Wintering Warblers Choose Agriculture Over Forest

CONDOR-17-180 S Valdez

Yellow Warbler Photo credit: S Valdez

Effective conservation for long-distance migrants requires knowing what’s going on with them year-round—not just when they’re in North America during the breeding season. A new study from The Condor: Ornithological Applications uncovers Yellow Warblers’ surprising habitat preferences in their winter home in Mexico and raises questions about what their use of agricultural habitat could mean for their future.

Large areas of natural forest in the Mexican lowlands have been converted to agriculture, and without enough habitat to go around, researchers speculate that the biggest, oldest birds might claim the choicest spots. To find out, Simon Fraser University’s Simón Valdez-Juárez and his colleagues studied Yellow Warblers wintering in western Mexico, counting how many birds were using each of three different types of habitat—riparian gallery forest, scrub mangrove forest, and agricultural land—as well as capturing birds to determine their age, sex, size, and likely point of origin. To their surprise, they found the highest density of warblers in agricultural habitat rather than either type of natural forest. There was also little evidence that a bird’s body size, sex, or age influenced where it ended up, although females’ habitat use differed depending on where they had spent the breeding season.

Irrigated agriculture may be attractive to Yellow Warblers as an alternative to the naturally dry forest habitat, which becomes even drier as winter goes on, or the limited availability of natural land cover may force most birds to occupy agricultural areas. Either way, this preference for agriculture could lead to problems if stressors such as pesticide use reduce the birds’ survival. “The implications for migratory bird populations depend on whether the condition and survival of birds wintering in agriculture is lower than that of birds wintering in natural habitats or not,” says Valdez-Juárez. “If it is lower, it might cause localized declines, as females from the contiguous U.S. and western Alaska were more likely to use agricultural habitats and lower female survival has been implicated in the population declines of other warblers.”

“With increasing agricultural intensification across the ranges of many long-distance migratory songbirds, it is critical to determine if they are using these new habitats. This new study shows that yellow warblers overwintering in western Mexico not only use agriculturally dominated habitats but are more common there than nearby sites with more natural landcover,” adds University of Manitoba Assistant Professor Kevin Fraser, an avian behavior and conservation expert who was not involved in this research. “They also report that more southern breeding females are more likely to use agricultural habitats than more northern breeding females, suggesting any impacts of overwintering habitat use may differ by breeding latitude. These results are important, as they highlight the need to investigate whether regions undergoing agricultural intensification are providing viable habitat for overwintering migrants, and how their use may carry-over to impact survival or fecundity in subsequent seasons.”

Use of natural and anthropogenic land cover by wintering Yellow Warblers: The influence of sex and breeding origin will be available May 2, 2018, at (issue URL

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.

Where do birds molt their feathers? New research indicates molting grounds are discrete from breeding and wintering sites.

nashville warbler

Nashville Warbler, one of 140 species whose post-breeding movements to molting areas were examined in a new study by The Institute for Bird Populations. Results indicate that Nashville Warblers in western North America disperse upslope from breeding areas to molt, whereas in the East, they move downslope and south. Many such east-west differences within species were found, reflecting differing climate and insect availability across North America. This photo was taken in August, when Nashvilles are on their molting grounds. (Photo by Frank D. Lospalluto via Flickr Creative Commons).

A new study by The Institute for Bird Populations finds that many North American birds do not molt on their breeding territories, as previously thought, but disperse or migrate in late summer to discrete molting locations before continuing their migration to wintering areas. The locations and habitats at these molting sites — and their conservation status — remain largely unknown.

Effective conservation of migratory birds depends on the protection of habitats used during all phases of their life cycle. New research by The Institute for Bird Populations, a non-profit bird conservation organization, has shed light on a key phase of the annual cycle: where birds go to molt after they breed. The study was published May 2, 2018 in the journal The Auk: Ornithological Advances.

Molt is an energetically taxing process during which birds shed worn or broken feathers and grow new ones. During this time, a bird’s flight ability is reduced, so they become harder to find than at other times of year. “Molt has been under-studied by ornithologists. Birds become very retiring at this time, as if on vacation, to recover from the breeding season,” said Peter Pyle, lead author on the study. Despite its importance to a bird’s survival, the subject has received a lot less attention than other factors such as breeding, migration, and wintering habitats. “We still have no clue where most North American landbirds undertake their post-breeding (prebasic) molt, and we need more information on the particular habitats or foods that are most important, and the conservation status of these areas.”


The new study relied on 17 years of records from The Institute for Bird Populations’ Monitoring Avian Productivity and Survivorship (MAPS) Program, a continent-wide bird banding effort assisted by thousands of citizen scientists and professional biologists. Data from more than 760,000 capture records of 140 species of landbirds collected at 936 MAPS bird-banding stations across North America were used to generate the results. The authors leveraged this extensive data set to estimate what proportion of birds known to breed at a station were also captured molting there, and vice versa. Dashed lines indicate delineation of western and eastern North America.

Although some bird species are known to undergo “molt-migrations” — movement to a molting location that is neither where they nested nor where they will winter — to take advantage of wetter conditions or more abundant food in areas such as the Sierra Nevada mountains in California or the late summer monsoonal rain area in the desert Southwest, most species have been assumed to molt on or near their breeding territories. This turns out not to be the case.

The study demonstrated some surprising findings, including widespread evidence of molt-migration for many species previously thought to molt only on their breeding grounds. The researchers analyzed spatial differences between breeding and molting locations, and found evidence that, across North America, birds shifted in nearly every compass direction, and with some going higher in elevation and some lower to find suitable molting areas. “In some cases this may be hundreds of kilometers away, in others it may be down the block,” Pyle noted.

bird molt

Most of the 140 species examined (indicated by 4 letter codes abbreviating their common names) showed movements from breeding to molting areas. Species lower/further left on each graph show a lower probability of being recaptured during molt at a site where it bred. In the graph, species names in orange were known to molt away from breeding grounds, those in blue were thought to molt on breeding territories, and those in black are residents, presumed to molt on or near breeding territories. The new study largely validates these results for species marked in orange and black, but many species previously presumed to molt on breeding grounds (those in blue) appear to undergo molt migrations.

Although western North American species and populations have previously been reported to undergo more molt-migration than eastern species, this study found similar evidence of molt-movement between the two regions, although many species in the west migrate longer distances.

molt migration map

Two examples of molt migration movements by North American birds. Bluer areas represent higher probability of a bird breeding relative to molting at that site and redder areas represent higher probability of molting relative to breeding. As with Nashville Warbler (see above), Orange-crowned Warblers in the West moved significantly further south and higher in elevation, largely into the Sierra Nevada mountains of California, likely to take advantage of cooler and moister habitats at these locations in late summer. Within the MAPS area, Swainson’s Thrushes in the East appeared to shift west and south, largely into the Mississippi Valley, but did not appear to shift elevations.

These findings have significant implications for conserving birds. Although molting areas appear to be crucial for completing a species’ annual life cycle, more study is needed to determine if such areas are receiving the conservation attention or protection they need. The new research also indicates that individuals within a species make various molt-movement choices, even on an annual basis, in response to breeding season success or environmental and food conditions each year. “Rather than characterizing molt as occurring on discrete breeding or wintering grounds, it can perhaps best be thought of as a process occurring along a continuum, with most some species, populations, or individuals showing some level of movement from breeding territories to molt,” said Jim Saracco, another author on the study. “Our findings highlight the need for better understanding of molt-movement patterns and habitat needs during molt to better inform full life-cycle conservation strategies.”


Evidence of widespread movements from breeding to molting grounds by North American landbirds is available at For more information about this study, contact Peter Pyle (

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.

Is the Rufous Hummingbird a Mexican monsoonal molt migrant?


Rufous Hummingbird, Photo credit: T Hopwood

Many species of North American hummingbirds are highly migratory, with some traveling over 2,000 km. These migratory birds risk more feather wear due increased solar exposure on an annual basis compared to non-migratory species. Hummingbirds, like all birds, need their feathers to be in good condition to survive and therefore have to undergo molt.

All North American migratory hummingbirds undergo complete molts on wintering grounds in Mexico before they migrate north. In first-year birds, this complete molt has been referred to as the performative molt and in all subsequent years, it is called the definitive prebasic molt. First-year males were also known to replace some throat feathers during the fall, replaced again during the complete overwinter molt.

However, a recent study by Dittmann and Cardiff suggested that molting in migratory hummingbirds may be more complex than simply the annual molt (2009, Birding 41: 32–35). They examined molt of Ruby-throated Hummingbirds during their fall migration from June to September on the Gulf Coast of Louisiana and found evidence of molting at this time in both adults and young. They called this molt an inserted molt, separate from the overwinter complete molt.

Curious whether inserted molts might occur in other migratory hummingbird species, we examined molt in the Rufous Hummingbird, a Mexican species that migrates to the northern United States and Canada to breed. We examined 346 specimens of Rufous Hummingbirds from three different specimen collections in California. We looked at each specimen for signs of molt in four feather regions: the crown, the back, the throat (including the gorget in adult males), and the underparts. We considered a bird as showing evidence of molt if we found any replaced feathers contrasting with worn feathers, indicating two generations of feathers in a single region, or if we found pin feathers present in any of the regions.

As expected, the majority of the specimens collected in the winter from southern Mexico showed signs of the complete molt that was known to take place on the wintering grounds. However, we also found signs of molting in both young birds and adults during southbound migration in the fall. These inserted molts occurred in all four feather regions but were more limited in extent than those of Ruby-throated Hummingbirds. Many (but not all) molting specimens were collected in the Mexican monsoonal region of northwestern Mexico. The Sierra Madre Occidental, a mountain range that lies within the monsoonal region attracts Rufous Hummingbirds because of the high flower abundance that occurs there, and we infer that the hummingbirds take advantage of this food resource to molt in this area, as do many western North American species of passerines.

Surprisingly, we found that the inserted throat feather molt in Rufous Hummingbirds varied greatly among ages and sexes. Adult males showed no evidence that their iridescent throat feathers were replaced in fall, whereas some adult females and all young birds of both sexes replaced at least some throat feathers during the inserted molt. New iridescent throat feathers might give young birds an advantage when defending territories by showing dominance. While it is still unclear why the highly migratory Ruby-throated and Rufous Hummingbirds undergo inserted fall molts in other body regions, one reason could be that their feathers become more worn compared to non-migratory hummingbirds.

We compared molts of Rufous Hummingbirds to the molts of basal Apodiform taxa, including Vaux’s Swift and Rivoli’s Hummingbird. We propose a terminology that would consider the inserted molts as the definitive prealternate molt in adults and the preformative molt in young birds. Therefore, we propose that Rufous Hummingbirds undergo a prealternate and preformative molt migration to the Mexican monsoonal region.

Our work on Rufous Hummingbirds is part of The Institute for Bird Populations’ Avian Molt Research Program, aimed at understanding molt strategies of all species of birds, standardizing molt and plumage terminologies worldwide, and applying findings to age-related demographic studies, habitat requirements for molt, and, ultimately, the conservation of birds.

~Post written by authors: Desmond Sieburth and Peter Pyle


Evidence for a prealternate molt-migration in the Rufous Hummingbird and its implications for the evolution of molts in Apodiformes is available, at (issue URL

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.

Rethinking the Umbrella Species Concept

CONDOR-17-200 J Carlisle brewer's sparrow eggs

Brewer’s Sparrow’s nest. Photo credit: J Carlisle

According to the “umbrella species” concept, preserving and managing habitat for a single high-profile species also benefits a whole suite of other species that share its habitat—but how well does this really work? Not all species that share the same general habitat necessarily have the same specific needs, and a new study from The Condor: Ornithological Applications finds that habitat management to benefit Greater-Sage Grouse in Wyoming can actually harm some of its songbird neighbors.

Shrub mowing is sometimes done to benefit sage-grouse during their chick-rearing season, when they favor habitat with fewer shrubs and more grass and forbs. The University of Wyoming’s Jason Carlisle (now at Western EcoSystems Technology) and his colleagues collected data on the abundance and nesting success of three songbird species before and after shrub mowing in central Wyoming, as well as at un-mowed sites for comparison. Two of the species, Brewer’s Sparrows and Sage Thrashers, are “sagebrush obligates” that rely heavily on shrub habitat. The researchers found no Brewer’s Sparrow or Sage Thrasher nests in mowed patches, where it may be decades until shrubs have regrown enough to be used for nesting, and the mowing treatment also reduced the overall abundance of Sage Thrashers by around 50%. Vesper Sparrows, on the other hand, are more flexible in their habitat use and were actually more abundant in patches where mowing was most extensive.

“The umbrella species concept is an appealing shortcut,” says Carlisle. “However, when conservation practitioners go beyond protecting the umbrella species’ habitat and start manipulating habitat conditions to meet the needs of the umbrella species, they risk harm to other species that also rely on those areas.” The mowed area in the study was only about five square kilometers, and cost and logistics make it unlikely that this treatment will be implemented on a large scale, so the negative effects of these habitat manipulations are likely more than offset by the positive effects of broader-scale sage-grouse conservation efforts. Still, Carlisle and his colleagues suggest that more monitoring is needed in general of how non-target species fare under umbrella-species management.

“It is clear that the observed effects on ‘background’ species are not consistent across the range of potential beneficiaries, and that the assumed benefits do not accrue to many,” adds Russel Norvell, an Avian Conservation Program Coordinator at the Utah Division of Wildlife Resources, who was not involved with this research. “It is likely these effects will persist, given the relatively slow growth rates in this arid system. While the conservation of Greater Sage-Grouse habitats across its range is likely to encompass much good, local habitat manipulations driven by unexamined assumptions or uninformed by landscape context have much to prove.”

Nontarget effects on songbirds from habitat manipulation for Greater Sage-Grouse: Implications for the umbrella species concept is available at (issue URL

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.

Endangered Petrels and Trawl Fishing Clash in Tasman Sea

CONDOR-17-179 T Poupart

Photo Credit: T Poupart

Today’s shifting environmental conditions are creating an uncertain future for many top predators in marine ecosystems, but to protect the key habitat of a species, you first have to know where that habitat is and what threats might be affecting it. A new study from The Condor: Ornithological Applications looks at where New Zealand’s endangered Westland Petrel forages during its breeding season and shows that its range overlaps more with trawl fishing activity than conservationists realized.

The Museum of New Zealand Te Papa Tongarewa’s Susan Waugh and her colleagues outfitted 73 petrels with GPS loggers over the course of four breeding seasons to track where they went during their foraging trips in the Tasman Sea. The results show that the birds’ core feeding areas were consistent from year to year, located within 250 kilometers of their breeding colonies and focused on highly productive areas where the seafloor is steeply sloped. These sites often overlap with areas of significant trawl fishing activity, and further data is needed to see whether this co-occurrence translates into bird mortality. The species is the tenth most at-risk species from the impacts of New Zealand commercial fishing, but it appears that its actual ranking may be even higher as a result of the finding that the species is exposed to more fisheries activity than was previously understood.

“Our work on Westland petrels started in 2010, with a desire to understand how this species was faring demographically, as well as the key influences on it,” says Waugh. “Our work highlights a key factor in the birds’ ecology that has strong implications for conservation—these birds predictably use the same waters year in and year out, regardless of El Niño cycles, and they are therefore a great candidate for a marine protected area to create protection of their trophic relationships. We feel this Westland Petrel foraging data will provide a high-quality information source to help define key areas for marine conservation that will also provide protection for a whole suite of species.”

“A remarkable and important aspect of this study is that they performed this investigation during a six-year period, encompassing a variety of environmental conditions. Whatever the sex or the breeding stage considered, Westland Petrels consistently foraged in the same core areas from year to year,” adds Christophe Barbraud of the French National Centre for Scientific Research, a seabird conservation expert who was not involved in the study. “Since these areas were also consistently and heavily used by trawl fisheries, these results call for the implementation of marine spatial management tools, such as marine reserves or restrictions and monitoring of interactions between individual Westland Petrels and trawl fisheries, to ensure the conservation of this endangered petrel species.”


Environmental factors and fisheries influence the foraging patterns of a subtropical seabird, the Westland Petrel (Procellaria westlandica), in the Tasman Sea is available at (issue URL

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.

Will increasingly frequent warm-water events in the Gulf of California reduce seabird populations? The case of Heerman’s Gull

Heermann's Gull w chicks_Enriqueta Velarde

Photo Credit: Enriqueta Velarde

[Video link:

Oceanic warm-water events in the Gulf of California have increased in frequency during the last three decades, passing from a historic mean of one or two warm anomalies per decade to five events in the 2007–2016 period. This can lead to massive failures in seabird nesting, as anomalously warm waters accumulate in the ocean’s surface, preventing the upwelling of colder, nutrient-rich waters from the ocean bottom, which in turn deprives seabirds of their food.

A recent study by Enriqueta Velarde from the University of Veracruz and Exequiel Ezcurra from the University of California Riverside, published in The Condor: Ornithological Applications, analyzed four decades of nesting dynamics of the threatened Heermann’s Gull (Larus heermanni), and modeled population growth under normal and high sea-surface temperature conditions.

Heermann’s Gulls exhibit the main characteristics of seabirds.  They can live for decades, but take many years to reach breeding age, and produce few young.  These traits allow their populations to resist oceanographic anomalies of up to one event every five years, but population size may decline rapidly if the frequency of warm-phase anomalies is over two per decade.  Under normal conditions, predicted population growth is around 4%, with production of fledglings contributing to the population increase. Under anomalous warm-water conditions, population growth drops to -15% and adult survival becomes the key condition for the survival of the species. Simulations project a rapid population decline if warm anomalies maintain the high frequencies they have shown during the last decade.

These results underscore the need to understand the dynamics of warming ocean waters under current large-scale environmental change, as well as the importance of following the demographic dynamics of seabirds as indicators of oceanic conditions. Monitoring seabird populations may provide us with warning signals about the health and the future of our marine ecosystems.


Are seabirds’ life history traits maladaptive under present oceanographic variability? The case of Heermann’s Gull (Larus heermanni) 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. In 2016, The Condor had the number one impact factor among 24 ornithology journals.

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

AUK-17-240 HOWA

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

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.