Timing of Migration Is Changing for Songbirds on the Pacific Coast

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Volunteers collect data on captured songbirds. Photo credit: San Francisco Bay Bird Observatory

Changes in the timing of birds’ migration can have serious negative effects if, for example, they throw the birds out of sync with the food resources they depend on. A new study from The Condor: Ornithological Applications uses a long-term dataset from the Pacific coast and shows that the timing of bird migration in the region has shifted by more than two days in both spring and fall over the past two decades.

Gina Barton and Brett Sandercock of Kansas State University used 22 years of data from the San Francisco Bay Bird Observatory in northern California, where researchers captured and marked migrants as they passed through each year. Examining how migration timing was related to three indices of climate variation, including El Niño, they focused on five species—the Pacific-slope Flycatcher, Orange-crowned Warbler, and Wilson’s Warbler, which are short-distance migrants, and the Swainson’s Thrush and Yellow Warbler, which are long-distance migrants. They found that, over time, birds had been migrating earlier in spring and later in fall. About one third of this change in timing was explained by variation in climate indices, suggesting that some western songbirds can respond flexibly to changing environmental conditions. However, the two long-distance migrants had smaller changes in timing that were not as closely linked to climate indices.

The study of the timing of cyclic natural phenomena such as migration is known as phenology. “Long-term studies are valuable for understanding changes in the seasonal phenology of migratory birds and other organisms,” says Sandercock. “Our results join growing evidence that long-distance migrants may be less able to cope with rapid environmental change. However, the potential impacts of changing phenology on songbird population dynamics are difficult to predict, in part because our understanding of the migratory behavior of western songbirds remains incomplete.”

Sandercock adds that long-term projects like this one wouldn’t be possible without the commitment of many funders, technicians, and volunteers. “Our project was made possible by the dedicated efforts of many volunteers who assisted the staff of the San Francisco Bay Bird Observatory with systematic mist netting at the Coyote Creek Riparian Station. The long-term continuity could not have been maintained without stable financial support from funding by the Santa Clara Valley Water District, and the Pacific Gas and Electric Company.”

Long-term changes in the seasonal timing of songbird migration on the Pacific Flyway is available at http://www.bioone.org/doi/full/10.1650/CONDOR-17-88.1.

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, 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.

Gina is now a Director for Cheesemans’ Ecology Safaris, an ecotourism company in Los Gatos, California. Brett is now a Senior Research Scientist with the Norwegian Institute of Nature Research in Trondheim, Norway.

Invasive Frogs Give Invasive Birds a Boost in Hawaii

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Invasive coqui frogs are affecting nonnative bird populations in Hawaii. Photo credit: R. Choi

Puerto Rican coqui frogs were accidentally introduced to Hawaii in the 1980s, and today there are as many as 91,000 frogs per hectare in some locations. What does that mean for native wildlife? Concerns that ravenous coquis could reduce the food available for the islands’ native insect-eating birds, many of which are already declining, spurred researchers to examine the relationship between frog and bird populations—but their results, published in The Condor: Ornithological Applications, weren’t what they expected.

Working at fifteen sites on the island of Hawaii in 2014, Utah State University’s Robyn Smith, Karen Beard, and David Koons determined whether coquis were present at each site by listening for their calls at night and then estimated coqui density in invaded plots through a visual search.  Bird surveys at all sites recorded 20 species, of which only 5 were native. To the researchers’ surprise, native birds showed no response to coqui density or presence, but three of the nonnative species were more abundant in plots with coqui. The overall abundance of nonnative birds ranged from an average of 57 birds in coqui-free plots up to 97 where coqui density was highest.

Coquis forage mostly in leaf litter, while Hawaii’s native insect-eating birds forage mostly in canopy and understory, so perhaps they aren’t in direct competition for insects. Nonnative birds may be getting a boost directly by eating adult or juvenile coquis, or indirectly through coqui’s effects on the ecosystem—coquis may alter nutrient cycling and even increase fly populations thanks to the extra biomass their excrement and carcasses create. “I was very surprised with the results for birds. It had been hypothesized before our study that coquis would compete with birds, particularly natives, because we know that coquis reduce insects where they invade,” says Beard. “In retrospect, I guess it’s not too surprising that predation is a more important interaction than competition—that is a common finding in invaded systems—but it was definitely not what we went in to test. The response that we see for Common Mynas and Red-billed Leiothrix is pretty convincing. We’re pretty sure that some of this increase is due to these species eating live or dead coquis, and we’re suggesting that some non-native birds are likely consuming coquis and this novel resource appears to be increasing their populations.”

“The findings presented in the study by Smith, Beard, and Koons, where abundances of some species of non-native birds in Hawaii are correlated with invasive coqui frogs, are very intriguing,” according to the USDA’s Aaron Shiels, an expert on invasive species in island ecosystems. “Furthermore, their interesting findings beg for future experimental manipulations that would uncover the causal factors that explain the patterns that they have observed.”

Invasive coqui frogs are associated with greater abundances of nonnative birds in Hawaii, USA is available at http://www.bioone.org/doi/full/10.1650/CONDOR-17-109.1.

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, 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.

Translocated Hawks Thrive in Hispaniola

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A Ridgway’s Hawk on the island of Hispaniola. Photo credit: R. Thorstrom

Species translocation—capturing animals in one place and releasing them in another—is a widely used conservation method for establishing or reestablishing populations of threatened species. However, translocation projects often fail when the transplanted animals fail to thrive in their new home. A new study from The Condor: Ornithological Applications demonstrates how close monitoring of the animals being released into a new area is helping wildlife managers gauge the success of their effort to save the Ridgway’s Hawk of Hispaniola.

Ridgway’s Hawk is a critically endangered raptor endemic to the Caribbean island of Hispaniola. Since 2009, the Peregrine Fund has translocated 104 nestlings from the species’ stronghold in a national park to a protected resort area called Punta Cana 130 kilometers away. They monitored the birds after their release, tracking their survival and breeding success, as well as collecting the same data on 36–110 breeding pairs per year in the original national park population. Survival rates were high in both locations, and more young hawks began breeding on the resort property, probably because more territories were available due to the birds’ low numbers.

Lead author Chris McClure of the Peregrine Fund and his colleagues believe that the “soft release” method they use with the translocated birds, involving an adjustment period where young birds are allowed to come and go freely but are provided with food, has contributed to the success of the project. Post-release monitoring has been a valuable tool for gauging their success and planning future conservation efforts, and they hope that more wildlife managers will consider incorporating this into their plans when seeking funding for translocation projects. “Our results show that the new population of Ridgway’s Hawks in Punta Cana is taking hold, but we only know that because of the hard work of our staff and volunteers in tracking these birds,” says McClure. “I think our study is a great example of monitoring to ensure that conservation goals are actually achieved.”

“The paper is a good example of the important role of post-release monitoring in translocation projects,” adds Massey University’s Doug Armstrong, an expert on reintroduction programs who was not involved in the study. “It is easy for people to invest a lot of effort into well-meaning but ineffectual translocations projects if they aren’t monitored. This paper illustrates how state-of-the-art methods can be applied to provide clear and useful information of post-release survival and recruitment.”

Successful enhancement of Ridgway’s Hawk populations through recruitment of translocated birds is available at http://www.bioone.org/doi/full/10.1650/CONDOR-17-77.1.

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, 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.

AUTHOR BLOG: Migrating Birds That Eat Northern Spicebush Berries Are Fat and Healthy Birds

Yushi Oguchi

Linked paper: Fruits and migrant health: Consequences of stopping over in exotic- vs. native-dominated shrublands on immune and antioxidant status of Swainson’s Thrushes and Gray Catbirds by Y. Oguchi, R.J. Smith, and J.C. Owen, The Condor: Ornithological Applications 119:4, November 2017.

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Collecting a small blood sample from a Gray Catbird to assess its health during stopover. Photo credit: Zak Pohlen

“We should not only conserve avian populations; we should conserve ‘healthy’ avian populations.”  – Dr. Jen C. Owen (my M.S. adviser, Michigan State University)

We investigated whether the health status of fall frugivorous Swainson’s Thrushes and the Gray Catbirds differed depending on their use of shrublands dominated by exotic versus native plants. In the process, we came to appreciate the value of one native fruit, northern spicebush (Lindera benzoin), a fruit we couldn’t even identify at first. Our story represents a unique insight that ecophysiology can bring to conservation science and habitat management.

Our prediction was that habitat may influence bird health through their frugivorous diet and that landbird migrants may be able to enhance their immune system by resting and refueling during stopover. In fall, many shrubs are loaded with fruit, and this fruit contains essential nutrients, including antioxidants, which help a bird neutralize reactive oxygen species produced during exercise (flight).

A question of conservation interest is whether birds that forage on exotic fruits and in exotic-dominated shrubland experience a deleterious effect on their health. Many exotic shrubs such as autumn olive and honeysuckle (introduced to the Midwestern U.S.A.) produce fruits that are generally lower in energy than native fruits such as northern spicebush, but some exotic fruits may have high antioxidants, including immunostimulatory carotenoids. We wouldn’t know unless we tested it!

A state-managed land in Michigan had the perfect habitat matrix—exotic-dominated shrubland and native-dominated shrubland occurring side by side, bisected by woodland. The exotic habitat was largely autumn olive, honeysuckle, and multiflora rose, and the native habitat was largely dogwood species, common winterberry, and northern spicebush. During fall migration in 2012 and 2013, we captured more than 800 individual birds, from which we collected blood samples for comprehensive health assessments.

Where Swainson’s Thrushes foraged during stopover had no impact on their health. On the other hand, Gray Catbirds using exotic shrubland experienced poor health; they lost mass and had reduced immune function and lower antioxidant capacity compared to catbirds using native shrubland. We also saw annual variation, with catbirds exhibiting more deleterious effects in 2013 compared to 2012. We further found that the pattern of habitat effect on catbird (but not thrush) health could be at least partly predicted based on the fruits they ate (math on fruit nutrition and bird fecal data).

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Fecal sample showing the remnants of northern spicebush. Photo credit: Jen Owen

It is through this nutritional analysis that we saw the value of northern spicebush. Spicebush fruit has very high energy and antioxidant capacity relative to other species. It was a preferred fruit by both bird species and likely contributed to the bulk of the nutrients they acquired in native shrubland. In exotic shrubland, thrushes consumed more exotic fruits like common buckthorn than catbirds did—so they got antioxidants from exotic fruits. Catbirds, apparently more “reluctant” than thrushes to eat exotic fruits, instead consumed other native fruits that were low in antioxidants. Not eating enough exotic fruits in exotic shrubland may be why catbirds experienced a dip in their health there.

To summarize, migrant health may be influenced by human alteration of habitat, and the effects may depends on the diet of the birds. And again, northern spicebush is a great fruit for birds if you are in eastern North America! In a future paper (accepted by The Condor), we will cover more on shrubland habitat use by the birds at our site.

Dr. Jen Owen continues to direct the Burke Lake Banding Station (BULA) where this research was conducted. She and her undergraduate researchers are doing more intensive monitoring of the fruit abundance and phenology at the site, particularly on the interannual variation in spicebush fruiting in relation to climate. BULA is now in its 7th year of operation, and it continues to provide research opportunities for many students as well as inspiring bird enthusiasts of all ages. Visitors are always welcome while they are open during fall and spring migration. Check them out on their website (burkelakebanding.com) or like them on Facebook (facebook.com/BULAbandingstation).

I’ve done a return migration to the University of Wisconsin–Madison for my Ph.D. this fall. Following my renewed interest in nutrient acquisition so key to health, I now study molecular mechanisms of digestive enzyme modulation in the avian gut.

AUTHOR BLOG: Citizen-Science Data and Capture-Mark-Recapture Models to Estimate Numbers of Rare Species

Andrew Dennhardt

Linked 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 by A.J. Dennhardt, A.E. Duerr, D. Brandes, and T.E. Katzner, The Condor: Ornithological Applications 119:4, November 2017.

Have you ever looked to the autumn sky above and wondered how many of your favorite feathered friends are out there, as they migrate southward each year? If so, then you are not alone. A common goal in ecology has been to estimate the abundance of wild populations. From basic counts to educated guesses, historically, people have tried it all. During and even before the work of Sir Ronald Fischer, the father of modern statistics, applied mathematics aided in population estimation—for humans and wild animals alike.

F.C. Lincoln (1930) is credited with one of the first modern attempts to approximate population abundance of wild animals based on a sample of marked individuals (Bailey 1952, Le Cren 1965). The premise of his approach, co-credited to C. G. Johannes Petersen (1894) and commonly called capture-mark-recapture (CMR), was simple: trap a random set of animals of the same species in an area, mark those you caught with a unique tag, release them back into the wild for a period of time, return to the same area once more and randomly trap individuals of that same species again, and record which previously tagged individuals, if any, came back to the area. In brief, when you know the proportion of individuals you recaptured the second time, you can assume that you trapped the same proportion of the total population when you trapped the first time. The math is straightforward, too: if you have an initial sample of animals captured, marked, and released back into the wild, M, and multiply that by the total number of animals sampled a second time, n, then you can divide that quantity by the number of marked animals in that second sample, m, and approximate true population size, N.

In our paper, we used modern statistical advancements in CMR to estimate abundance of Golden Eagles (Aquila chrysaetos), a species of conservation concern in the United States, using an unusual approach. In effect, we did not physically capture, mark, or recapture individual animals in the wild; rather, we did so virtually in a computing environment with the help of observational data collected by some savvy citizen-scientists.

Golden Eagles in eastern North America face lethal and sub-lethal threats, many of which are human caused. However, these eagles are rarely seen, broad-ranging, and difficult to capture in the wild because they often avoid areas of human activity. Despite this fact, citizen-scientists observe Golden Eagles frequently and regularly during their annual spring and autumn migrations in Pennsylvania, U.S.A. Moreover, Golden Eagle movements are highly stereotyped, especially in autumn, bringing them within a few hundred meters of hawk counters on ridgetops in the Appalachian Mountains. Better still, because of past telemetry studies, we know how fast Golden Eagles fly when they migrate. Hawk counters collect their data on Golden Eagles and other migrant species and archive their observations in an online database, hawkcount.org. With assistance from managers of the archive, our research team gained permission and access to download historic count data on migrating Golden Eagles observed using the Kittatinny Ridge during their peak migration period, November, over a 10 year period from 2002 to 2011.

Now, here’s where things get really exciting (well, they do for me, at least!). Because (a) historic hawk-count data included information on the timing of Golden Eagles migrating southward past monitoring sites along the Kittatinny, (b) telemetry data gave us an idea of how fast they fly while migrating in autumn, and (c) we could measure the distance between each of the monitoring sites they passed, we could then estimate how long it would take eagles to travel between pairs of sites. Using a customized computer program, we matched records of Golden Eagles together, from site to site, such that we could know when observers at one site counted a bird that had probably been counted at a previous site (i.e. eagles became “captured,” “marked,” and “recaptured”).

Using these virtual data on matched eagle observations, we developed what are commonly called encounter histories. In brief, encounter histories comprise a sequence of 1s and 0s unique to each eagle. In a given sequence, a 1 represents either the event of first “capture and marking” or a subsequent “recapture” after being previously marked and released back into the wild, and a 0 represents an event when an eagle was not “captured or recaptured” at a previous or subsequent migration monitoring site. For modern CMR approaches, extensions of Lincoln (1930) and Petersen’s (1894) early work, encounter histories are the necessary input data for a statistical model. Because we were interested in estimating eagle abundance in a particular area (i.e. the Kittatinny Ridge) over time, we chose the Population Analysis (POPAN) Jolly-Seber model—the right tool for the job.

Most exciting for us, our methodology worked. We produced population abundance estimates for Golden Eagles migrating along the Kittatinny Ridge each autumn. To boot, two sets of our estimates followed the necessary rules (e.g., statistical model goodness-of-fit tests) for us to consider them reliable by modern CMR standards. Together, our best models estimated that approximately 1,350 Golden Eagles migrated along the Kittatinny Ridge each November, 2002–2011.

In the end, we feel that we have produced a useful framework for evaluating other migratory bird populations based on similar data and known movement behaviors. Our proposed methodology not only builds upon the legacy of modern CMR work, but is also far more cost-effective than physical CMR and other costly survey techniques. In the case of fixed-wing aircraft surveys for Golden Eagles in the western United States alone, our work costs far less than the annual ~$320,000 necessary to implement such surveys. Most importantly, such an achievement was only made possible by the tireless work of numerous dedicated citizen-scientists, whose standardized and centrally managed data can provide wildlife researchers and managers with quality information useful in conservation decisions. To our friends at the Hawk Migration Association of North America, for all of their data collection and management year-in and year-out, our team is abundantly grateful—pun intended!

References

Bailey, N. T. J. 1952. Improvements in the interpretation of recapture data. Journal of Animal Ecology 21:120–127.

Le Cren, E. D. 1965. A note on the history of mark-recapture population estimates. Journal of Animal Ecology 34:453–454.

Lincoln, F. C. 1930. Calculating waterfowl abundance on the basis of banding returns. Circulation of the U.S. Department of Agriculture No. 118.

Petersen, C. G. J. 1894. On the biology of our flat-fishes and on the decrease of our flat-fish fisheries: with some observations showing to remedy the latter and promote the flat-fish fisheries in our seas east of the Skaw. Report of the Danish Biological Station No. IV (1893–94).

How Many Golden Eagles Are There?

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Photo credit: D. Brandes

For conservation efforts to be effective, wildlife managers need to know how many individuals of a species are out there. When species are spread out over large areas and occur at low densities, as is the case with the Golden Eagle, figuring this out can be tricky. However, a new study from The Condor: Ornithological Applications applies an old technique called “mark-recapture” in a novel way, eliminating the need to actually capture and mark eagles but instead, using math that allows scientists to turn individual observations into population estimates.

West Virginia University’s Andrew Dennhardt, Adam Duerr, and Todd Katzner and Lafayette College’s David Brandes used observations made by volunteer “citizen-scientists” of Golden Eagles migrating along a single, long mountain ridgeline in Pennsylvania to estimate the total number of eagles passing through the area each year. To do this, they developed a new way to apply a classic ecology tool called mark-recapture analysis—capturing and marking a portion of a population, and then counting the number of marked individuals in another group captured later. Helped by the fact that observers were often able to categorize individual eagles as either immature or adult birds, the scientists were able to identify instances of individual eagles being sighted at more than one location as they made their way south along the ridge over the course of a day, treating these subsequent sightings as “recaptures.”

Volunteers reported more than 3,000 sightings of Golden Eagles at five count sites along the ridge from 2002 to 2011. The analysis used in the study, which lets researchers estimate how many birds were missed as well as how many were seen more than once, suggests that these sightings represented between 2,592 and 2,775 individual eagles over the ten year period, with approximately 1,300 passing through the area on average in a given year. Past studies indicate that the total population of eagles breeding in Quebec and migrating through Pennsylvania is around 5,000, making this about a quarter of the larger population. Because the eagles are difficult to count on their breeding grounds, however, better methods for tracking their numbers during migration represent a significant advance.

“Conservation of Golden Eagles in eastern North America is a really important goal for lots of reasons—it is a small, historically declining population, at risk from anthropogenic threats and habitat loss. A central part of that conservation goal is figuring out how many of the darn things there are. Andrew’s work is the first empirical estimate of golden eagle population size,” says Katzner, now a Research Wildlife Biologist at the US Geological Survey. “Nothing quite like this has ever been done. We’ve taken a standard tool, mark–recapture, and turned it on its head to give us a new way to estimate population size.”

“For me, this was a dream come true, because I got to work on a project relevant to the conservation of the species that originally inspired me to enter the field of wildlife ecology and management,” says Dennhardt, now at Michigan State University. “Partnerships between researchers and citizen-scientists can help improve wildlife management decisions to address threats to migratory Golden Eagles and other species. I hope this work inspires future researchers to evaluate the populations of other migratory species, and that it encourages the greater scientific community to consider new and existing citizen-science programs and think about how such programs’ data might be used in their own research toward improving resource management and decision making.”

Applying citizen-science data and mark–recapture models to estimate numbers of migrant Golden Eagles in an Important Bird Area in eastern North America is available at http://www.bioone.org/doi/full/10.1650/CONDOR-16-166.1.

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, 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.

AUTHOR BLOG: How Canada Warblers Keep Up with the Joneses

Anjolene Hunt

Linked paper: Forestry and conspecifics influence Canada Warbler (Cardellina canadensis) habitat use and reproductive activity in boreal Alberta, Canada by A.R. Hunt, E.M. Bayne, and S. Haché, The Condor: Ornithological Applications 119:4, November 2017.

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Photo credit: Torin Heavyside

Could a behavioral phenomenon, like the tendency of birds to live near neighbors, change the way we think about the effects of habitat disturbance? Results from new paper in The Condor: Ornithological Applications suggest that for Canada Warblers, this may be the case.

These iconic, brightly colored boreal songbirds are declining in number. Habitat loss and degradation are likely the main culprits. While it is true that forestry changes the landscape across much of the Canada Warbler breeding range, it remains contentious whether this results in habitat loss or whether they will use and thrive in regenerating postharvest areas. We suspected this discrepancy in our understanding of forestry effects could be explained by social factors influencing how Canada Warblers choose where to live.

Just as you might choose a neighborhood based on criteria like access to green space, birds also assess their physical environment when choosing where to live. These species-habitat relationships help biologists understand which areas to protect.

But the physical environment is far from the only cue birds use to choose a place to live, nor is it the only cue we use to understand why they live where they do. Just as you might rely on friends’ opinions of a neighborhood to snatch up a good place before it’s off the market, birds may choose to live near members of their own species. If another bird picks an area, chances are it has high quality food, nesting areas, and mates. However, if everybody makes the same decision, problems can arise as they pack into the same location. When habitat is disturbed, choosing to live near neighbors can result in overcrowding in the remaining undisturbed areas and potentially force some birds into the outskirts of high-quality neighborhoods.

After returning from a long spring migration, Canada Warblers have limited time to find a breeding site (they have the shortest breeding season of wood warblers in Alberta!), and strangely, researchers always seem to find Canada Warblers clustered together in some areas while absent from others. Based on this information, we suspected that where Canada Warblers choose to breed might depend on where their neighbors chose to settle. If this is the case, it could explain why they will live in postharvest areas, and in turn, how we perceive the effects of forestry.

Taking quads down muddy trails and bushwhacking through dense shrubbery to our destination, we surveyed for Canada Warblers in boreal Alberta, Canada. We used a recorded song to mimic an intruding male and lure territory owners into our nets. Once captured, we attached color bands to their legs to distinguish between individuals and followed them throughout the breeding season. We documented their space use in and around postharvest areas, how close they were to neighbors, and whether they found a female mate and raised young. See what a day in the life of Canada Warbler biologist looks like here.

Our results showed that numbers of male Canada Warblers were much lower in postharvest areas compared to unharvested forest. The few males that did live partially in postharvest were typically at the edge of nearby unharvested forest. Males were also more likely to live nearer to neighbors rather than spacing out. Hence, Canada Warblers in the boreal forest may not prefer to live in postharvest areas, but may live there as a side effect of trying to be near neighbors in unharvested forest.

But are Canada Warblers reaping the benefits of living near neighbors, or are they feeling the pressure to keep up with the Joneses? We found males living in areas with more neighbors were more likely to be “single” (without a female mate) than males living in areas with fewer neighbors. Overcrowding may lead to increased male-male competition for limited females. These rivalries also mean that males have to spend more time and energy defending their territory, leaving less time and energy to court females.

Our results suggest that protecting large stretches of unharvested forest near sites occupied by Canada Warblers will be important to provide enough habitat and prevent crowding effects. It also goes to show that appearances can be deceiving when it comes to the use of disturbed habitat and that the influence of social behavior should not be underestimated.

Follow Anjolene on Twitter: twitter.com/AnjoleneHunt