Studying “song neighborhoods” within sabrewing leks

Juan Francisco Ornelas

Linked paper: Male relatedness, lekking behavior patterns, and the potential for kin selection in a Neotropical hummingbird by C. González and J.F. Ornelas, The Auk: Ornithological Advances.

A Wedge-tailed Sabrewing. Photo by Clementina González.

About 25 years ago, I started a research position at the Instituto de Ecología AC in Veracruz, Mexico. During my first explorations of the cloud forest there, I heard the marvelous sound that eventually became a key part of my work and that of my assistant Clementina González. After spending my graduate school years at the University of Arizona and learning the sounds of the Sonoran Desert, I decided to tape record the unfamiliar sound, a decision that resulted in a great journey.

When I was field hunting for the sound, holding my old tape recorder, I soon heard swooshing noises and the mystery sound coming from somewhere in the bushes and thickets of the understory, but at first I could not see what was making it. Suddenly, I spotted it: a Wedge-tailed Sabrewing! These birds are among the most common hummingbirds in the area, or at least the most belligerent. Males are polygamous and (like all male hummingbirds) do not participate in raising the young, but this species forms leks, which are areas where males sing to show off and females choose mates.

Years later, Clementina started recording songs from different groups, and she realized that they sounded different. Once we were familiar with the behavior of Wedge-tailed Sabrewings, we documented the structure and variation of their songs within and between singing groups in central Veracruz. What we found was very exciting. Locally known as fandanguero (because it sings all day long like local folks at the fandangos), the Wedge-tailed Sabrewing has a song that is complex and variable, composed of more than 20 different syllable types per individual. We found a total of 239 different syllable types among the eight singing groups we studied.

This became the basis of Clementina’s doctoral thesis. She started to record songs from leks across their geographic distribution and showed that their song varies throughout eastern Mexico, where populations of three subspecies (curvipennis, excellens, and pampa) are disconnected. When she looked at their DNA, lek members clustered into the three subspecies and could also be distinguished by their songs. We wondered, what caused the genetic and behavioral differences? Genetic drift (the accumulation of random mutations in a population) could partially explain their genetic and behavioral differences, because the members of populations of one subspecies don’t mate with the others. But the genetic drift alone is not enough to fully explain the observed song variation within and among leks of each of the subspecies. Some sort of selection might be occurring.

We wanted to explore more deeply what was happening inside the leks. Clementina realized that males within a lek also sang differently and that males with different songs were clustered spatially, with an introductory syllable in their songs as the signature of a song neighborhood. It is not clear at this point in our research how and why these unique and variable complex acoustic signals have evolved in some but not all lekking species of hummingbirds, but we’re working on figuring it out.

For our newest study, published in The Auk: Ornithological Advances, we asked whether lek formation and the existence of song neighborhoods within leks are due to kin selection. In this system, kin selection would occur when a member of a song neighborhood engages in self-sacrificial behavior that benefits the genetic fitness of its relatives in the neighborhood. To address this, we genotyped males in leks located across the distribution of Wedge-tailed Sabrewings as well as in a focal lek composed of song neighborhoods and estimated their relatedness — not an easy task! Most males within leks were unrelated, and song neighborhoods were not composed of related individuals. This means that kin selection is not acting on the formation of leks or song neighborhoods in this species, suggesting that membership in song neighborhoods is achieved by learning the song of the neighborhood, regardless of kinship.

Clementina González website

Juan Francisco Ornelas website

Singing Above the Urban Ruckus, If You’re a Mockingbird

Mitchell Walters

Linked paper: Urban background noise affects breeding song frequency and syllable-type composition in the Northern Mockingbird by M.J. Walters, R.P. Guralnick, N.J. Kleist, and S.K. Robinson, The Condor: Ornithological Applications.

Northern Mockingbirds adjust their songs to noisy urban environments. Photo by Mitchell Walters.

Imagine having a conversation with someone next to a very busy intersection at rush hour. You can’t hear one another at a normal volume, so what do you do? You could talk louder, maybe wait for the noise to die down, or simply pick up your belongings and move to a quieter setting to converse.

Our research shows that if you’re a Northern Mockingbird, one of North America’s most famous avian songsters and the state bird of five U.S. states, you adjust to noisy settings by conversing at a higher pitch. Northern Mockingbirds’ songs are incredibly complex, made up of hundreds of different elements, and we also found that urban mockingbird songs include more high-pitched elements.

So why raise your pitch? Noise in urban environments overlaps most strongly with lower-frequency bird vocalizations, so mockingbirds may be increasing the pitch of their songs to avoid being drowned out by urban background noise. However, mockingbirds also love to mock things, so it’s possible that individuals mimic higher-pitched sounds more often in urban environments. In any case, the Northern Mockingbird is a great example of a bird adapting to urban habitat. Previous research in our lab has shown that not only is this species more abundant in urban areas, it also adjusts its nesting, foraging, and anti-predator behavior in these settings. The acoustic flexibility we found in our study may add to their ability to coexist with urbanization and people.

Because mockingbirds love to mock other bird species in their environment, it also makes sense to ask how much of a role surrounding bird communities play in the characteristics of their songs. This is important, because a male who’s a prolific impressionist may be very attractive to female mockingbirds. Future research aims to look at whether urbanization affects the quality of mockingbird songs via changes in bird diversity. If this is true, then human disturbance could be indirectly impacting the evolutionary trajectory not only of Northern Mockingbirds, but of other mimicking species as well.

When I started my master’s degree at the University of Florida, I actually had no intention of working with mockingbirds. I originally planned to do a project overseas, but my advisor and I quickly realized that was unrealistic. After a few mild panic attacks and brainstorming sessions I turned to urban ecology, a topic I had recently developed an interest in. Our lab had done long-term studies on urban mockingbirds, but no one had looked at what was, in my opinion, the most fascinating part of mockingbird life history — their songs. So the stage was set, the field gadgets were bought, and the grant proposals were accepted, sort of. After two years of blistering Florida summer heat and several strange encounters with locals asking what the heck I was doing with recording equipment, the project was complete and the results are now in! I just want to thank AOS for publishing my first first-author manuscript; it’s been an absolute pleasure working with this amazing journal.

AUTHOR BLOG: Lifting hunting limits hasn’t solved the Snow Goose overpopulation problem

Robert Rockwell

Linked paper: Liberalized harvest regulations have not affected overabundant Snow Geese in Northern Manitoba by D.N. Koons, L.M. Aubry, and R.F. Rockwell, The Condor: Ornithological Applications.


A large flock of snow geese during spring migration in North Dakota. Photo by Craig Bihrle, North Dakota Game and Fish Department.

As many marshes along the Gulf Coast were drained in the late 1970s, Snow Geese that had used them as wintering areas began shifting to agricultural land instead. Leftover crops in farm fields provided them with a generous new winter and spring diet, and the population began growing at an unprecedented rate. While they have ample habitat in the south, the growing number of geese, coupled with their destructive foraging behaviors, has led to increasing and widespread habitat destruction in the Arctic coastal habitats where they breed.

In response to this habitat degradation and the resulting negative impacts on other species, the U.S. and Canada attempted to reduce the size of the population by liberalizing the regulations on Snow Goose hunting beginning in 1997. This included increasing bag and possession limits, expanding the hours during which hunting is permitted, allowing the use of unplugged shotguns and electronic callers, and instituting a spring conservation harvest. Adult mortality has a greater relative impact on the growth rate of this species than any other demographic variable, so wildlife managers hoped that removing more adults from the population through hunting would help bring it under control.

Severely degraded coastal and near-coastal habitat in Wapusk National Park outside of Churchill, Manitoba, Canada. Photo by Stephen Brenner, Nebraska Game and Parks Commission.

Adaptive Harvest Management is an approach used in North America that examines the impacts of management decisions and readjusts those decisions as necessary to meet management goals. We used our 50-year database on more than 100,000 Snow Geese banded on Canada’s Cape Churchill Peninsula to assess whether the new regulations were having the desired effect of increasing adult mortality. Using new analytical techniques, we could estimate the rates of both hunter and non-hunter deaths of adult geese and see whether those rates changed when the new hunting regulations were implemented.

Adult harvest mortality has stabilized at a low level, with only around 3% of adult geese being killed by hunters in a given year, probably because the sheer abundance of Snow Geese in this population has overwhelmed the number of hunters. The number of North American hunters has not increased in recent years, and most hunters are only willing to harvest enough geese to meet their family’s needs. Adult non-harvest mortality declined through the 1970s and 1980s to trivial levels (around 1% per year), but has increased recently and now fluctuates between 10% and 19% per year. Unfortunately, the combined mortality isn’t enough to limit the species’ growth, and the population continues to increase in size.

In light of our results, the next cycle of Adaptive Harvest Management needs to consider the potential causes of the increase in non-hunting mortality and its year to year variability, especially since such variation actually reduces overall population growth rate. We also suggest that management agencies further refine hunting regulations to increase the harvest per hunter and make efforts to increase public participation in hunting, perhaps through outreach to currently underrepresented groups.

The authors are all associated with the Hudson Bay Project. More about our work can be found at http://research.amnh.org/users/rfr/hbp.

AUTHOR BLOG: Speciation with gene flow in Northern Saw-whet Owls

Jack Withrow

Linked paper: Speciation despite gene flow in two owls (Aegolius ssp.): Evidence from 2,517 ultraconserved element loci by K. Winker, T.C. Glenn, J. Withrow, S.G. Sealy, and B.C. Faircloth, The Auk: Ornithological Advances.

Dorsal and ventral views of specimens of the two Saw-whet Owl forms, showing plumage differences. Photos by Jack Withrow.

Scientists have long thought that for two related populations of birds to evolve into separate species, they needed to be completely separated. This usually means the kind of total separation produced by isolation on islands or by features such ice sheets, mountain ranges, or rivers. However, the complex distributions and migratory nature of many birds mean that long-term total separation of bird populations, long the assumption in speciation research, is actually not necessary for speciation to occur. As we’ve tried to better understand how bird populations diverge, other ways of explaining speciation have begun to receive more attention, and it’s becoming increasingly clear that speciation can happen even with low levels of gene flow. Learning how this can happen in migratory taxa has become an important focus in ornithology.

Northern Saw-whet Owls (Aegolius acadicus) occur across much of North America. A small, non-migratory population is isolated on the islands of Haida Gwaii in British Columbia (A. a. brooksi), and the nominate mainland form (A. a. acadicus) passes through these islands during migration but does not stop to breed. So, these two populations overlap during some parts of their annual cycle but not during others. Presently considered a subspecies, brooksi feeds heavily on intertidal invertebrates in winter and has strikingly dark plumage compared to the migratory form, acadicus. Although their geographic context suggests the potential for gene flow, no one has ever found any intergrades between the two populations, suggesting that ecological factors are playing a role in maintaining their separation. Reconstructing how these owls diverged is difficult, because we can’t know the exact history of the distributions and ecological contexts that preceded what we observe today in this glaciated region. However, genomic data can be used to test hypotheses about their divergence and tell us what the long-term demographic history has been.

Recent technological advances have vastly increased the amount of genetic data that can be generated to look at divergence in populations. However, it’s important to examine the same parts of a genome so that you are making apples-to-apples comparisons. Ultraconserved elements (UCEs) are sections of the geneome that are relatively easy to find and can be used to assess divergence at population to family and order levels. They provide a similar set of core sequences that increase in variability the further from the UCE you go, allowing the assessment of both deep and shallow divergences at the same place in the genome. We examined over 2,500 separate UCEs to infer the processes under which these data were most likely to have evolved, including population sizes, levels of gene flow, and time of divergence. We found that the model that best fit the data included an initial split between the two populations with a low level of ongoing but skewed gene flow: an average of about 0.7 individuals per generation coming from acadicus into brooksi, and about 4.4 individuals per generation going the opposite direction.

These results support our hypothesis that the divergence between these two forms included low levels of gene flow rather than complete isolation. The low levels of gene flow from nominate acadicus into brooksi, coupled with strong evidence of ecological selection, suggests that the Haida Gwaii owls are on an independent evolutionary trajectory. Nominate acadicus are not staying on Haida Gwaii and breeding with the islands’ owls with the frequency we might expect, and we believe that selection is operating differently on the two populations due to the fact that they focus on resources that are distributed differently in time and space. Despite the history of gene flow between the two populations, brooksi appear to be a young biological species.

AUTHOR BLOG: Getting to the bottom of male Black-throated Blue Warblers’ migratory behavior

Jessica Deakin

Linked paper: Sex differences in migratory restlessness behavior in a Nearctic–Neotropical songbird by J.E. Deakin, C.G. Guglielmo, and Y.E. Morbey, The Auk: Ornithological Advances.

A female and male Black-throated Blue Warbler in captivity. Photo by Jessica Deakin.

Many species of migrant songbirds have a reproductive strategy called protandry, where males arrive at stopovers and breeding sites earlier than females. Ornithologists believe that males do this because it increases their mating opportunities and reduces competition among males for high-quality nest sites. Although it’s a common phenomenon, the question of how males arrive earlier is still unanswered for most species. One thing we do know is that the difference in arrival timing between males and females appears to be similar across years, suggesting that the behaviors that contribute to protandry are innate.

For this study, we evaluated potential innate contributors to protandry in the Black-throated Blue Warbler (Setophaga caerulescens). Black-throated Blue Warblers are small songbirds that spend the summer in North America and the winter in the Caribbean, migrating at night. They have a lazy, buzzy-sounding song, and males are easy to distinguish from females — the males are a brilliant blue, while the females are olive-brown. We caught Black-throated Blue Warblers at a stopover site in southern Ontario, Canada, during their fall migration and kept them at Western University’s Advanced Facility for Avian Research over the winter.

During migration, caged birds display a behavior called migratory restlessness, which is characterized by extensive hopping and wing whirring (the fluttering of wings while perched) at night. The timing of this behavior’s onset in the spring corresponds with when the birds typically depart from their overwintering grounds, and the intensity corresponds with the duration of their migration. We used automated video analysis software to quantify the onset and intensity of hopping and wing whirring of our captive Black-throated Blue Warblers when spring arrived. We found that migratory restlessness began earlier in males than in females, suggesting that males have an innate disposition to depart for spring migration. Surprisingly, we also found that males displayed higher intensity wing-whirring behavior than females, suggesting that the sexes may have innate differences in flight behavior that could influence their migration rate.

A tagged Black-throated Blue Warbler after release. Photo by Sue Bishop.

Next, we outfitted our Black-throated Blue Warblers with digital nano radio-tags and released them where they were caught the previous fall. The tags let us use the Motus Wildlife Tracking System to determine when they departed from the stopover site, and we also used manual telemetry to monitor their behavior. The birds behaved normally upon release, foraging, preening, and singing, and they departed in a normal northern spring migration direction. This suggests that migratory birds readjust quickly to the wild after being held in captivity for several months — important information for wildlife rehabilitators.

Overall, this study demonstrated that males and females have different migratory restlessness behavior. We think males might make longer flights than females between stopovers. Our future studies will include looking at sex differences in flight characteristics, such as wing morphology and energy use during migratory flight.