Tracing the Evolution of Bird Reproduction

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Avian reproductive strategies have gone through a series of stages, from dinosaurs to today. Image credit: D. Anduza

What really did come first—the chicken or the egg? Birds’ reproductive biology is dramatically different from that of any other living vertebrates, and ornithologists and paleontologists have long wondered how and when the unique features of bird reproduction originated. A new Review in The Auk: Ornithological Advances examines answers from three sources—modern birds, fossils of primitive birds, and fossils of the dinosaurs from which birds are descended—to shed new light on the subject.

All modern birds share certain reproductive features, such as a single functional ovary and the practice of incubating their eggs through direct contact. Analysis of the bird family tree also suggests that early birds built simple, open nests on the ground and that their young were “precocial,” meaning they were well-developed and almost ready to fend for themselves when they hatched. Those dinosaurs close to the ancestry of birds shared some of these traits, but they had two functional reproductive tracts, and their eggs were smaller relative to their body size and more elongated than those of modern birds.

Fossils of primitive birds and eggs from the Mesozoic era place them midway between their dinosaur ancestors and their modern descendants, with eggs between those of pre-avian dinosaurs and modern birds in term of size and shape. In this way, David Varricchio and Frankie Jackson of the Montana State University are able to trace the evolution of bird reproduction through a series of distinct stages, from pre-avian dinosaurs to the birds of today.

“Reproduction in modern birds is distinct among living vertebrates. Many aspects of this reproduction mode trace their origin to theropod dinosaurs such as Oviraptors and Troodontids, but not really beyond them to more distantly related dinosaurs,” according to Varricchio. “Interestingly, reproduction in the most common group of Mesozoic birds is very similar to that of these dinosaurs, and so still differs from modern birds. Consequently, modern birds stand apart from Mesozoic birds, and perhaps this contributed to their surviving the end-Cretaceous extinction event.”

Reproduction in Mesozoic birds and evolution of the modern avian reproductive mode is available at http://www.aoucospubs.org/doi/full/10.1642/AUK-15-216.1.

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. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

Shorter Telomeres Reveal Stress in Migratory Birds

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Dark-eyed Juncos carry the stress of migration in their DNA. Photo credit: C. Billman

The stress of birds’ continent-spanning annual migrations, it appears, leads to faster aging and a potentially earlier death. A new study in The Auk: Ornithological Advances reveals that telomeres, structures on the ends of chromosomes that shorten with age, are shorter in migratory birds than in their non-migratory counterparts.

Migration lets birds take advantage of abundant food resources at high latitudes during the breeding season while escaping the region’s harsh winters. However, it’s also an enormous undertaking, and the benefits that birds gain from it come with a cost. Carolyn Bauer of North Dakota State University and her colleagues compared the telomeres—bits of non-coding DNA that shorten during cell division and stress—of migratory and resident birds from the same species, the Dark-eyed Junco. They found that the migrants had significantly shorter telomeres than birds that stayed put year-round, suggesting that the migratory birds were aging at a faster rate and that the stress of a migratory lifestyle may actually shorten birds’ lifespans.

“Whenever our cells divide, we lose a little bit of DNA on the ends of our chromosomes, and telomeres are simply non-coding regions that act as ‘protective caps,” explains Bauer. Once they reach a certain threshold of shortness, the cell dies. Importantly, exposure to stress can also make telomeres shorten faster. For their study, Bauer and her colleagues collected blood samples from 11 migratory and 21 resident juncos in Virginia, using only first-year birds to ensure that any telomere differences were not simply due to age. “I’ve been interested in measuring telomeres since I was undergraduate at the University of Washington,” says Bauer. “I remember my introductory biology professor lecturing about telomeres and how environmental stress could cause them to shorten.”

If migrating is so stressful, why keep doing it? Bauer and her colleagues believe that the costs of migration must be balanced out by the reproductive boost birds get from nesting in resource-rich northern habitats. They hope that future studies will determine whether shorter telomeres reflect the stress of migration itself or if they’re the result of decreased self-maintenance, as well as whether telomere length is negatively correlated with migratory distance.

A migratory lifestyle is associated with shorter telomeres in a songbird (Junco hyemalis) is available at http://www.aoucospubs.org/doi/full/10.1642/AUK-16-56.1.

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. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

The Double-Edged Sword of Wildlife-Friendly Yards

CONDOR-16-26 Steve Hager

Hundreds of millions of birds are killed in collisions with windows each year in the U.S. alone, and although high-rise buildings tend to be the biggest individual culprits, the vast number of suburban homes across the continent means that even a few deaths per house add up fast. A new study in The Condor: Ornithological Applications examines the factors that affect window collision rates at homes and shows that yards that are more attractive to birds are also the sites of more collisions.

Working with Alberta homeowners who collectively contributed more than 34,000 days’ worth of collision data, Justine Kummer of the University of Alberta and her colleagues found that the presence of a bird feeder, whether a house was in an urban or rural area, and the height of the vegetation in the yard were the most important predictors of collisions. Of Alberta’s 421 bird species, 53 were represented in the data, mostly common urban species.

“Although each typical residential dwelling only causes one or a few bird–window collisions per year, the enormous number of these buildings means we are killing far more birds in our collective backyards than are dying at large office buildings and skyscrapers,” according to Scott Loss of Oklahoma State University, lead author of a landmark 2014 review on the subject. “Kummer et al. provide an excellent example of how the power of citizen scientists can be harnessed to address this major conservation issue.”

The data was collected through a citizen science project launched in 2013 that recruited homeowners in Alberta to walk the perimeters of their houses daily and report evidence of bird–window collisions. “Conducting a citizen science project had a number of challenges,” says Kummer. “Unlike some other projects, I didn’t spend my time collecting data; I spent it trying to recruit homeowners and educate the public about the issue.”

Most homeowners would not want to remove their feeders and wildlife-friendly vegetation. Instead, the authors suggest that mitigation efforts should focus on evaluating the effectiveness of products such as tape and film that can be applied to window panes to prevent collisions. “As homeowners don’t want to reduce the number of bird in their yards,” says Kummer, “I think the next step will be to determine the best window deterrents they can use at their homes.”

The use of citizen science to identify the factors affecting bird-window collision risk at houses is available at http://www.aoucospubs.org/doi/full/10.1650/CONDOR-16-26.1.

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.

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Habitat Needs of Nestling & Fledgling Songbirds

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A juvenile Acadian Flycatcher with a radio transmitter. Photo credit: J. Jenkins

Both before and after they leave the nest, baby birds face a host of challenges. A new study in The Condor: Ornithological Applications examining songbird survival in the nestling and fledgling stages finds that even in the same habitat, different species face different risks and survive at different rates.

Ovenbirds and Acadian Flycatchers are migratory songbirds that nest in similar habitats, but they have very different nesting and foraging strategies. Julianna Jenkins of the University of Missouri and her colleagues tracked the survival of young birds of both species before and after they fledged. They found that flycatcher survival at both stages was related to mature forest, while Ovenbirds did best in mature forest as nestlings but sought areas with dense understories after fledging. Post-fledging survival was lower for Ovenbirds than for Acadian Flycatchers, with more than half of the tracked Ovenbird fledglings dying within ten days of leaving the nest.

Habitat information like this can be crucial for conservation biologists trying to address songbird population declines, because they can take action through land management to boost birds’ survival at multiple life stages. “It is my hope that by investigating what affects both nesting and postfledging survival, we can make management decisions that are effective for the entire breeding season,” says Jenkins.

Jenkins and her colleagues monitored nests at three sites in central Missouri, fitting nestlings with radio transmitters shortly before they fledged so they could continue to track their survival. From 90 Ovenbird and 264 Acadian Flycatcher nests, they tracked 50 Ovenbird fledglings and 45 flycatcher fledglings. “Tracking radio-tagged fledglings was the highlight of my day,” says Jenkins. “Without transmitters, I doubt we could have relocated many fledglings, if any. I was amazed at how far from the nest newly fledged Ovenbirds could travel, even without the ability to fly.”

In addition to the difference in survival between the two species, fledglings’ success also varies between regions. According to the study’s authors, this highlights the need for tracking postfledging survival for a variety of species and landscapes rather than assuming that birds nesting in similar habitat face similar risks. “Jenkins and her colleagues provide insight into factors related to post-fledging survival and how those factors might influence population trajectory,” according to David Andersen of the University of Minnesota, an expert on bird population ecology who was not involved with the study. “Their results shed light on the complex interactions between fledgling songbirds, the landscapes in which they exist, and how this important life stage influences population dynamics.”

Contrasting patterns of nest survival and postfledging survival in Ovenbirds and Acadian Flycatchers in Missouri forest fragments is available at http://www.aoucospubs.org/doi/full/10.1650/CONDOR-16-30.1.

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.

Alaska’s Shorebirds Exposed to Mercury

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A Pectoral Sandpiper, one of the shorebirds included in the study. Photo credit: B. Lagasse

Shorebirds breeding in Alaska are being exposed to mercury at levels that could put their populations at risk, according to new research from The Condor: Ornithological Applications.

Thanks to atmospheric circulation and other factors, the mercury that we deposit into the environment tends to accumulate in the Arctic. Mercury exposure can reduce birds’ reproductive success and sometimes even be lethal. Shorebirds may be particularly vulnerable because they forage in aquatic environments where mercury is converted into methylmercury, its most dangerous form. Marie Perkins of the Biodiversity Research Institute (BRI) and her colleagues investigated the level of mercury in Alaska’s shorebirds and found that some birds breeding near Barrow, at the state’s northern end, have mercury concentrations upwards of two micrograms per gram of blood.

“These species already face a lot of tough new challenges, from climate change to disappearing stop-over habitat, so throwing a neurotoxin in the mix that can reduce reproductive success is likely to harm their populations,” according to Dan Cristol of the College of William & Mary, an expert on mercury in birds who was not involved with the new study. “The mercury concentrations reported in this paper are likely to reduce reproduction, but not catastrophically, based on what we know from other species. What may be even worse, though, is that these mercury levels probably spike when they leave the breeding grounds and start burning their reserve fuel, making their already arduous continent-jumping trips even harder.”

To assess the birds’ mercury exposure, Perkins and her colleagues collected blood and feathers from nine shorebird species breeding and staging for their southward migration at sites throughout Alaska. In addition to the troubling results from birds breeding near Barrow, they found that mercury levels depended on a species’ foraging habits—shorebird species that foraged in upland areas, away from methylmercury-rich wetlands, had the lowest blood mercury concentrations.

More work is needed to determine how much mercury various shorebird species can handle before they suffer adverse effects. “These results have encouraged me to expand my research on mercury exposure in Arctic shorebirds,” says Perkins. “I am currently pursuing my PhD at McGill University, where I am working in collaboration with BRI and the Arctic Shorebird Demographics Network to closely examine mercury exposure in multiple shorebird species breeding across the North American Arctic.”

Mercury exposure and risk in breeding and staging Alaskan shorebirds is available at http://www.aoucospubs.org/doi/full/10.1650/CONDOR-16-36.1.

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.

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Feather-Munching Bacteria Damage Wild Bird Plumage

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Carolina Chickadees were among the many species sampled for the study. Photo credit: C. Kent

A new study in The Auk: Ornithological Advances links feather-degrading bacteria to damaged plumage on wild birds for the first time, offering new insights into how birds’ ecology and behavior might affect their exposure to these little-studied microbes.

Scientists know surprisingly little about the diverse community of microbes that lives on bird feathers. A few of these species of bacteria can actually break down keratin, the material feathers are made of, but few studies have looked at how feather-degrading bacteria actually affect birds in the wild. Combining data from a decade’s worth of bird-banding studies, Cody Kent and Edward H. Burtt of Ohio Wesleyan University found that tail-feather wear was strongly correlated with the presence of feather-degrading bacteria—the first time this relationship has been demonstrated in live, wild birds.

Kent and Burtt’s dataset included more than 3,500 birds from 154 different species captured between 1996 and 2005, in locations ranging from a rural Ohio backyard to Canada’s Bay of Fundy to a Louisiana bayou. Researchers carefully wiped each bird’s plumage across petri dishes containing a growth medium to culture any microorganisms living in the feathers. Ground-foraging, aerial-foraging, and fly-catching birds had a higher prevalence of feather-degrading bacteria, while nectivorous, tree-probing, and marine-foraging birds were less likely to harbor the microbes.

“In vitro work has shown that many organisms are capable of breaking down bird feathers, and several studies have linked bacterial load to factors with evolutionary implications, such as breeding success and survival,” says Kent, now a PhD student at Tulane University. “However, we have little knowledge on what factors actually influence bacterial load on wild birds or even clear evidence that feather-degrading bacteria actually break down feathers in live, wild birds. These are important factors to understand before we can make strong arguments for their evolutionary importance, and this paper begins to answer these questions.”

“Kent and Burtt’s new study is a tour de force,” says Dale Clayton, a University of Utah ornithologist who was not involved with the study. “It underscores the potential importance of feather-degrading bacteria as selective agents that may influence the evolution of birds in important ways. Tragically, Jed Burtt passed away prior to final publication of the paper, which is a fitting tribute to his lifelong love of birds and his pioneering work on the creatures that live on them.”

Feather-degrading bacilli in the plumage of wild birds: Prevalence and relation to feather wear is available at http://www.aoucospubs.org/doi/full/10.1642/AUK-16-39.1.

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. 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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Campgrounds Alter Jay Behavior

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A Steller’s Jay steals a peanut from a campground. Photo credit: W. Goldenberg

Anyone who’s gone camping has seen birds foraging for picnic crumbs, and according to new research in The Condor: Ornithological Applications, the availability of food in campgrounds significantly alters jays’ behavior and may even change how they interact with other bird species.

Steller’s Jays are intelligent, adaptable birds, and in California’s Redwood National and State Parks, the ready availability of food draws them to campgrounds. Tracking the behavior of jays near living near campgrounds and comparing it with jays living in more natural areas of the same parks, Will Goldenberg of Humboldt State University and his colleagues found that having easy access to food made jays living near campgrounds less territorial. Campground jays spent less time foraging and more time near the ground than their counterparts without access to campgrounds.

A federally threatened seabird, the Marbled Murrelet, nests in the highest canopy old-growth redwood forest, and jays are known to eat murrelet eggs when they encounter their nests. Though jays living in campgrounds spent more time on the ground, 40 meters below where murrelets nest, Goldenberg and his colleagues still worry that the concentrated foraging activity of jays could put murrelets near campgrounds at risk. “The results of our study should give researchers and conservationists new tools to make informed decisions regarding corvid management,” according to Goldenberg.

They carried out the study in 2010 and 2011 by capturing and radio-tagging jays to follow their habits. “Working with Steller’s Jays in campgrounds really keeps you on your toes. Individual jays were quick to learn our trapping techniques and would avoid any traps that they had previously experienced,” says Goldenberg. “In addition, our study sites contained the tallest trees on the planet and more above ground biomass than any other forest, making our radio tracking efforts an extreme challenge.”

“This study by Goldenberg et al. breaks new ground by providing detailed observations of Steller’s Jay behavior in and out of campgrounds,” according to John Marzluff of the University of Washington, an expert on corvid behavior. “Though Steller’s Jays still pose a serious threat to sensitive species that nest well above ground, such as the Marbled Murrelet, their use of anthropogenic foods that occur most frequently at ground level in camps may reduce chance encounters between jays and murrelets within the camp proper.”

Steller’s Jay (Cyanocitta stelleri) space use and behavior in campground and non-campground sites in coastal redwood forests is available at http://www.aoucospubs.org/doi/full/10.1650/CONDOR-15-187.1.

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.

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