AUTHOR BLOG: Not Too Many Sperm, Not Too Few

MaleZB and LTF

Male Zebra Finch and Long-tailed Finch. Photo credit: L. Hurley


Laura Hurley

Linked paper: Variation in the number of sperm trapped on the perivitelline layer of the egg in three species of estrildid finch by L.L. Hurley, K.V. Fanson, and S.C. Griffith, The Auk: Ornithological Advances 119:4, October 2017.

When you crack open your morning egg, you see the familiar yolk with its little white circle staring at you. That little white circle, the germinal disk, is the target sperm are aiming for to fertilize the big yolky ovum, but in birds one sperm is not enough to turn the egg into a chick. Multiple sperm must fuse with the ovum for this to happen, so lots of sperm are present at fertilization, and those that don’t fuse can become trapped between the two delicate layers that surround the yolk.

Hope I didn’t ruin breakfast for you. However, too many sperm reaching the egg can cause the development of the chick to fail, so there’s a bit of a Goldilocks situation—just the just right number of sperm are needed. The size of bird eggs vary widely—from hummingbirds to emus—and so does the number of sperm that reach their ovum. In general, the number of sperm varies with body size, but there is a lot of unexplained variation between species of similar size, within species, and even within a clutch of eggs. In our current paper, we explore variation in three similarly sized birds from a family of Australian finches to help us better hypothesize about what could be influencing sperm numbers.

This is part of a larger body of avian ecology work looking at how genetic, social, and environmental factors influence and regulate reproduction, development, and population dynamics ( This work involves a number of Australian species in both wild and captive settings, including Gouldian Finch, Zebra Finch, Long-tailed Finch, and Chestnut-crowned Babbler, as well as the invasive House Sparrow. We also use historical records to build models to help us understand the life history of birds across the whole of Australia—for example, opportunistic breeding ( and nest structure (

AUTHOR BLOG: Tracking the Preen Gland Over Time

UNI Bielefeld

Researchers tracked changes in Zebra Finches’ preen glands during breeding to glean new insights about the gland’s function.


Sarah Golüke

Linked paper: Sex-specific differences in preen gland size of Zebra Finches during the course of breeding by S. Golüke and B.A. Caspers, The Auk: Ornithological Advances 119:4, October 2017.

Almost all birds possess a preen gland that produces a waxy secretion used by birds for feather maintenance. Several studies have found that the preen gland is enlarged during breeding, but it is currently not clear why.

We investigated the temporal pattern of gland size changes during breeding in a captive population of domesticated zebra finches. Zebra finches are small passerines, only weighing 13 grams on average, and the gland is therefore quite small. Additionally, the gland itself is really flexible, and measuring the gland manually with a caliper is therefore quite challenging and error-prone. So, how to measure precisely this flexible, heart-shaped gland in such small birds?

We took pictures of preen glands and calculated the gland surface area using digital picture analysis. This method worked out well, and we started to investigate gland size changes over the breeding period. We measured preen gland sizes of breeding pairs at key points that are relevant during the reproductive period, such as pre-mating, egg laying, hatching, rearing and independence of the chicks.

We found that gland sized increased in both parents—that is, they produced more secretion—during breeding. The maximum size of the gland was reached around the time the chicks hatch. We assume that the need for secretions is highest at this point. After breeding, the gland size was similar to what it was before the reproductive season, indicating that the size increase is due to breeding. Similarly, at the same time, non-reproducing birds showed no size increase.

More specifically, our results show that the temporal pattern of gland size increase differs for males and females, with males already enlarging the gland around the time of egg-laying, while females start increasing the gland size later.

Knowing the temporal pattern of preen gland size enlargement of males and females allows us to think about different factors that might explain the enlargement pattern we observed. First, an investment in gland secretions might reduce the odor of the birds inside the nest, which is advantageous against olfactory-hunting predators. Second, in a breeding-related context, the ingredients of preen gland secretions might be necessary for self-protection and/or to protect offspring against harmful microbes. The nest is an ideal environment for microbial growth, which could reduce plumage condition and health and could further impact egg viability and hatching success. There is evidence that transferring gland secretions to the plumage and onto eggs or offspring might reduce harmful microbes. Third, there might be a different need for chemical communication during breeding. As the gland secretions are spread on the plumage during preening, they might be an essential contribution to a bird’s body odor. In our group, we are especially interested in the role of odors for social communication.

Nesting in Cavities Protects Birds from Predators—to a Point

AUK-17-51 M Arndt

A Marsh Tit brings nesting material to a cavity. Photo credit: M. Arndt

Nesting in cavities provides birds with some protection from predators—but it isn’t foolproof. A new study from The Auk: Ornithological Advances explores how Poland’s cavity-nesting Marsh Tits deal with predator attacks and finds that while tactics such as small entrances and solid walls do help, adaptations like this can only take the birds so far.

Wrocław University’s Tomasz Wesołowski has spent nearly thirty years monitoring Marsh Tit nest cavities in Poland’s Białowieża Forest, comparing nests that are destroyed with nests that are attacked but survive. He has found that a nest’s chance of survival depends on the predator’s technique—broods are least likely to survive (10%) when the predator manages to get into the cavity through the existing entrance, more likely (29%) when the predator uses its paws or beak to pluck out the nest contents, and most likely to survive (39%) when the predator tries to enlarge the opening or make a new one. Tits’ antipredator tactics vary in their effectiveness depending on the predator; attacks by Great Spotted Woodpeckers were successful only 60% of the time, while forest dormice were 100% successful.

The results show that despite the constant pressure of natural selection, Marsh Tits can only improve their antipredator tactics so much—there are limits to adaptation. Small, narrow entrances don’t work against small predators and are only effective when combined with cavity walls made of solid (not decomposing) wood; nests that were deep in a cavity, out of reach of the entrance, are safest, but birds seldom place their nests that way, suggesting that cavities that are too deep may cause other problems for Marsh Tit parents.

The Białowieża Forest, one of the last remaining tracts of old-growth forest in Europe, is an ideal place to study cavity-nesting birds, full of cavities of every size and shape for Marsh Tits to choose from. However, the fieldwork was not without its difficulties. “The Białowieża Forest still contains fragments of primeval origin,” says Wesołowski. “The work is challenging, as the old-growth stands are very tall. Marsh Tits breed at very low densities, and on average one has to search five to seven hectares of this forest to find a single breeding cavity. It requires much patience and determination.”

“To understand the evolution of nesting behaviors, many ornithologists attempt to quantify the trade-offs that birds face in warding off nest predators. Usually we do this by comparing nests that fail versus nests that succeed, but that approach is limited because we can’t tease apart the multiple factors, including chance, that contributed to making a nest successful,” according to Kristina Cockle of the National Scientific and Technical Research Council of Argentina (CONICET), an ornithologist not involved with the study who has worked extensively on nest cavities. “The new study by Wesołowski compares, instead, nests that were depredated to nests that were attacked but survived. With this approach, the author was able to identify the physical attributes of tree cavities that foiled a suite of nest attackers from woodpeckers to dormice.”

Failed predator attacks: A study of tree cavities used by nesting Marsh Tits (Poecile palustris) for security is available at

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.

2016 Journal Impact Factors Released

The 2016 impact factors for peer-reviewed journals were released last week, and both journals published by the American Ornithological Society saw a boost in their numbers—The Condor is up to 2.654 from 1.427 and is now #1 of the 24 ornithology journals ranked, and The Auk is up to 2.096 from 1.871 (ranking #4). This means that the American Ornithological Society now publishes half of the top four ornithology journals in the world.

Impact factors are calculated based on the number of citations received in a year by articles published in a journal during the two preceding years and are considered to be an important measure of a journal’s prominence in its field. The AOS publications team wants to thank all of our Associate Editors, authors, and reviewers, as well as everyone who reads and cites The Auk and The Condor!


Muscle Fibers Alone Can’t Explain Sex Differences in Bird Song

Male birds tend to be better singers than females—but does the basis for this difference lie in the brain or in the syrinx, the bird equivalent of our larynx? The researchers behind a new study from The Auk: Ornithological Advances analyzed the muscle fibers in the syrinxes of male and female birds from a range of species and found, to their surprise, that the amount of “superfast” muscle wasn’t typically related to differences in vocal ability between the sexes.

Most muscle fibers are one of two types—fast, specialized for short, intense bursts of activity, or slow, specialized for endurance. However, some animals, including birds, have a third type called superfast muscle that can contract around 200 times per second. Ron Meyers of Weber State University and his colleagues hypothesized that superfast muscle fibers in the syrinx might explain the greater singing ability of male birds, but when they analyzed the syringeal muscles of male and female birds from a range of species, they found that the amount of superfast muscle fiber didn’t differ between the sexes in most species. Instead, their results suggest that the role of superfast muscle is more complicated than they expected and may be related to the entire range of vocalizations of a species rather than song alone. Even though females of some species don’t sing, their superfast muscle fibers appear likely to play a role in the calls they use for other types of communication.

The researchers collected syringeal tissue from a total of ten bird species, some wild-caught and some from a University of Utah aviary. All species had both fast muscle and superfast muscle fiber in their syrinxes, but there was a clear sex difference in fiber type composition in only two species studied, Bengalese Finches and Zebra Finches. Based on this, the researchers speculate that the need for superfast muscle may be related to the entire vocal repertoire of each sex, not just singing behavior. Calls made by Zebra Finch females don’t have acoustic features that would require rapid muscle control, but in other species females may produce calls that require the muscle control provided by superfast fibers even if they don’t sing.

“The data really surprised us,” says Meyers. “Based on our first species studied, starlings and Zebra Finches, we went into this thinking that superfast fibers were related to singing in males. Zebra Finch males sing and females don’t, and males have 85% of the syrinx muscles made up of superfast fibers. In starlings, both male and females sing, and they both had about a 65% make-up of superfast fibers. But as the number of species we looked at grew, we had to totally change our perception of the role of superfast fibers in singing and the role they actually play in vocalizing.”

“Most of the research investigating the mechanisms of bird song focuses on the brain. However, research has begun to suggest that peripheral structures like the syrinx influence song divergence, which of course is an important factor that contributes to avian biodiversity,” according to Wake Forest University’s Matthew Fuxjager, an expert on superfast muscle. “This study therefore provides an exciting starting point to address this issue from a physiological perspective, and it shows that muscle fiber content in the syrinx might not be a strong predictor of avian vocal diversity. But then what is? I would argue that we’re still working this out, and that this study will provide an intriguing framework from which more work in this area can be conducted.”

Is sexual dimorphism in singing behavior related to syringeal muscle composition? is available at

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.

Which Extinct Ducks Could Fly?

AUK-17-23 J Watanabe

Fossils of extinct ducks and geese provide new clues about flightlessness. Photo credit: J. Watanabe

We’re all familiar with flightless birds: ostriches, emus, penguins—and ducks? Ducks and geese, part of a bird family called the anatids, have been especially prone to becoming flightless over the course of evolutionary history. However, it can be difficult to determine from fossils whether an extinct anatid species could fly or not. A new study from The Auk: Ornithological Advances takes a fresh approach, classifying species as flightless or not based on how far their skeletal proportions deviate from the expected anatomy of a flying bird and offering a glimpse into the lives of these extinct waterfowl.

Kyoto University’s Junya Watanabe painstakingly measured 787 individual birds representing 103 modern duck and goose species. From this data, he developed a mathematical model that was able to separate flightless and flying species based on their wing and leg bones—flightless species, the math confirmed, have relatively small wings and relatively large legs. Applying the model to fossil specimens from 16 extinct species identified 5 of the species as flightless, ranging from a land-dwelling duck from New Zealand to a South American duck that propelled itself underwater with its feet.

“I really enjoyed measuring bones in museums and appreciate the hospitality given to me by museum staff. One of the most exciting things was to find interesting fossils that were previously unidentified in museum drawers,” says Watanabe. “What is interesting in fossil flightless anatids is their great diversity; they inhabited remote islands and continental margins, some of them were specialized for underwater diving and others for grazing, and some were rather gigantic while others were diminutive.”

“Dr. Watanabe has developed a valuable statistical tool for evaluating whether a bird was capable of powered flight or not, based on measurements of the lengths of only four different long bones. His method at present applies to waterfowl, but it could be extended to other bird groups like the rails,” according to Helen James, Curator of Birds at the Smithsonian Institution’s National Museum of Natural History. “Other researchers will appreciate that he offers a way to assess limb proportions even in fossil species where the bones of individual birds have become disassociated from each other. Disassociation of skeletons in fossil sites has been a persistent barrier to these types of sophisticated statistical analyses, and Dr. Watanabe has taken an important step towards overcoming that problem.”

Quantitative discrimination of flightlessness in fossil Anatidae from skeletal proportions is available at

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: ‘Bare Parts’ are an Important but Underappreciated Avian Signal


Two female American Goldfinches in an antagonistic interaction. Bill-color, derived from carotenoids, is a signal of dominance among female goldfinches but not among males. Image credit: K. Tarvin

Erik Iverson

Linked paper: The role of bare parts in avian signaling by E.K. Iverson and J. Karubian, The Auk: Ornithological Advances 134:3, July 2017.

Birds are well-known for being among the most colorful of all animals, with many species displaying striking, brightly-colored feathers. Scientists have long wondered why color is so important to fitness, and hundreds of studies have been published on the relationships between plumage and traits such as age, physiological condition, reproductive success, and attractiveness to mates. However, there is a growing awareness that plumage is not the only important site of coloration among birds; there is also considerable variation within and between species in the color of bills and in bare skin such as legs, feet, ceres, or wattles. Yet compared to plumage, these ‘bare part’ ornaments have received relatively little attention; a 2006 review of carotenoid coloration in birds, for instance, identified only 14 studies of bare parts versus 130 studies of plumage.

Unlike plumage, bare part color has the potential to be highly flexible. For example, carotenoid-based bare parts can lose their color within days of food deprivation or within hours of stress. Amidst growing suggestions that changes in bare part color could have important implications for signaling, one of the authors, Jordan Karubian, was studying Red-Backed Fairywrens (Malurus melanocephalus) in Australia. In this species, males either acquire a territory and display black breeding plumage and bills, or stay dull and serve as helpers at the nest. Jordan noticed that when a breeding male died and a dull male took over its vacancy, the dull male’s bill would darken within several weeks. Experiments confirmed this effect and showed that dull males with newly black bills also had testosterone levels comparable to birds with black plumage. I joined Jordan’s lab as an undergraduate and studied fairywrens as well, and when I was looking for a topic for an honors thesis Jordan suggested that bare parts were an expanding area in need of a review. That thesis grew and grew, eventually becoming my master’s work and encompassing 321 published studies of bare-part coloration and signaling.

Our review shows that despite the research focus on plumage, bare part signals might be more common than plumage-based ones and are an important visual signal in many species that lack bright plumage altogether. Carotenoids, melanin, and structural colors are all flexible in bare parts, and rapid blood flushing through skin can change color even more rapidly. Bare part color provides up-to-date information about a signaler, allowing competitors, mates, and offspring to adjust their strategies and maximize their fitness. Carotenoid-signaling with bare parts may also be less costly than with plumage, allowing signaling by females and non-breeding males. In species where both plumage and bare parts of the same color exist, the two are likely to be ‘multiple messages,’ conveying different aspects of condition or targeting different audiences. We believe that more careful and extensive characterization of bare part coloration will contribute greatly to our understanding of this underappreciated dynamic signal, and help inform a more inclusive theory of animal communication.