I am pleased to announce that I have a new paper in the Journal of Field Ornithology that describes the year-round migratory movements of Cassin’s Vireos. The study can be viewed and downloaded here. This paper was a side project initiated during my PhD, motivated by a simple question: where do Cassin’s Vireos go in the latter half of summer?
Bird migration is typically described as movements to and from the wintering/breeding grounds. Under this paradigm, a bird might be expected to be on a breeding territory from spring until fall, before migrating south. In the course of field work for my PhD, however, I noticed that my field site was practically devoid of Cassin’s Vireos from about July onward, despite the birds not being expected to migrate until September. So, where do they go in July and August?
I answered this question using two methods: light-level geolocators and eBird citizen science data. I attached geolocators to 22 birds in 2016, and was able to retrieve data from four individuals in 2017. The geolocator data clearly demonstrated that birds were remaining in the Sierra Nevada mountains of California during July and August. This ruled out the possibility that the absence of birds at my study site may have been due to early departures for long-distance migration. Instead, they must have traveled somewhere nearby.
The geolocator data was also useful for identifying the winter locations of the tracked individuals. They spent the winter in different parts of Mexico:
Despite the insights provided by the tracking data, the question of where the birds spent July and August remained. Geolocators only provide location estimates with an accuracy of a few hundred kilometers, so these data were unsuitable for assessing smaller-scale movements. For this I turned to eBird data.
eBird is a website which allows citizen scientists to contribute their observations of birds. In return, the website tracks the users’ life lists, and allows users to share sightings and easily track their observations through a phone app. The website has been a huge success, and now hosts more than half a billion bird observations. Better yet, the data can be used by anyone.
I downloaded the eBird data pertaining to Cassin’s Vireos from the Sierra Nevada mountains, and investigated the possibility that the species moves to higher elevations in late summer. The results were clear:
In panel a) of the figure, it is evident that the overall trend of Cassin’s Vireo observations moves upslope throughout the summer. In particular, note how there are very few observations of the species above 2000 m in late May, but many more in June and July. Meanwhile at low elevation, there were very few observations after July 1. More convincingly, panel b) shows that the proportion of checklists with Cassin’s Vireos drops from about 6% to about 1% from May to July at low elevations. This is in line with my observations from my PhD field site, which is at about 700m elevation – no matter how much time I spent searching, I could not find birds in late summer. At high elevation (>2000m), in contrast, the rate of observations is quite constant, about 6% of checklists. One might expect that, as singing rates decline in late summer, the species may become less detectable in late summer. If so, the steady rate of observations at high elevation may indicate an influx of less vocal birds to high elevation in late summer.
All this is to say that this study supports the notion that the species moves to higher elevation in late summer, and moves to wintering locations in about September. This fits into a broader emerging literature showing that post-breeding elevational movements to cooler, high-elevation habitats is likely more widespread in songbirds than previously thought, especially in western North America. Viewed through the lens of conservation, this only further complicates the challenge of conserving migratory songbirds: efforts can no longer just focus on “breeding grounds” and “wintering grounds”, but may also have to grapple with conserving high-elevation “post-breeding grounds”.
One shortcoming of the study was the inability to resolve individual movements in late summer, which meant I had to use the coarser, population-level inferences available with eBird data. It would be fascinating to know the nature of late-summer movements. Do all individuals move upslope, or do some move upslope and some move downslope or remain on territory? Do movements involve a single, directed flight from lower to higher elevations, or do individuals wander haphazardly and just happen to end up at higher elevations where conditions are more suitable? Are there particular habitats preferred by the species in late summer? The technology needed to answer these fine-scale, individual-based questions is currently lacking. However, technology is rapidly evolving – stay tuned, and maybe soon we’ll have answers.