birds sing, to share resources, and to disseminate their findings. You don’t need to be a professional ornithologist in order to help expand our knowledge of female song, either—Odom has created a website where any birdwatcher can upload their observations. “If you hear a bird singing, do not assume it’s a male,” she says. “If you observe a female bird singing, document it by uploading field notes, audio, or video to the collections on our website, femalebirdsong.org. Make sure to indicate how you recognized the bird was female.”

“Odom and Benedict have written an excellent appeal to document and record more female bird song,” adds Leiden University’s Katharina Riebel, a former collaborator of Odom’s. “They rightly point out that the extent of female bird song has been starkly underestimated, as almost by default we assume that a singing bird must be the male of the species. As a consequence, we might have missed out many aspects and the dynamics of male and female vocal signaling in songbirds—clearly, there is still lots to discover! I am confident that ornithologists in the field can make substantial contributions toward these questions by sharing their observations and recordings, as I very much hope this article will encourage them to do.”

A call to document female bird songs: Applications for diverse fields is available at http://www.bioone.org/doi/full/10.1642/AUK-17-183.1.

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: Recognizing the Importance of Female Birdsong

Karan Odom & Lauryn Benedict

Linked paper: A call to document female bird songs: Applications for diverse fields by K.J. Odom and L. Benedict, The Auk: Ornithological Advances 135:2, April 2018.

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House Wrens are among the familiar birds in which females sing. Photo credit: J. Hudgins/USFWS

Can you name ten North American or European bird species in which females sing? Can you name twenty? Fifty? That may seem like a lot, but in fact it’s only a small percentage of temperate-zone bird species with female song. There are at least 144 North American passerine species with female song, and many more non-passerines with elaborate vocalizations that could be classified as song (defining “song” is a topic we won’t even go into here!). Across all avian species, approximately 64% have female song, but these estimates are rough. The true numbers could be much higher. Why are the estimates so rough? Because documentation and reports of female song are lacking. We highlight this problem in our paper “A call to document female bird songs: Applications for diverse fields.” We ask all of you to help us address the deficit.

We know that the data are out there; we regularly have conversations with ornithologists and citizen scientists who tell us that they have observed singing females in myriad species. Our response: Publish it! Archive it! We’ve chatted with many senior researchers who have years of data including observations and recordings of female song in their study populations, but who haven’t published these data because female song is rare or is not their main research focus. We’ve also heard from students working as field assistants whose cohorts regularly observe singing females, but those observations are seldom documented. On field projects with many technicians, word-of-mouth data can be extensive and highly informative, but staff turnover means that known population traits never get put down on paper (or audio).

Citizen scientists frequently tell us about singing females, and many of them have taken the next step to document their observations through The Female Bird Song Project. Contributors have recorded female song in species as diverse as the Mexican Sheartail (Doricha eliza), Black-goggled Tanager (Trichothraupis melanops), Saffron Finch (Sicalis flaveola), and Cerulean Warbler (Setophaga cerulea), all of which seem to be the first documentation of female song in their species!

Each of these contributions plays a role in understanding the distribution of species with female song – data that researchers can use to address a wide range of biological questions. A complete picture of when and how female birds sing will offer insights into the biological mechanisms, evolution, and applications of avian vocal signals. Neurobiologists can ask how bird brains perceive and produce these variable signals, and whether that differs by sex. Evolutionary ecologists can ask why songs differ among species with different ecology and life-history traits. Conservation biologists can use songs to census and monitor the presence of males and females across populations.

What can you do? Don’t assume that a singing bird is a male. Look, listen, and document without bias. Teach your students to do the same. In 73% of all bird species we lack enough published information to even determine whether females sing. We are confident, however, that in some of those species females do sing because we have talked to colleagues about them. Common knowledge suggests that female song is rare, but our experiences make us question that: if all ornithologists talked to each other about female song the way that they talk to us, then that assumption would change. Your random observations of a female warbler singing can probably be backed up by the observations of many others. Females of temperate-breeding species may not sing as often as males, but when we pool all our knowledge and observations it’s likely that we’ll find more parity than we expect.

AUTHOR BLOG: Latitudinal Gradient in Bird Survival

Gonçalo Ferraz

Linked paper: Age effects on survival of Amazon forest birds and the latitudinal gradient in bird survival by A.P. Muñoz, M. Kéry, P.V. Martins, and G. Ferraz, The Auk: Ornithological Advances 135:2, April 2018.

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A Guianan Warbling Antbird, Hypocnemis cantator, banded and ready for release. Photo credit: E. Johnson

Bold, recurring patterns are fascinating, and more so if they are unexplained. For biologists living in the tropics, perhaps the most striking examples are the various latitudinal gradients in community and life history metrics. Science excites our curiosity in the search for universal rules, but large-scale latitudinal patterns hint that some things may be fundamentally different in different parts of the world. Could there be really such a thing as “tropical biology”?

One of the best-documented of these latitudinal gradients is the tendency for avian clutch size to increase with increasing latitude. Since there is no evidence that high-latitude populations grow faster than equatorial ones, it’s been widely assumed that some other life history trait must also change with latitude to compensate for the variation in clutch size. Thus, throughout the second half of the 20th century, the idea that adult tropical birds live longer than their temperate counterparts became widely accepted among ornithologists.

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A Rufous-throated Antbird, Gymnopithys rufigula. Photo credit: E. Johnson

The acceptance of a latitudinal gradient in survival, however, was more based on common sense than hard data—so much so that when Karr et al. (1990, The American Naturalist 136: 277-291) challenged the idea in a paper subtitled “Will the dogma survive?” there were not many voices sounding in defense of the “dogma.” Indeed, two decades later, Corey Tarwater and colleagues suggested that, if there is no latitudinal trend in the survival of adult birds, there could be a trend in the survival of juveniles (2011, Ecology 92: 1271-1281). Their work drew on detailed analysis of age-dependent survival in one Central American passerine species and proposed that tropical juveniles survive more than temperate juveniles. If adult survival shows no relationship with latitude this implies relatively little difference between adult and juvenile survival in the tropics.

For a paper just published in The Auk, Alejandra P. Muñoz, myself, and our colleagues used bird banding data to tackle the problem of the latitudinal gradient in bird survival on two fronts. First, we quantified the effect of age on survival for forty species of Amazon forest passerines, showing that adults at our tropical site had substantially higher survival probabilities than juveniles. Second, we compared our adult survival estimates with estimates from 175 species from Peru to Alaska and found that survival does decrease with increasing latitude. This latitudinal effect persists even after accounting for effects of migration mode, phylogeny, and time of data collection, and we conclude that the latitudinal gradient in survival is a fact after all, at least as seen among New World forest passerines.

Our work benefited a great deal from two recent advances. First, there is an ongoing transformation in how tropical ornithologists assess bird age. The Wolfe-Ryder-Pyle molt-cycle system, which we employed, is extremely useful for aging birds in populations that have poorly delimited breeding periods, which is typical of tropical regions. This was central for quantifying the effect of age on survival. Second, we tapped into the phenomenal database of Vital Rates of North American Birds made available by the Institute of Bird Populations (IBP), enabling us to include data from as far north as Alaska. The IBP estimates, combined with a wealth of tropical passerine survival estimates published since 1990, made it possible to take up Karr et al.’s (1990) challenge anew.

From the analytical perspective, we used a multi-species Cormack-Jolly-Seber (CJS) approach, a standard method for estimating apparent survival from capture-recapture data on open populations, to model our Amazon data. We treated each of the forty species in our dataset as a random draw from a wider distribution of species, and the inference about age effects was made at the level of this wider distribution, strengthening the generality of the conclusions. The CJS accounts for the possibility of capture failure, and a “mixture” component of the model, developed by co-author Marc Kéry, makes it possible to incorporate birds of unknown age in the analysis. Our work thus addresses a variety of sources of uncertainty before reaching its final conclusions.

The observation of a latitudinal trend in survival probability helps explain the maintenance of a latitudinal trend in clutch size, but it need not be the only explanation. It’s also possible, for example, that the number of clutches laid each year also varies with latitude in association with the length of the breeding season. With or without latitudinal change in number of clutches, one cannot tell whether the variation in survival we documented is a sufficient explanation for the current variation in clutch size. What’s more, even if one explains the maintenance of current variation, there is still the question of how that variation evolved. There is certainly still much to explore in the latitudinal variation of bird life history traits.