Feather replacement or parental care? Migratory birds desert their offspring to molt

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This is a male Hooded Warbler delivering food to his three 6-day-old nestlings. When replacing their tail feathers during molt, many males desert their nestlings and fledglings, leaving the female responsible for all remaining parental care. Photo Credit: Ron Mumme

A new study shows that when feather replacement and parental care overlap in time, migratory songbirds make a striking trade-off; they desert their offspring, leaving their mates to provide all remaining parental care.

This radical solution to conflict between parental care and the annual molt provides “a nice illustration of the complex lives that migratory songbirds lead,” says Ron Mumme, the study’s author and Professor of Biology at Allegheny College in northwestern Pennsylvania. “They have to negotiate conflicting demands and make difficult trade-offs and compromises, just like we do.”

Migratory songbirds that breed in North America confront serious issues with time management. After spending a relatively leisurely winter and early spring luxuriating in warm tropical climates, they migrate north for a brief but highly eventful summer in North America, during which they must complete three energetically demanding and time-consuming tasks: (1) they must build nests, lay eggs, and provide for their offspring until the young reach independence, (2) they must completely replace all the feathers in their plumage as part of the annual molt, and (3) they must prepare for the fall southward migration by eating prodigiously and storing the body fat that will fuel their long-distance flights.

Completing all three tasks during a short summer is challenging; when the tasks overlap in time, migratory songbirds may be forced to make compromises and trade-offs.

The new study, published in The Auk: Ornithological Advances, a journal of the American Ornithological Society, examines the trade-offs that birds make when parental care overlaps with the annual molt. It focuses on the Hooded Warbler, a songbird that spends a brief summer nesting in the forests of eastern North America before migrating south to winter along the Caribbean coast from southern Mexico to Panama.

Hooded Warblers have an unusual combination of traits that makes conflicts between parental care and molt likely. First, their prolonged nesting season extends into August, a time typically reserved for feather replacement. Second, their diet consists largely of flying insects that are captured in an unusual way; they use their white tail spots and tail-flicking behavior to flush prey hidden in vegetation. Third, the birds employ the odd strategy of molting and replacing all their tail feathers simultaneously at a time when they are also replacing the primary flight feathers on each wing.

Hooded Warblers simultaneously engaged in both parental care and tail molt therefore find themselves in a difficult position; they have to find enough food to feed themselves and their offspring at a time when their own nutritional requirements are high and their ability to fly and forage is significantly compromised.

So what happens when a molting Hooded Warbler loses all its tail feathers while it is still feeding nestlings or fledglings? About 70% of the time it deserts, leaving its mate responsible for all remaining parental care.

“For that one week when they are without a tail, Hooded Warblers are the goofiest looking birds you can imagine,” says Mumme. “But the big problem is flying and foraging are just not easy during that time. Molting birds typically hunker down in thickets and keep a low profile, and that usually isn’t compatible with parental care.”

Males arrive on the breeding grounds earlier than females and nearly always begin molt earlier than their mates. As a result, most cases of desertion involve molting males deserting nestlings and recently fledged juveniles, but rare cases of single-parent desertion by molting females also occur.

One intriguing aspect of the study is that it identifies two factors that explain why some molting parents do not desert; age of the dependent offspring, and age of the parent. If the offspring are older and close to the age of independence, which occurs about 35 days after hatching, a tailless molting parent may continue providing care. If the offspring are younger and more demanding, however, a molting parent nearly always deserts. Similarly, old veteran males are less likely to desert than are younger males with fewer years of previous breeding experience. These observations indicate that Hooded Warblers are acutely sensitive to the costs of providing parental care during molt. “If the costs of continuing care are relatively modest — for example, if it’s an experienced male with offspring close to independence,” says Mumme, “a molting parent may elect not to desert.”

A second intriguing finding is that females deserted by males and forced to provide all remaining parental themselves appear to have about the same chance of surviving to the following year as non-deserted females. In fact, in cases where both a deserting male and his mate return the following year, the female will more likely than not re-mate with the same male that ditched her the previous year. “Male desertion doesn’t seem to be a big problem for females,” says Mumme. “It may just be an unavoidable consequence of the prolonged nesting season and early male molt, and females are able to deal with it just fine.”

Mumme’s research on molt and desertion is continuing. He is currently investigating the effects of single-parent desertion on growth and survival of offspring, and how females adjust their molt and migration schedules to deal with male desertion.

The trade-off between molt and parental care in Hooded Warblers: Simultaneous rectrix molt and uniparental desertion of late-season young  is available at http://www.bioone.org/doi/10.1642/AUK-17-240.1

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

After a Volcano Erupts, Bird Colonies Recover

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Photo Credit: G Drew

Where do seabirds go when their nesting colony is buried by a volcano? In 2008, the eruption of the Kasatochi volcano in the Aleutian archipelago provided a rare opportunity to track how the island’s Crested and Least auklet populations responded when their nesting colony was abruptly destroyed. As a new study from The Auk: Ornithological Advances shows, the birds were surprisingly adaptable, establishing a new colony on freshly created habitat nearby in only four years.

Crested and Least auklets rely on habitat that must be maintained by continual disturbance—they nest in crevices in talus slopes formed by rock falls, which eventually become unusable when they’re filled in with soil and debris. The volcano’s 2008 eruption buried all of the suitable nesting habitat for the 100,000 Crested Auklets and 150,000 Least Auklets that had been nesting on Kasatochi.

The U.S. Geological Survey’s Gary Drew and his colleagues surveyed the island and its bird community by boat twice prior to the eruption and five times in the first eight years afterward, as well as deploying time lapse cameras at two locations on Kasatochi to monitor the auklets’ activity. Eleven months after the eruption, birds were sitting on the thick layer of ash covering their former nesting site, with no sign of any successful nests; the number of auklets turning up at the site declined each year. However, in 2012, Drew and his colleagues found a new auklet colony at a recently formed talus field north of the original colony site. Surveys of birds at sea indicated that some may also have moved to another nearby island.

“We were surprised at the speed at which the auklets were able to shift and make use of the new colony site. These birds typically nest in very large colonies, so there may be a tipping point where newly available habitat shifts rapidly from being a site of no or low density nesting to a site of high density nesting,” says Drew. “Fortunately, both Crested and Least auklets are currently doing well and we do not have any immediate concerns regarding the status of these two species. That said, these findings provide us a potential template for predicting the trajectory of auklet populations in response to habitat loss and interpreting auklet behaviors following future disturbance events.”

“The volcanic eruption at Kasatochi in 2008 provided the rare opportunity to document the response of a colonial seabird to the sudden and complete destruction of their nesting habitat. This study capitalized on that opportunity and gives us a glimpse into the ability of these species to disperse to nearby colonies and colonize new habitat,” adds the University of New Brunswick’s Heather Major, an expert on Aleutian seabirds who was not involved in the study. “This study is therefore important to our understanding of dispersal and habitat selection, and more generally, the ability of these two species to respond to large disturbances at their nesting colonies.”

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Biological responses of Crested and Least auklets to volcanic destruction of nesting habitat in the Aleutian Islands, Alaska is available at http://www.bioone.org/doi/full/10.1642/AUK-17-180.1 (issue URL http://www.bioone.org/toc/tauk/135/3).

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.

How to improve habitat conservation for migrating cranes

Every year, North America’s critically endangered Whooping Cranes travel back and forth along a 4,000-kilometer corridor linking their nesting grounds in Canada and their winter home in Texas. Habitat in their path through the northern Great Plains is being lost at an alarming rate to agriculture and other development, but the birds’ widely dispersed movements make identifying key spots for protection a challenge. Now, researchers behind a new study from The Condor: Ornithological Applications have created a model of Whooping Crane habitat use with the potential to greatly improve the targeting of conservation efforts during their migration.

Researcher Neal Niemuth and his colleagues used a database of Whooping Crane sightings in the region since 1990 to examine cranes’ habitat use in North and South Dakota. Analyzing the spatial patterns of the sightings, they found that Whooping Cranes prefer habitat that includes a mix of croplands and wetlands and are more attracted by a single large wetland basin than multiple smaller basins. Their results also show the effects of different conservation strategies across the region. East of the Missouri River, where efforts have been specifically targeted toward waterfowl conservation, lands under conservation management were more likely than other locations to be used by Whooping Cranes. West of the river, however, this was not the case.

Niemuth and his colleagues hope that their model can help to guide the siting of new wind, oil, and electrical transmission infrastructure to minimize potential conflicts with Whooping Cranes, as well as identifying opportunities for wetland restoration. According to the article, approximately $50 million per year is spent for habitat protection in the region, with much funding coming from sales of Duck Stamps. Because of their endangered status, Whooping Cranes have always been a priority in the area, but the quality and resolution of existing tools for targeting conservation and avoiding conflicts were low. The model presented in this publication provides biological linkages and increased spatial resolution that will increase effectiveness of Whooping Crane conservation efforts.

“Research on Whooping Crane habitat use throughout the migration corridor is crucial in helping us ensure that we are restoring and protecting habitat for a growing population of Whooping Cranes in the right places,” states Wade Harrell, the U.S. and Wildlife Service’s Whooping Crane Recovery Coordinator. “It is positive to see that the prairie pothole habitat in the Dakotas that the U.S. Fish & Wildlife Service is actively protecting for breeding waterfowl is also benefiting endangered species like the Whooping Crane.”

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Opportunistically collected data reveal habitat selection by migrating Whooping Cranes in the U.S. Northern Plains is available at http://www.bioone.org/doi/full/10.1650/CONDOR-17-80.1 (issue URL http://www.bioone.org/toc/cond/120/2).

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. In 2016, The Condor had the number one impact factor among 24 ornithology journals.

About the U.S. Fish and Wildlife Service: The mission of the U.S. Fish and Wildlife Service is working with others to conserve, protect, and enhance fish, wildlife, plants, and their habitats for the continuing benefit of the American people. The findings and conclusions in the forthcoming article are those of the authors and the U.S. Geological Survey and do not necessarily represent the views of the U.S. Fish and Wildlife Service.

Nominees for BioOne Ambassador Award

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

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

The Condor has nominated Andrew Dennhardt, lead author of the paper Applying citizen-science data and mark–recapture models to estimate numbers of migrant Golden Eagles in an Important Bird Area in eastern North America (press release).

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

Terns Face Challenges When They Fly South for Winter

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

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

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

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

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

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

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

Long-Term Study Reveals Fluctuations in Birds’ Nesting Success

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

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

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

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

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

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

About the journal: The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology that began in 1884 as the official publication of the American Ornithologists’ Union, which merged with the Cooper Ornithological Society in 2016 to become the American Ornithological Society. In 2009, The Auk was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

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

Bri Benvenuti and Adrienne Kovach

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

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

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

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

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

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

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

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