Curious Nature: Who flew the coop?
Walking Mountains Science Center
Migration occurs each year as thousands of avian species move great distances in pursuit of suitable environments. What triggers this response varies — climate, season, or availability of food, to name a few. But even within these migratory groups, individuals make the decision to participate or not.
We see this play out in the Corvid family, where crows have been studied to exhibit “partial migration” when a fraction of a population migrates and the rest remains at their usual residence. While the science behind this unique behavior widely remains a mystery, the evolution of this habit has provided insight into the adaptability of species to respond to the changing climate.
In 2014, ornithologist Andrea Townsend and her colleagues from Hamilton College took to the field to study crow migration by tracking populations in both New York and California for four years. Tracking devices were put on 18 individuals, allowing the team to monitor who would move and who would not.
Data showed that 73% of western crows and 86% of eastern crows migrated in some capacity, returning to the same territory each year. An interesting result, however, was that whether or not individuals migrated was consistent from one year to the next — they didn’t switch strategies depending on environmental conditions. These findings, published in The Auk: Ornithological Advances, are not only important to the advancements of understanding migration, but also to the critical focus on the effects of climate change.
Crows and other city-dwelling birds not only see the rush of increased urbanization but the direct impacts of climate change. With rising global temperatures, urban areas develop “heat islands” characterized by the inability for dark surfaces such as pavement and buildings to deflect heat. A milder winter, for example, may serve as an easier winter for a crow, encouraging them to become sedentary in such a place. We see birds being forced to adapt their daily and seasonal behaviors, and as a result, these disruptions can cause imbalances in thermoregulation, reproduction, and timing of breeding and migration.
Whether partial migration can be seen as a stepping stone to coping with the impacts of disturbances like this one, we don’t know for certain. Townsend suggests that the flexible nature of this species could benefit them in the long run, however.
Rather than being tied down to one area, modified migration strategies might give these birds the upper hand in being able to shift their range to more suitable sites as conditions change. Even if individual crows are not completely switching between migratory and sedentary behavior in a single season, they might be able to conditionally adjust the distance that they travel to their overwinter sites in response to climate conditions.
As we have transitioned to fall here in Eagle County, the sounds of summer songbirds have left us with the rustling of golden aspen leaves. The harsh “caw” of an occasional crow overhead may remind you of the plight of this migratory species.
While so much remains unknown, the collection of data stays a powerful tool. Consider joining your fellow citizen scientists in observing local bird communities and behaviors in your area. Who has been here? Where did they go? Visit eBird.org to join researchers, amateur birders, and Walking Mountains staff making the effort to log real-time data in hopes to better understand our feathered friends.
Bridget Whyte is the outgoing lead naturalist at Walking Mountains Science Center. She can often be found dancing with her friends (both humans and birds) and planning the next weekend adventure.
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