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

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.

PRESS RELEASE: Do Songbirds Pay a Price for Winter Wandering?

The greater the magnitude of nuthatch’s winter irruption, the lower the next summer’s breeding population.

In years when winter conditions are especially harsh, birds that depend on conifer seeds for food are sometimes forced to leave their homes in northern forests and wander far from their normal ranges to find enough to eat. A new study published in The Auk: Ornithological Advances uses citizen science data to show for the first time that these winter movements—called “irruptions”—lead to a decline in birds’ population density in their breeding range the following summer, suggesting that irrupting birds succumb to the difficulties of avoiding predators and finding food in unfamiliar landscapes.

Many birdwatchers love irruptions, because they can temporarily bring seldom-seen boreal birds south in large numbers. However, we know very little about how these journeys into unfamiliar territory actually affect bird populations. Red-breasted Nuthatches are a useful species in which to study this, because they return to the same core breeding areas even after winters with massive irruptions, making it possible to track how their breeding populations are doing from one year to the next.

Environment Canada’s Erica Dunn checked more than fifty years of records from Ontario’s Long Point Bird Observatory (LPBO) against citizen science data from Project FeederWatch, the Christmas Bird Count, and eBird to confirm that fall irruptions at Long Point are a good indicator of what’s going on with nuthatches across North America in any given year. Then, she used Breeding Bird Survey data to see how nuthatches fared in summers following major irruptions and found that breeding population density tended to dip noticeably after a winter where nuthatches had wandered more widely than usual.

This is the first study to demonstrate a correlation between the magnitude of birds’ winter irruption and their population density during the following breeding season. While these large-scale winter movements may be a necessity for birds in years where food is scarce, the rigors of travel, exposure to predators, and need to find food in unfamiliar places might take a toll.

“This paper actually had its genesis over 30 years ago, when I was running LPBO’s Ontario Bird Feeder Survey and noticed that feeder watchers were reporting more nuthatches in winters following large fall irruptions at Long Point. When the biggest irruption ever at LPBO occurred in 2012, I was inspired to use their fifty-plus years of data to investigate that old observation in more detail,” says Dunn. “It was truly a project without a particular goal or hypothesis—I simply had a great dataset and wanted to see what I could learn from it. Citizen science data are great for this kind of exploration, because the datasets are so large and are freely available to anyone who wants to work with them.”

Dynamics and population consequences of irruption in the Red-breasted Nuthatch (Sitta canadensis) will be available April 15, 2019, at https://academic.oup.com/auk/article-lookup/doi/10.1093/auk/ukz008.

About the journal:The Auk: Ornithological Advances is a peer-reviewed, international journal of ornithology published by the American Ornithological Society. The Auk commenced publication in 1884 and in 2009 was honored as one of the 100 most influential journals of biology and medicine over the past 100 years.

AUTHOR BLOG: Are homebody warblers more likely to sing together?

Liam Mitchell

Linked paper: The evolution of vocal duets and migration in New World warblers (Parulidae) by L.R. Mitchell, L. Benedict, J. Cavar, N. Najar, D.M. Logue, The Auk: Ornithological Advances.

Family tree of duetting and migrating warbler species.

Scientists who want to study the evolution of behavior face a fundamental problem: unlike bones, behavior generally doesn’t fossilize. However, that doesn’t mean that extinct species’ behavior doesn’t leave any evidence. The behavior of living or “extant” species can give us clues about the behavior of their ancestors, and we can use the behavior of living species, the evolutionary relationships among species, and computational modelling to make inferences about extinct species’ behavior.

In our study, we used this approach to study the evolution of vocal duetting and migration in New World warblers. Vocal duetting is when a mated pair of birds sings together. Duets are a cooperative behavior, because they communicate that the duetting pair will cooperatively defend their shared territory against intruders. It’s hard for mated pairs to stay together through migration, so non-migratory birds tend to have longer-lasting pair bonds than migratory species. These longer pair bonds mean that non-migratory birds may have more to gain from cooperative behaviors like duetting, so we might expect duetting to be evolutionarily associated with the absence of migration. 

We tested whether migrating and duetting are correlated in the evolutionary history of New World warblers. Essentially, we were looking to see if duetting and the absence of migration show up in similar places on the birds’ family tree. We collected data on each warbler species and determined whether or not they performed duets and whether or not they migrated. We used these data to perform our analyses.

Our primary analysis generated an evolutionary tree that shows duetting and migration over evolutionary time. We borrowed an existing phylogenetic tree of New World Warblers (Lovette et al., 2010) and used a technique called Markov Chain Monte Carlo (MCMC) to simulate trait evolution (Revell, 2013). MCMCs are computer simulations that calculate the frequency at which a given node (a point where the tree branches) exhibits a specific characteristic over a number of simulated generations. The likelihood that a given generation exhibited a characteristic is informed by the other nodes on the tree, especially the nearby ones. The frequency with which a node exhibits a characteristic in the simulation can be interpreted as the probability that the ancestral species at that node exhibited the characteristic. For example, if we calculate one million simulations for the most ancestral node on the tree, and 800,000 of those simulations exhibit the characteristic “migration,” we can say there is a high likelihood (0.8) that the last common ancestor of all living warblers migrated.

This let us calculate the evolutionary correlation between duetting and migration. Our analysis showed that migration and duetting are indeed negatively correlated over evolutionary time. In other words, duetting is associated with a non-migratory lifestyle, as we predicted. These methods allow us to get a quantitative look at the evolution and loss of certain behaviors over time. We can then draw informed conclusions about the nature of these complex behaviors and open the way for studying the factors that may have influenced their changes over time.

Read more on the lab website.