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.

AUTHOR BLOG: Fly like an eagle? Topography tells us how high Golden Eagles soar

Adam Duerr

Linked paper: Topographic drivers of flight altitude over large spatial and temporal scales by A.E. Duerr, T.A. Miller, L. Dunn, D.A. Bell, P.H. Bloom, R.N. Fisher, J.A. Tracey, and T.E. Katzner, The Auk: Ornithological Advances.

A Golden Eagle soars over a line of ridgetop wind turbines. Photo by Dave Brandes.

Like many people, I am fascinated by bird flight. Unlike most people, I get to study flight of Golden Eagles for a living. These large birds move through the landscape primarily by soaring—a style of flying where they hold their wings outward and rarely flap, saving them considerable energy. Instead of flapping, they rely on rising air currents to gain altitude.

Two types of rising air currents provide most lift for soaring eagles. The first, thermal updrafts, form when energy from the sun heats air at the Earth’s surface and causes it to rise. Eagles circle within these columns of rising air to gain great altitude and then glide out of the thermals to move across the landscape. The second, orographic updrafts, form when winds are deflected upward by structures such as ridges or hills. Eagles can then soar at relatively low altitude above and along these structures.

Although updraft formation depends on the interaction of weather and topography, our goal for this study was to determine if topography alone can explain how high eagles soar. To do this, we used telemetry systems that we placed on the backs of 91 Golden Eagles in California, which recorded the eagles’ locations every 15 minutes. For each of the almost 180,000 locations we recorded of eagles in flight, we compared the eagle’s altitude with the characteristics of the topography below.

We found a strong relationship between topography and flight altitude for the Golden Eagles in our study. In places where the topography made the formation of orographic updrafts likely, eagles were more likely to fly at lower altitudes, while in places where the topography made the formation of thermal updrafts likely, eagles were more likely to fly at higher altitudes. We also found that the effects of some topographic features depended on their region within California, which may be due to regional differences in weather patterns, land cover, or a variety of other factors that we did not include in our analysis. Our topography-based model of flight altitude is much simpler than other models of avian flight altitude, thanks to the fact that it lets us ignore weather conditions, which are constantly changing. Instead, we can simply estimate how high a Golden Eagle is likely to be flying as it crosses over any point of interest in California. Wildlife managers can use this type of information to predict where eagles may collide with wind turbines and power lines; therefore, making these predictions in an accurate and straightforward way is critical for Golden Eagle conservation.

AUTHOR BLOG: How do traits change across a scrub-jay hybrid zone?

Devon DeRaad

Linked paper: Phenotypic clines across an unstudied hybrid zone in Woodhouse’s Scrub-Jay (Aphelocoma woodhouseii) by D.A DeRaad, J.M Maley, W.L.E. Tsai, and J.E. McCormack, The Auk: Ornithological Advances.

Devon DeRaad lines up Woodhouse’s Scrub-Jays (left) and Sumichrast’s Scrub-Jays (right) to compare back color between the forms. Photo by John McCormack.

Where should we draw the line between species? Biologists have debated this question for over 100 years. For much of that time, Ernst Mayr’s Biological Species Concept, which defines a species as a group of individuals that is reproductively isolated from other groups, has dominated the conversation. The BSC, as it has come to be abbreviated, led to more conservative species definitions, with many distinctive forms lumped together as single species because of actual or even potential interbreeding. Recently, however, more and more evidence of hybridization between species has accumulated, especially in birds. The question now is often not whether there is gene flow between what we would consider species, but how much is too much for them to still be considered separate? And when gene flow happens, how does it affect the array of traits that we can see in hybridizing forms?

The New World Jay genus Aphelocoma – which includes well-known species such as the California Scrub-Jay (A. californica) and the Mexican Jay (A. wollweberi) – has proven to be a good study system for investigating the role of gene flow in the early stages of speciation. Because Aphelocoma species don’t tend to move around much, many geographically-isolated, locally-adapted forms have evolved, but they have not become so different that they don’t hybridize when they come into contact with each other. Several of these contact zones have been well studied, like the one between the California Scrub-Jay and Woodhouse’s Scrub Jay (A. woodhouseii), which used to be considered a single species called the Western Scrub-Jay. In fact, the species was split based in part on the fact that although gene flow was occurring, it seemed to produce traits that were selected against outside of the small area where the two groups overlap and hybridize.

I was an undergraduate researcher at the Moore Laboratory of Zoology at Occidental College when I learned of another, largely unstudied Aphelocoma hybrid zone, between the northern form of Woodhouse’s Scrub-Jay and a distinct southern lineage called Sumichrast’s Scrub-Jay. The largest collection of Mexican birds in the world, the Moore Lab has an extensive collection of Woodhouse’s Scrub-Jay specimens from throughout Mexico. Using this valuable collection as well as specimens loaned from other natural history museums, we set out to discover whether there really was evidence for gene flow between these two groups and, if so, how much.

I measured the tail, wing, tarsus, bill length, bill width, and bill depth of 133 specimens from throughout Mexico. I also measured the intensity of blue on the back feathers of each specimen with a spectrophotometer, a tool that captures the wavelength of light reflected off of a surface, as back color seemed to be a major difference between the two forms. Our results confirmed that Sumichrast’s Scrub-Jay is significantly larger and has brown back feathers, as opposed to the blue-gray back feathers of Woodhouse’s Scrub-Jays from northern Mexico. A new analytical method for visualizing geographic transitions in traits called HZAR also showed that while body size transitioned gradually, the transition in back color was much more rapid, suggesting potential selection on this trait.

We were excited by our results not only because it was fascinating to see two traits, size and color, behaving differently across a hybrid zone, but also because the confirmation of the hybrid zone presents another opportunity to study speciation in action in Aphelocoma jays. And while we’re proud to publish a study on hybrid zones that does not include genetic data, providing another example of the modern value of museum specimens in their own right, we do hope to collect more specimens from the contact zone in the future and use genetic data to see how the genomes of both forms are responding to hybridization. These future genetic studies, combined with behavioral, vocalization, and ecological data, will provide an integrated portrait of these divergent lineages that have come back into contact and will help us make an informed decision about how best to recognize the taxonomic distinctiveness of these lineages—that is, whether the species once known as the Western Scrub-Jay should be split yet again.

AUTHOR BLOG: Tracking cultural evolution in House Finch song, Part 2

David Lahti

Linked paper: Four decades of cultural evolution in House Finch songs by C. Ju, F.C. Geller, P.C. Mundinger, and D.C. Lahti, The Auk: Ornithological Advances 136:1, January 2019.

(Read part one here.)

So what did we learn about how House Finch songs have changed since the 1970s—the equivalent of a millennium of cultural evolution in human terms? Here are the main results, and how we interpret them. We have to be careful with interpretation, though, as we cannot be sure that the differences we observed represent consistent trends; it’s possible that the birds have been fluctuating through the years and we merely caught two random points.

  1. All the main features of House Finch song in 2012 (such as song length, pitch, and syntax) are within the same range as they were in 1975.

Because the basic characteristics of House Finch song have remained consistent across the decades, birds today would probably still recognize old recordings as being from their own species. We’re soon going to test this to find out for sure!

  1. Roughly half of the individual syllables that were around in 1975 were around in 2012, too. The more common the syllables were in 1975, the more likely they were to still be in use by 2012.
  2. However, none of the particular songs (that is, sequences of syllables) that we recorded in 1975 were sung by any bird in 2012.

These results are to be expected. Since House Finch syllables are learned whole, they can be preserved from generation to generation; perhaps birds even reinvent the same syllables over time. However, young birds individually assemble syllables into songs each generation, and there are millions of combinatorial possibilities.

  1. The population of songs is more diverse (there are more different syllables in use) in 2012 than there were in 1975.
  2. Although birds shared songs with each other in 1975, the birds in our 2012 sample didn’t share any songs with each other, despite being the same distances from their neighbors.

We know that the population of House Finches generally grew and expanded between 1975 and 2012, although a nasty outbreak of conjunctivitis was decimating the population for a while. A larger population means more neighbors to listen to and more individuals to create new syllable modifications as they learn. Both of these factors should eventually cause greater overall song variety, which is what both of these results show.

As is typical in science, we also found results we cannot readily explain:

  1. Birds in 2012 do not repeat their songs as reliably as they did in 1975—they are more likely to skip syllables, add new ones, or switch them around.
  2. Individual songs in 2012 have fewer different kinds of syllables than they did in 1975 (despite there being more total syllable types in use in the population as a whole!).
  3. In 2012, the syllables that are more common tend to be the ones that are more complex—they change pitch more rapidly and more often. They also tend to be higher pitched. This was not the case in 1975.

We’re developing some ideas to explain these curious results—hypotheses that will inspire our next round of field and laboratory work. The House Finch researchers in our lab are taking some exciting next steps, looking at such things as song similarity over geographic distance, changes on islands, early song development, social networks, and sex differences.

Unfortunately, Paul Mundinger passed away while this study was being conducted, and he never got to see the results. But it is because of his early work that we were able to chronicle changes in these songbirds over nearly four decades, and his song recordings (which are voluminous) will continue to provide us with interesting baseline data and prompt new research for years to come.

See more about the Lahti lab at

Photo by César Castillo.

AUTHOR BLOG: Tracking cultural evolution in House Finch song, Part 1

David Lahti

Linked paper: Four decades of cultural evolution in House Finch songs by C. Ju, F.C. Geller, P.C. Mundinger, and D.C. Lahti, The Auk: Ornithological Advances 136:1, January 2019.

The first bird song I ever recorded was that of a House Finch. When I was a kid growing up in Leominster, Massachusetts, the bird that nested behind my front porch lamp would fly out to a particular birch tree or the telephone wire and belt out a complex four-second warble over and over again. That sound became emblematic of summertime for me and my siblings. One day when I was in my room holding my tape recorder against the radio speaker to record songs (human songs, that is!), I heard the little red fellow outside start doing his thing, and I promptly stuck my recorder out the window for an acoustic memento. I actually ran across this cassette tape for the first time in nearly four decades a couple of months ago, coincidentally just as my first scientific paper on House Finch song was about to be accepted for publication in The Auk.

Here are a couple of examples of House Finch song. Read it like sheet music, with time on the horizontal axis and pitch on the vertical—it’s composed of a bunch of notes, or syllables.

House Finch Song 1
House Finch Song 2

The reason my research collaborators and I are interested in House Finch songs today is because these songs change over time and space—we’d like to know how and why they change the way they do. Most animals simply inherit the noises they make, and so the sounds don’t change much from generation to generation. About half of the world’s birds, however, learn how to “speak” as juveniles from older members of their own species, just as we humans do.

The youngsters don’t always imitate perfectly the songs they learn, and so over the generations small changes in these songs can accumulate. These changes result in noticeable song differences across time and space, just as we humans diverge in our accents and languages. For this reason, bird song is an important animal model system for the study of cumulative change in socially learned traits, what’s known as “cultural evolution.”

Long-term changes in bird song are rarely studied, because research projects don’t often last for decades. However, even as I was listening to that House Finch from my bedroom, Dr. Paul Mundinger, a professor at Queens College at the City University of New York, was recording them on western Long Island, in high quality and accompanied by meticulous field notes. Paul had just published a paper in The Condor showing that House Finches can have different song dialects. He had also indicated that young House Finches learn their songs by listening to a bunch of singing neighbors and assembling chunks of syllables from several of them, like an acoustic collage. The end result is two to four songs that an individual will sing consistently for the rest of its life.

Fast forward 37 years, and I, a new professor at the same college, became Paul’s friend and colleague. He was pleased to hear that I wished to pick up where he had left off with House Finch song in the 1970s (after which he had moved on to research on the canary). I was excited to compare his early House Finch recordings to the songs sung by local birds today. Because birds’ generations are so much shorter than those of humans, this would be like comparing our English to that used a millennium ago in the epic story Beowulf, which is so different from our modern language that it would be unintelligible to most English speakers today.

The main two steps in this study would be (1) to see what songs these Long Island House Finches are singing today, and (2) to find a reliable way to compare songs across time. Two doctoral students in my lab stepped up to the task. Franny Geller loves observing and recording birds, and so she recorded as many House Finches as she could find in western Long Island in 2012, and Chenghui Ju is a computational wiz who programmed software specifically to characterize and compare House Finch songs in different times and places. This study became part of Chenghui’s recent doctoral dissertation.

What did we learn? Find out in Part 2!