I am very excited that my paper that starts by quoting English poetry has been published in The Auk: Ornithological Advances --- our first two lines are: "Percy Bysshe Shelley wrote in 1821 that the nightingale “sits in darkness and sings to cheer its own solitude with sweet sounds.” However, unlike English romantic poets, ornithologists know that nightingales sing not to stave off despair, but for sex—a primary reason birds sing is to attract and maintain mates." Graham Montgomery and I (ok, mainly Graham) have completed a huge number of playback experiments in the past year. Our aim is to study how songs evolve, and our protocol is to measure whether territorial individuals respond to (angrily approach the speaker) or ignore song from related populations that live somewhere else. We assume that song discrimination is a proxy for premating (not "permeating", as autocorrect wanted me to type) reproductive isolation. That is, two related populations that live in different places and ignore each others' song are unlikely to mate with each other should they ever come into contact. In this paper, we highlight 21 cases where related populations ignore each others' songs but are classified as a single species. We think this means the populations deserve species status; thus, the "21 new species" title! We also argue that playback experiments that "ask the birds themselves" are a better method to measure how different songs are than using computer software to compare audio recordings. Last, though this paper took a ton of fieldwork (again, mostly Graham there), it's not too hard to conduct these kind of playback experiments. We hope that our study motivates folks around the world to get out into the field with smartphones and wireless speakers in hand - we need this data to shed light on which populations should be considered distinct biological species!
Evolutionary ecology is messy; we usually talk about patterns, and only the strongest patterns get labeled as "rules" (and even these "rules" tend to be more along the lines of "strong patterns"). One such example is Bergmann's rule, which describes the pattern that organisms living at high latitudes tend to be larger than organisms living at low latitudes. The likely interpretation is that larger body size is an adaptation to colder environments. In humans, think of the short squat body types of Arctic peoples (good for staying warm!) versus slender and tall East Africans (good for dissipating heat?). But temperature is not the only factor that differs along latitudinal gradients - so does seasonality, species richness, and a host of other abiotic and biotic factors. If Bergmann's rule is really about adaptation to temperature, we should see similar patterns along tropical elevational gradients, where it is cold at high elevations but warm at low elevations. However, I show that in tropical birds, body mass is unrelated to elevational distribution. This lack of pattern holds across regions and comparisons (intraspecific vs. interspecific, and closely related species vs. entire passerine assemblages), suggesting temperature exerts minimal influence on body size evolution in tropical montane birds. At least for tropical birds, Bergmann's rule is not even a pattern!
Just back from a great Evolution conference in Austin, Texas. I presented brand new data from this spring's playback experiments, talked science with friends old and new, and initiated (at least) two new collaborations. All while having a grand time in the mid June heat and humidity of Texas' capital city.
There are many cases where closely related bird species live at different elevations. So why doesn't the lower elevation species live higher on the mountain, or the higher elevation species lower? After all, they have wings...
It is tempting to think that interspecific competition is responsible for this pattern. But there are several other factors that can generate parapatric distributions, and inferring interspecific competition in the absence of any further information is dubious. We decided to measure interspecific aggression in five species-pairs of elevational replacements of New Guinean songbirds. We could use playback experiments to measure interspecific aggression, and aggression is a likely mechanism of interspecific competition. It took a lot of hard work traversing steep muddy slopes to find territories of our target species and conduct playbacks. In the end, we found interspecific aggression in three of the five species-pairs; in each case, the interspecific aggression was asymmetric, with the lower elevation species more aggressive to the upper than vice versa. In sum: some evidence that interspecific aggression (and presumably interspecific competition) is one important factor shaping the elevational distributions of New Guinean songbirds. Read the paper for a more nuanced view, or, better yet, check out this blog post describing our study: http://www.bou.org.uk/freeman-elevation-aggression/.
I am pleased and honored to be the recipient of a generous research award bestowed by the American Ornithologist's Union. The grant will fund an ambitious slate of field playback experiments to quantify population's behavioral responses (or lack thereof) to songs from related, allopatric populations. I will then use this data to measure rates of premating reproductive isolation in tropical birds. The concept for this line of inquiry comes from last year's Advanced Tropical Field Ornithology course. Students in this course completed a series of playback projects to measure how White-breasted Wood-Wrens respond to genetically divergent, allopatric populations, and recently published the results. Read the study here; a recap is here. Now we'll massively expand this project to include a whole raft of Costa Rican species. Thanks to the AOU for giving this exciting project a jumpstart!
Excited that this paper is out. It's pretty straightforward -- there's no evidence that high elevation species in New Guinea (that experience extra cold temperatures) are better able to deal with cold than montane birds that live at warmer elevations, at least when using two common metrics that describe thermal physiology (lower critical temperature and thermal conductance). A somewhat surprising result. Maybe there are other aspects of physiological adaptation to cold that are more important (or, for birds that live above ~ 2,500 m, adaptations to hypoxia), or maybe these birds' elevational distributions are simply more influenced by food, diseases and competitors than by abiotic conditions. A big thank you to Brian McNab -- all the physiological data came from his studies, and he provided encouragement and advice when I first began this project.
Former Cornell undergrad Graham Montgomery and I just published our paper (open-access! thanks Condor!) describing the results of playback experiments we conducted in 2014 -- Swainson's Thrushes in the Adirondacks exhibit asymmetric interspecific aggression towards threatened high elevation Bicknell's Thrushes. This is consistent with the possibility that Swainson's Thrushes are behaviorally dominant, and we suggest Bicknell's Thrushes, already limited to mountaintop environments, may be threatened both by climate change and by interspecific competition (and the interaction between these factors). Read the great press release on the Condor blog!
Teaching Advanced Tropical Field Ornithology to a group of talented and committed Cornell undergrads was one of the highlights of my 2015. It was fantastic to guide students through the process of generating scientific hypotheses and testing them in the field, and I am very pleased that the first project from this course was recently published in PLoS ONE. Thanks to all the hard work by our awesome class, and especially to Teresa and Reid who took the lead on analysis and writing the manuscript. Great job everyone! We hope our project inspires others to conduct more field playback trials, as we argue they provide often critical information needed to assess the taxonomy of allopatric populations.
Last week I presented my dissertation research in a seminar titled "Elevational specialization in tropical montane birds." Following the talk, I completed my B exam and my committee signed off on my dissertation. Thanks to everyone - friends, family, colleagues at many institutions - who has helped me over these past five years! Here I am writing my initials next to the dent produced in the ceiling tile (just above the "B") from the champagne cork. A Cornell tradition maintained despite room rearrangements that require temporary ceiling tile removal! (The tiles with dents & initials are now located in the departmental office, and had to be brought over to the celebration room...)
The Chestnut-backed Jewel-babbler is found in New Guinean foothill forests, while its sister species lives in lowland forests. This exemplifies the typical pattern for sister species of tropical birds to live at different elevations when they are found in regional sympatry.Read More
What factors limit bird distributions? After all, they can fly, and sometimes it seems like perfectly good habitat is available but unoccupied (at present) by a species. Nick Mason and I investigated the drivers of distributional limits in the Acorn Woodpecker in South America. This clown-like Woodpecker is found only in the forests of the Colombian Andes. Surprisingly, it has never been recorded from nearby Ecuador. We first tested the possibility that the Colombian Andes have a special climate that is not present in Ecuador, but did not find any support for this idea. Instead, we found that Acorn Woodpecker distribution is closely tied to the presence of Colombian Oaks; this is evidence that Acorn Woodpecker distribution in the Northern Andes is limited by a tree, not by climate. Foraging data for Acorn Woodpeckers in Colombia is scarce, but it is most likely that they use Colombian oaks as a food resource. However, Acorn Woodpeckers in Colombia seldom consume acorns and do not store them in granary trees, as Acorn Woodpeckers in North America do. Instead, preliminary evidence suggests that Acorn Woodpeckers in Colombia may feast on the sap of Colombian oak trees. It seems likely that other birds have specialized relationships with particular plants that are food resources, and Nick and I hypothesize that similar specializations limit the distributions of other tropical birds.
Our Advanced Tropical Field Ornithology course just returned from an excellent field course at La Selva Research Station in the hot and humid Caribbean lowlands of Costa Rica. There were birds aplenty, from gaudy Great Green Macaws and the can-you-believe-it all white Snowy Cotinga to tinamous and curassows that walked up to us in the forest. Naturally, we focused on the relatively less-spectacular species for our studies, conducting a large set of playback trials on forest species (especially wrens). I look forward to moving onwards with data analysis on this project!
Ornithologists love to study clutch sizes. Eggs are easy to count, and eggs are important-the more you lay, the more offspring you will have (provided, of course, that you can feed them and steer clear of predators). The name of the evolutionary game is to produce offspring, so there is intense selection on birds to optimize their clutch size. Thus the interest. For example, many dozens of excellent studies investigate the reasons why tropical birds lay small clutches (typically two eggs; their temperate zone counterparts often lay clutches of five or more!). I call myself an ornithologist, so it's no surprise that I've begun to study the clutch sizes of tropical birds. I recently teamed up with Andy Boyce from the University of Montana (and his labmate Adam Mitchell and advisor and breeding biology guru Tom Martin) to study how clutch size varies along the slopes of tropical mountains. We found that high elevation species lay smaller clutch sizes than low elevation species in three very different regions; the tropical Andes of Venezuela, the mountains of New Guinea, and Borneo's high peaks. It remains unclear why exactly high elevation tropical birds lay small clutches (often just a single egg!), but it is clear this is a general pattern. There is something about high elevation tropical environments that selects for small clutches, a new mystery for enterprising ornithologists to unravel. Read our paper at http://aoucospubs.org/doi/abs/10.1642/AUK-14-150.1, or email me for a pdf.
We found our fair share of nests in New Guinea during fieldwork, and noticed that a surprising number of species lay clutches of only a single egg. While tropical forest birds commonly lay two eggs (temperate zone species typically lay much larger clutches), single-egg clutches are rare even in the tropics. Back in the library, I conducted a systematic review of the breeding biology of New Guinean passerines to build a database of clutch sizes. This database confirmed my suspicion that single-egg clutches are prevalent in New Guinean passerines. I then teamed up with fellow graduate student Nick Mason to demonstrate that small clutch sizes are associated with New Guinea and not an intrinsic feature of the lineages found on the island; species that live in Southeast Asian rainforests lay larger clutches than their relatives inhabiting New Guinea. The mechanisms that drive this pattern remain unknown, but could include rates of nest predation or parasitism in New Guinea that are abnormally high.
The paper can be found at http://www.publish.csiro.au/?paper=MU14023. Or email me for a copy.