Many birds use songs in the processes associated with acquiring mates (e.g., territoriality and courtship). Songbirds (Oscine passerines) are known to learn their songs in processes that resemble how we learn language. By and large, the research on this song learning process has focused on the cognitive and neurobiological side of things–how do they learn songs, and can we learn something about how we learn language by studying this process?
Starting around 2010, I got interested in the evolutionary implications of song learning–particularly in the connections between how birds learn songs and how they diversify across evolutionary time. Can understanding learning lead to a better understanding of why and when birds speciate? Conversely, can evolutionary processes help explain why different birds learn differently? I got to kick-start this line of research as a postdoc with Trevor Price at University of Chicago. Soon after I started my own lab at University of Nebraska, Emily Hudson took the basic system I had set up to a new level with her series of field studies.
We focused on the Golden-crowned Sparrow (Zonotrichia atricapilla) for a number of reasons. First, the genus Zonotrichia had long been a model system for song learning and song divergence–particularly the White-crowned Sparrow (Z. leucophrys) made famous by Peter Marler, Luis Baptista, Doug Nelson and many others. This meant that we had a lot of previous knowledge to work off of. For example, the learning process (e.g., timing of learning, species specificity, etc.) of North American sparrow species (including members of other genera such as Song Sparrows and Swamp Sparrows) were pretty well-resolved already. For White-crowned Sparrows in particular, Soha & Marler (2000) had shown that the introductory whistles could act as a species-specific identifier that induces young sparrows to learn that song.
The second reason we focused on Golden-crowned Sparrows was that I had already been working with them on their wintering grounds in California. And in fact, my first field season as a graduate student was spent attempting to study their mating system and plumage signal function on their breeding grounds in Alaska. (That attempt failed because… too many bears. It’s a story for another post.) So I had some prior knowledge of the system.
So the first thing I had to do was to find populations I could study. Golden-crowned Sparrows tend to live at timberline habitats throughout Western North America from the Washington-British Columbia border out to the Alaska (though up at the Northwestern end of their distribution they also occupy coastal shrubs). This led me to a continental-scale road trip that took me to as many timberline habitats as I could easily get to within a short hike from a road. This took me to many beautiful spots!
Chuck Creek Trail, near Chilkat Pass 
camping off trail, Stone Mountain Provincial Park, BC 
Exit Glacier, Seward, AK 
Salmon Glacier Summit Viewpoint, outside of Hyder, BC 
near White Pass, South Klondike Highway
In the end, most places were more beautiful than practical for detailed study. But in the meantime, I was able to collect enough song samples to describe the range-wide geographic variation in songs and describe 5 main song types (aka ‘dialects’) in a collaborative study (Shizuka et al. 2016) with Glen Chilton and Ross Lein (who had collected more golden-crowned sparrow songs in the 90s). Two interesting patterns emerged from this. (1) All golden-crowned sparrow songs start with an introductory whistle, like the white-crowned sparrow. But unlike the white-crowned sparrow, the golden-crowned intro whistle always has a descending tone. (2) There is a clear break in songs of Alaskan golden-crowned sparrows and Canadian golden-crowned sparrows: Canadian sparrows almost always end their songs with a trill, while Alaskan sparrows almost never do.
Figure 2 from Shizuka et al. 2016 
Figure 1 From Shizuka et al. 2016
This road trip also yielded another important find: a great study site where I would end up returning over the years to conduct an in-depth study on how song learning works in the context of evolution. This study site–Hatcher Pass near Palmer, Alaska–became our main base for our studies in 2011-2017. This site has it all: it is accessible by road, and harbors a dense population of Golden-crowned Sparrows and White-crowned Sparrows (though the Golden-crown population is much more dense). Here, I first conducted a study showing that we can induce behavioral responses to songs from nestling sparrows at about 7-9 days old (a couple of days before they fledge from the nest).
Shizuka Lab 