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 (

Prairie-Chicken Nests Appear Unaffected by Wind Energy Facility

CONDOR-17-51 L Powell

Prairie chickens nests appear unaffected by the proximity of a small wind energy facility. Photo credit: L. Powell

Wind energy development in the Great Plains is increasing, spurring concern about its potential effects on grassland birds, the most rapidly declining avian group in North America. However, a new study from The Condor: Ornithological Applications suggests that for one grassland bird species of concern—the Greater Prairie-Chicken—wind energy infrastructure has little to no effect on nesting. Instead, roads and livestock grazing remain the most significant threats to its successful reproduction.

Prairie-chickens are thought to avoid tall structures such as wind turbines because they provide a perch from which raptors can hunt. To learn more, the University of Nebraska–Lincoln’s Jocelyn Olney Harrison and her colleagues gathered data on the effects of an existing small wind energy facility (36 turbines) in Nebraska. They captured 78 female prairie-chickens at breeding sites, or leks, ranging from less than a kilometer from the wind energy facility to more than twenty kilometers away, and fitted the birds with transmitters to track them to their nests. Monitoring their nesting success and collecting data on the habitat characteristics of each nest site, they found little evidence that the wind energy facility affected nest site selection or a nest’s chances of survival. Instead, vegetation characteristics, driven by land use practices such as grazing, had the greatest influence on prairie-chicken nests. Birds also avoided nesting near roads.

“When comparing previous studies to our own, it appears that the effects of wind energy facilities on prairie grouse are often site- and species-specific,” says Harrison. “Therefore, it’s important to consider the results of our study in the context of the size and location of the wind energy facility, as well as the prairie grouse species investigated. We suggest that livestock grazing and other grassland management practices still have the most important regional effects on Greater Prairie-Chickens, but we caution future planners to account for potential negative effects of roads on nest site placement.”

Private landowners were key to completing the study, Harrison adds. “Our radio- and satellite-tagged Greater Prairie-Chickens made larger than expected movements while we were tracking them, which led us to require permission from new land owners on almost a weekly basis during our field seasons. Landowners throughout our field study area were always extremely welcoming and helpful, and genuinely interested in our work. Our project was a success due to more than 50 landowners who granted us access to their private lands.”

Nest site selection and nest survival of Greater Prairie-Chickens near a wind energy facility is available at

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.

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.

Fifty Years On, the Breeding Bird Survey Continues to Produce New Insights

Special Collection of Research Papers Highlights Latest Findings


Prothonotary Warblers (Protonotaria citrea) inspect a possible nest site at the Patuxent Wildlife Research Center, Laurel, Maryland. Prothonotary Warblers are one of more than 500 species monitored by the North American Breeding Bird Survey. Photo credit: W.A. Link

In 1966, a U.S. Fish and Wildlife Service biologist named Chan Robbins launched an international program designed to measure changes in bird populations using volunteers recruited to count birds on pre-set routes along country roads. The result, the North American Breeding Bird Survey or BBS, is still going strong more than five decades later. This month The Condor: Ornithological Applications is publishing a special set of research papers to honor the program’s fiftieth anniversary.

Unassuming but visionary, Robbins had studied DDT’s effects on birds—his reports were edited by Rachel Carson—and he wanted to devise a way of monitoring the health of the continent’s bird populations on a large scale. The simple field protocols he developed, able to be carried out by volunteer birdwatchers, have remained largely the same since the program’s inception. Today, there are more than 4100 survey routes spanning North America from Alaska to Newfoundland, Florida, and northern Mexico.

The BBS provides long-term data for 424 species, with more limited data for an additional 122. Since data collection began in the 1960s, significantly more species have been declining than increasing. Looking at patterns of change in groups of birds sharing common attributes can be especially useful; for example, only 8 of 24 grassland bird species have seen increases. However, in the short term the picture is slightly rosier—since the survey area was expanded in 1993, 56% of the species surveyed have showed positive trends. Today, modern statistical techniques are letting ornithologists glean more insight from BBS data than ever before.

“The BBS is the only source of long-term, multi-scale population change information for more than 500 species of North American birds,” according to the USGS’s John Sauer, who has worked with the BBS since 1986 and was one of the co-editors for the special section along with Keith Pardieck and Colleen Handel, also with the USGS. “BBS results have allowed conservationists to identify bird species and regions undergoing population declines, alerting the public and scientists to population changes and facilitating the development of initiatives to better understand declines.”

The papers that make up the special section in The Condor include:

  • Prioritizing areas for conservation by combining six years of BBS data with remotely sensed environmental data to model the predicted distribution of seven grassland bird species in the Northern Great Plains based on their habitat needs.
  • Statistical approaches for model selection in BBS analyses.
  • Combining BBS with off-road surveys to estimate population changes for birds that breed in Alaska, where habitats are being rapidly altered due to climate change.
  • Using long-term BBS data to rank the vulnerability of more than 460 landbird species, set population objectives, and track progress toward meeting conservation goals.
  • Analyzing how well road-based BBS routes represent larger landscapes, using data from 2011 National Land Cover Database, with the conclusion that any land-cover–based roadside bias in BBS data is likely minimal.
  • Combining BBS data with separate demographic data to estimate the size of the Atlantic Flyway’s Wood Duck population.
  • Plus, a review of how the BBS has informed North American bird conservation since its inception.

The papers grew out of a research symposium held at last summer’s North American Ornithological Conference in Washington, DC, to commemorate 50 years of the BBS. “The BBS provides a fundamental tool for understanding breeding bird distribution and abundance. We’re pleased to publish these papers that celebrate Chan Robbins’s vision and the hard work of thousands of volunteers through the latest results and analyses,” said Philip Stouffer, Editor-in-Chief of The Condor: Ornithological Applications.

The special section on the Breeding Bird Survey is available at

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.

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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.

Seaside Sparrows Caught Between Predators and Rising Seas

CONDOR-17-11 E Hunter

Fledgling Seaside Sparrows. Photo credit: E. Hunter

Sea-level rise may be a big problem for salt marsh birds, but so is predation, and birds sometimes find themselves caught between a rock and a hard place: They can place their nests lower in the vegetation to avoid predators, putting them at greater risk of flooding, or move them up to keep them dry but risk getting eaten. A new study from The Condor: Ornithological Applications finds that greater pressure from predators increases the risk of flooding for Seaside Sparrow nests—but the upside is that protecting them from predators could also mitigate the worst effects of climate change.

The University of Georgia’s Elizabeth Hunter (now at the University of Nevada–Reno) created a mathematical model to simulate Seaside Sparrow’s nesting behavior and success rates, based on nesting data collected on the coast of Georgia.  Her model shows that predation risk has had a much greater effect than flooding risk on nest survival rates. While flooding risk had essentially no effect on predation rates over this time period, predation risk did affect flooding rates—that is, because birds moved their nests down to avoid predators, they increased their risk of flooding. Under future sea level rise scenarios, flooding risk increases, but predation risk is still almost seven times as important for determining nest survival rates.

“Nest predation rates are so high right now that even under extreme sea level rise conditions, more nests are likely to be eaten than flooded,” says Hunter. “However, predation and flooding threats act synergistically, meaning that any estimates of the negative effects of sea level rise on the nesting success of Seaside Sparrow or other species are likely underestimates if they do not also consider the negative effects of predation on flooding risk. The flip side of this is that management actions to reduce nest predation could also reduce the risk of nest failures from flooding.” If measures such as fencing nest sites to exclude predators are taken, birds may place their nests higher in the salt-marsh vegetation, avoiding flooding from extreme high tides.

“Elizabeth Hunter’s research highlights both the risks that sea-level rise poses for coastal wildlife and the complexity of understanding those risks in light of other threats to their survival,” according Chris Elphick of the University of Connecticut, an expert on tidal marsh birds who was not involved with the study. “The study nicely illustrates the importance of understanding the behavior of individual birds when trying to devise strategies to mitigate threats such as predation and tidal flooding. Regardless of the threat, it is increasingly clear that tidal marsh birds and their habitats are in trouble, and that we need to explore a range of potential solutions to find ways to help them persist in light of the many ways that humans are changing coastal habitats.”

How will sea-level rise affect threats to nesting success for Seaside Sparrows? is available at

About the journal: The Condor: Ornithological Applications is a peer-reviewed, international journal of ornithology. It began in 1899 as the journal of the Cooper Ornithological Club, a group of ornithologists in California that became the Cooper Ornithological Society, which merged with the American Ornithologists’ Union in 2016 to become the American Ornithological Society.

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Ornithology & Social Media – A Perfect Partnership

This blog is one of the ways that the publications staff behind The Auk and The Condor connects with the journals’ readers (a.k.a. you!). Another? Social media.

Each journal has its own Twitter account (@AukJournal and @CondorJournal), where we share the latest ornithology research—from other publications as well as our own—and news that’s relevant to our community. The American Ornithological Society, our parent organization, also uses social media—in addition to Twitter, they’re on Facebook and Instagram. You don’t need an account on any of these sites to read the content posted there, and checking them out periodically will help you keep up with the latest developments in the American ornithology world.

If you’re interested in learning more about social media, whether you’re an avid user looking to get some new tips or you’re thinking of wading in for the first time, you should consider attending the social media symposium at American Ornithology 2017, the joint annual meeting of AOS and SCO (the Society of Canadian Ornithologists) that’s coming up in Michigan. The symposium begins at 10 a.m. on Wednesday, August 2, and will cover how using social media for science communication can benefit your research and your career. Hope to see you there!