AUTHOR BLOG: To the Grasshopper Sparrow, the Grass May Be Greener on the Other Side

Emily Williams

Linked paper: Patterns and correlates of within-season breeding dispersal: A common strategy in a declining grassland songbird by E.J. Williams and W.A. Boyle, The Auk: Ornithological Advances 135:1, January 2018.

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SY-GD, or silver-yellow, green-dark blue, is a male fiercely defending his territory on his favored signpost perch. Photo credit: D. Rintoul

Late in the summer of 2013, when Alice Boyle, a new faculty member at Kansas State University, was embarking on studies of grassland birds at the Konza Prairie Biological Station in northeastern Kansas, she noticed something really curious: Individual Grasshopper Sparrows she had color-banded earlier in the season were suddenly popping up in new places, singing their hearts out in locations far from where they were originally captured. Whether this was a weird one-off or a predictable and common behavior of grassland birds, Boyle didn’t know.

I started at Kansas State University as Alice’s first graduate student the next fall. She told me the story of the Grasshopper Sparrows and the strange things they did over the summer. With my fondness for movement ecology and a taste for novelty, I opted to base my master’s thesis on the rogue Grasshopper Sparrows seeking greener pastures.

To rewind a little bit, I should explain why this kind of behavior was surprising. The Grasshopper Sparrow is a small, grassland-obligate migratory songbird that spends its winters along the Gulf Coast and Northern Mexico and travels to the Great Plains to spend its summers. Migration is energetically costly, requiring a lot of time and preparation. Once birds arrive at breeding grounds, they have a relatively short window of time to set up a territory, find a mate, build a nest, raise young, and feed fledglings, and then undergo molt, feed, and prepare for the long journey back to the wintering grounds. All of this has to be accomplished in a span of a few short months. Given the constraints of time, resources, and energy, you’d think that they would stick pretty close to their original territory for the whole breeding season. That is what most migrant birds do, after all. The fact that Grasshopper Sparrows would switch territories, duplicating their efforts of setting up another territory, finding a potential new mate, and trying to nest again—it seems like it wouldn’t be worth it. The fact that Grasshopper Sparrows are indeed doing this—changing territories once, twice, maybe three times, even—makes them apparently unusual compared to their migratory counterparts and begs the question, why go to all that effort?!

Before we could examine why Grasshopper Sparrows move around during the breeding season, we first needed to determine just how common this kind of behavior was. We also wanted to find out the distances over which they traveled, where were the new places they chose to settle, and how frequently they moved locations. Following that initial season in 2013, we set out to answer these questions and looked for this behavior in full force. In the next three seasons of field work, we banded 779 Grasshopper Sparrows, outfitted 19 individuals with radio-transmitters to follow their movements, and searched for color banded birds throughout our study area every week to keep track of territory holders and their whereabouts throughout the season.

What we found, we couldn’t have predicted: Within-season breeding dispersal behavior in Grasshopper Sparrows was way more common than we expected. Depending on which of the different metrics we calculated, between 33% and 75% of males disperse at least once within a single breeding season. The scale of movement between territory locations was also remarkable; one individual moved 9 km between breeding attempts—a movement considered pretty large to a bird that defends an average territory size of 43 meters in diameter! If we had not been systematically looking for this behavior, we might have easily missed it; in many areas, densities of Grasshopper Sparrows remained constant throughout the breeding season, but the identities of territory holders changed, sometimes more than once over the summer.

The fact that these birds are moving around a lot during the breeding season introduces its own list of new questions. Now that we detailed the patterns of this behavior, we could begin to answer the questions of why. Why do they do this? What determines why some leave, and some stay? And what determines where they settle next? Could this be a common strategy of other birds occupying similar habitats?  While trying to determine whether this movement was truly unusual by digging into the literature, I actually found quite a bit of evidence for such movements. While the terminology is not consistent, it seems that within-season breeding dispersal could be more common in grassland birds than elsewhere.

The answers to some of these questions formed the rest of my MS research, and some remain as ones we are still working on. But now that the first piece of the puzzle is in place, the next steps are to explore the evolutionary and ecological causes of within-season breeding dispersal in such an interesting little brown job.

To find out more regarding this Grasshopper Sparrow movement story, visit and follow us on social media.

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Grassland Sparrows Constantly Searching for a Nicer Home

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A Grasshopper Sparrow with a radio transmitter. Photo credit: E. Williams

Some birds regularly move to new territories between years, depending on factors including habitat quality and the presence of predators, but what about within a single breeding season? Grassland ecosystems are particularly dynamic, continuously shaped by fire and grazing, and a new study from The Auk: Ornithological Advances confirms that one particular grassland bird moves frequently each summer in search of the best territories. For Grasshopper Sparrows, the grass really does look greener on the other side.

Emily Williams and Alice Boyle of Kansas State University captured 647 male Grasshopper Sparrows over the course of three breeding seasons and marked them with identifying color bands, surveying territories weekly to track their movements. The results indicate that about 75% of males changed territories at least once per season, with a third of banded defending new territories at least 100 meters away from where they were originally sighted. Additionally, 9 of 19 birds fitted with radio transmitters established new territories as far as 1200 meters away from their original locations.

“We had many plots where the density remained relatively stable over the entire breeding season, which could appear as if individual birds remained settled in the same areas over time. However, what we found was the complete opposite—individuals were blinking in and out of territories the entire time,” says Williams, who has since moved on to a position at Denali National Park and Preserve. “So while an onlooker could see a male Grasshopper Sparrow singing on a single patch of ironweed for months at a time, the identity of the individual claiming that ironweed as his own could change two or three times in a single summer.” Without careful observations, researchers could completely miss these dynamic movements happening over the course of a season.

This high turnover implies that while some birds might perceive a patch of habitat as no longer suitable, others see the same area as a good place to settle, perhaps because they base decisions on their individual experiences of nest success or failure. High mobility may benefit grassland birds by helping them locate isolated patches of high-quality habitat and colonize newly created or restored habitat, but could also challenge researchers’ ability to accurately track survival over time.

“Many avian ecologists have probably anecdotally noticed within-season shifts in breeding territories, yet this is one of the first attempts to actually quantify this phenomenon. The extent to which territorial turnover occurred and the fairly extensive distances moved by males within a season are intriguing!” says the University of Wyoming’s Anna Chalfoun, an expert on grassland birds who was not involved in the study. “I am left wondering if this behavior is more common than ornithologists have previously acknowledged and what drives proximate shifts in breeding territories. The results certainly have implications for habitat management for territorial birds of concern and for the accuracy of survival and site fidelity analyses.”

Patterns and correlates of within-season breeding dispersal: A common strategy in a declining grassland songbird 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.

AUTHOR BLOG: Flooding, Predators, and an Imperiled Sparrow

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A banded adult Saltmarsh Sparrow female foraging in Scarborough, ME. Photo credit: D. Hitchcox

Kate Ruskin

Linked paper: Demographic analysis demonstrates systematic but independent spatial variation in abiotic and biotic stressors across 59 percent of a global species range by K.J. Ruskin, M.A. Etterson, T.P. Hodgman, A.C. Borowske, J.B. Cohen, C.S. Elphick, C.R. Field, R.A. Longenecker, E. King, A.R. Kocek, A.I. Kovach, K.M. O’Brien, N. Pau, W.G. Shriver, J. Walsh, and B.J. Olsen, The Auk: Ornithological Advances 134:4, October 2017.

Ecologists have long hypothesized that the factors that affect a species vary over its geographical range. For example, cold climates may limit survival at higher latitudes, while competition with other species may be more important at lower latitudes. Scientists have proposed that this sets up a tradeoff for each species, favoring individuals that are physiologically hearty to harsh abiotic conditions at higher latitudes and individuals that are good competitors at lower latitudes.

With the help of 14 coauthors scattered across the northeastern U.S., I collected demographic data on Saltmarsh Sparrows to test whether this pattern was supported. Our team, known as the Saltmarsh Habitat and Avian Research Program (SHARP), conducted coordinated demographic research on Saltmarsh Sparrows at 23 sites in 7 states from Maine to New Jersey. We searched for nests, revisited them every few days throughout the breeding season, and classified each as successful or failed due to various causes.

Saltmarsh Sparrows breed exclusively in high marsh habitat, which is the zone of tidal marshes that typically floods monthly during the astronomical high tides. Saltmarsh Sparrows build their nests in the short grasses of the tidal marsh, just a few inches above the ground. As a result, nests often fail due to flooding during the high monthly tides. Most nest failure in Saltmarsh Sparrows is caused either by this nest flooding, or by depredation.

Footage captured by University of Connecticut graduate student Samantha Apgar.

Using monitoring records from 837 nests collected across our study sites, we observed patterns in the factors that limit nest survival that varied predictably across hundreds of kilometers. We found that the biotic stressor, nest depredation, increased toward lower latitudes, which is consistent with the Asymmetric Abiotic Stress Limitation (AASL) hypothesis. AASL proposes that populations are limited by biotic stressors like nest depredation at the lower latitudes of their range, while abiotic stressors such as climate limit populations at higher latitudes. Conversely, we observed that the abiotic stressor, nest flooding, did not vary with latitude. Instead, nest flooding was best predicted by indicators for regular monthly flooding as well as irregular flooding events, which varied independent of latitude. Our results suggest that stressors to Saltmarsh Sparrow reproductive success vary systematically across its range, but independently from each other. Therefore, we did not observe the tradeoff between physiological heartiness at higher latitudes and competitiveness at lower latitudes that is predicted by the AASL hypothesis.

In addition to the insight this example provides into how different stressors limit species across their ranges, the patterns of biotic and abiotic stress that we observed provide information relevant to conservation of the Saltmarsh Sparrow. The Saltmarsh Sparrow is considered threatened by the International Union for the Conservation of Nature, and SHARP researchers have found that the Saltmarsh Sparrow population is small, declining, and expected to go extinct this century. For example, our results suggest that predator control may be an effective method for improving Saltmarsh Sparrow fecundity toward the low latitudes of its range, but not farther north.

This new article in Auk: Ornithological Advances is the latest in a series we have written about the Saltmarsh Sparrow and other tidal marsh birds found in northeastern North America, many of which are facing population declines and habitat change. Learn more about tidal marsh birds and SHARP’s research at our website ( and Facebook page (

A First Look at Geographic Variation in Gentoo Penguin Calls

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A calling Gentoo Penguin. Photo credit: M. Lynch

Vocal communication is central to the lives of many birds, which use sound to attract mates and defend territories. Penguins are no exception, but we know little about how or why penguin vocalizations vary geographically between isolated populations. A new study from The Auk: Ornithological Advances takes a broad look at vocalizations across the range of Gentoo Penguins and concludes that while their calls do vary from place to place, we still have a lot to learn about the processes at work.

The Gentoo Penguin’s “ecstatic” call, consisting of repeated pairs of short syllables, is used to attract and contact mates. Maureen Lynch and her PhD advisor Dr. Heather Lynch (no relation) of Stony Brook University recorded ecstatic calls at 22 Gentoo Penguin colonies across the Antarctic Peninsula, southern Argentina, and nearby islands. While they found variation in call frequency and duration both within and between colonies, no clear patterns emerged based on latitude, region, or subspecies. An algorithm based on their data was able to classify calls to correct colonies better than random, but with a high error rate.

Their results suggest that the vocal characteristics of colonies drift independently of each other over time. Within colonies, it may be beneficial for individuals to differ in their calls so that they can tell each other apart. “There is so much that we still do not know about penguin vocal behavior,” says Heather. “We see this as being very much the beginning, rather than the end, of understanding how penguins communicate, how and if such communications play a functional role in protection against predators, choice of mates, and breeding site selection.”

“Work in the Antarctic is always challenging, and this project was time- and data-intensive, with data collection over three field seasons,” adds Maureen. “Unexpected challenges came from flying birds rather than penguins. The recording units hold up well in the Antarctic elements and can even record over winter, but I learned the hard way that if I leave a Song Meter unattended in the Falkland Islands, the Striated Caracaras will eat the windscreens off the microphones and can actually pull the microphones off.”

“Understanding the drivers of population differentiation is increasingly important for species such as penguins that are being impacted by climate change,” according to Fordham University’s J. Alan Clark, a penguin behavior researcher who was not involved in the study. “This study, the largest of its kind, takes a creative and rigorous approach to exploring the role of vocalizations in population differentiation across a wide geographic range and across populations with known intraspecific genetic variation. The results of this study provide practical insights that help set the stage for future research on interactions between speciation processes and climate change.”

Variation in the ecstatic display call of the Gentoo Penguin (Pygoscelis papua) across regional geographic scales 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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Social Environment Matters for Duck Penis Size

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Penis size in Ruddy Ducks has a complex relationship with the birds’ social environment. Image credit: P. Brennan

Most birds lack genitalia, but male ducks are known for their long, spiraling penises, which have evolved through an ongoing cat-and-mouse game with females. A new study from The Auk: Ornithological Advances looks at whether these impressive organs are affected by the social environment—that is, whether male ducks that face more competition grow bigger penises. While this appears to be true for some species, in others the relationship between social environment and penis growth is more complex.

Patricia Brennan of Mount Holyoke College and her colleagues tested their hypothesis in two species: Ruddy Ducks, which are very promiscuous, do not form pair bonds, and have relatively long penises, and Lesser Scaup, which form seasonal pair bonds and have relatively short penises. Keeping captive ducks in either pairs or groups during the breeding season over two years, they found that Lesser Scaup had longer penises on average when housed in groups with other males, as predicted. For Ruddy Ducks, the effects were more complicated—many males failed to reach sexual maturity until the second year of the experiment, and when they did, the smaller Ruddy Duck males housed in groups grew their penises faster than males housed in pairs, but grew out of sync with each other and stayed in reproductive condition for only short periods of time.

Small Ruddy Ducks males faced with intense competition may strategically offset their development from each other to reduce the costs of male–male aggression and make the best of a bad situation. Additionally, since Ruddy Ducks already have relatively long penises on average compared to other waterfowl species, their ability to grow even larger based on social cues may be limited. In any case, the study shows that the level of competition that individual male ducks experience can have a big effect on their genitals.

The biggest challenge during the study, says Brennan, wasn’t measuring the ducks—it was simply keeping them housed and fed. “Keeping ducks in captivity is expensive,” says Brennan. “We were lucky to partner with the Livingstone Ripley Waterfowl Conservancy in Litchfield, Connecticut, where their expert personnel kept the ducks healthy and in beautiful, naturalistic enclosures year-round.”

“This is an excellent experimental study of penis morphology, looking at the effects of social environment on penis size in two duck species that have different mating systems,” according to Queen’s University’s Bob Montgomerie, an expert on reproductive strategies who was not involved in the study. “The question now is whether the observed increase in penis size in Lesser Scaup under the threat of sperm competition actually gives males a competitive advantage. Like all good studies, this one will undoubtedly stimulate more research, as it provides both methodologies and a clear focus on interesting questions.”

Evidence of phenotypic plasticity of penis morphology and delayed reproductive maturation in response to male competition in waterfowl 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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Song Experiments Reveal 21 Possible New Tropical Bird Species

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Playback studies indicate that two populations of Buff-throated Foliage-gleaners do not recognize each other’s songs. Photo credit: B. van Doren

Birds often choose their mates based on song, making it a key factor in separating species. However, analyzing spectrograms can only tell us so much—the characteristics that birds hone in on when identifying potential mates may not be the same ones scientists notice in audio recordings. A new study from The Auk: Ornithological Advances uses field experiments to “ask the birds themselves” and uncovers as many as 21 previously unrecognized species.

Benjamin Freeman of the University of British Columbia and Graham Montgomery of Cornell University compared these two methods—analysis in the lab and experiments in the field—for 72 pairs of related but geographically separated bird populations in Costa Rica, Panama, and Ecuador. In addition to analyzing more than a thousand song recordings for seven variables, they used playback experiments to test birds’ real-world reactions to recordings of their relatives, observing whether or not they approached the speaker. Their results show that when the divergence between the characteristics of the recordings is high, birds consistently fail to recognize recordings of their relatives in the field, but when divergence is low, birds’ discrimination is much less consistent. In other words, analyzing recordings can’t accurately predict how birds will act when presented with songs just slightly different from their own.

Many pairs that failed to recognize each other are currently categorized as members of the same species, suggesting that current taxonomy does not reflect actual bird behavior when it comes to song. Freeman and Montgomery propose that 21 such pairs should be recognized as separate species based on song discrimination and that playback experiments should be the standard for assessing whether song divergence between populations is a barrier to interbreeding. “It is abundantly clear to anyone familiar with the amazing diversity of Neotropical birds that there are many cases where populations that sing very different songs are classified as the same species,” says Freeman. “These populations look the same—they have similar plumage and are similar in size and shape—but assuming that populations that sing differently tend not to interbreed, this means that species-level diversity in the Neotropics is underestimated.”

“Playback experiments between geographically isolated taxa provide key data on how populations might perceive each other in terms of ‘same’ or ‘different’ if they were in actual contact,” according to Louisiana State University’s J.V. Remsen, an expert on Neotropical birds who was not involved in the research. “Hopefully, this pioneering study will catalyze a wave of similar studies around the globe as a way to approach the always-thorny problem of species limits in these birds.”

Using song playback experiments to measure species recognition between geographically isolated populations: A comparison with acoustic trait analyses 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.

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 (