As 2018 concluded, a wealth of new research shed light on the intriguing world of corvids, our favorite family of birds. This roundup highlights five pivotal publications that offer deep insights into their behavior, migration patterns, intelligence, and conservation status. These studies, while sometimes condensed for brevity, provide a crucial glimpse into the complex lives of crows and ravens.
Understanding Crow Migration Patterns
Where do winter crows go? Characterizing partial migration of American Crows with satellite telemetry, stable isotopes, and molecular markers by Townsend AK et al. (2018) delves into the fascinating phenomenon of partial migration in American Crows. The study addresses the question of whether crows migrate, revealing that the answer varies significantly by geographic location. In regions like Seattle, crows are present year-round, leading residents to believe they don’t migrate. Conversely, in southern Canadian provinces, a noticeable decrease in crow numbers during winter suggests migratory behavior. This variation highlights that American Crows are a “partial migrant species,” meaning some individuals within a population migrate while others remain resident. This behavior plays a significant role in understanding the evolution of complete migration. The research explored whether this migratory behavior is fixed or flexible, its environmental influences, and how species adapt to changing conditions.
The study focused on two overwintering crow populations: one in Ithaca, New York, and another in Davis, California. Researchers employed a combination of intrinsic (originating within the body) and extrinsic (originating outside the body) markers to track the movement and origin of 18 tagged subjects over a period of 2-4 years. Intrinsic markers included molecular and stable isotope data, while extrinsic markers involved satellite tracking devices attached via lightweight backpacks. Stable isotopes helped identify the place of origin by analyzing unique properties in the local food and water that become embedded in an individual’s tissues. Molecular data was used to determine the sex of the birds and establish relatedness.
A surprising key finding was that approximately 78% of the tagged crows across both east and west coast populations were migratory. The distances traveled varied significantly, with some birds migrating as little as 280 km (173 miles) and others as far as 1095 km (680 miles). Resident birds, however, were observed to stay within 25 km (15.5 miles) of their breeding sites. Importantly, both resident and migratory individuals demonstrated strong loyalty to their breeding sites, returning to the same territory year after year. This consistency suggests that individuals did not change their migratory behavior from year to year. These findings offer valuable insights into how crows might adapt to environmental changes induced by climate and urbanization.
Advanced Tool Construction in Crows
Compound tool construction by New Caledonian crows by von Bayern AMP et al. (2018) published in Nature Scientific Reports, challenges the long-held belief that tool use is exclusively a human trait. While it’s now established that various animals use tools, and a select few even create them, the construction of compound tools—those made by combining separate non-functional parts—remained largely unobserved in wild animals. This study investigated the capabilities of New Caledonian crows, a species known for its tool-making prowess. Understanding their ability to create compound tools could provide insights into the evolution of human executive functions.
The experiment involved eight wild-caught, captive New Caledonian crows. The study progressed through multiple phases. Initially, birds were provided with a long stick and a baited test box, demonstrating their ability to use a single tool to access food. In the second phase, the crows were presented with the same test box but were given a hollow cylinder and a thinner cylinder that needed to be combined to form a tool long enough to reach the food. Subsequent phases involved novel combinable objects and tests to determine if the birds understood the necessity of combining elements or if they acted out of other benefits. The final phase tested their ability to combine more than two elements.
The results were remarkable. All birds successfully used the single long stick. In the second phase, half of the subjects (four crows) managed to combine the two elements after minimal failed attempts, and they successfully transferred this skill to novel objects. When presented with a choice of tracks, birds primarily constructed compound tools only when necessary, indicating a cognitive understanding of the task. While only one bird succeeded in creating a tool from more than two elements, these findings position New Caledonian crows as on par with, and potentially exceeding, the known capabilities of great apes in compound tool use. The study opens avenues for further research into whether this complex behavior stems from cognitive mapping or chance, with the authors leaning towards cognition due to the crows’ other innovative tool-use behaviors.
Decoding Raven Communication
Raven food calls indicate sender’s age and sex by Boeckle M, Szipl G, and Bugnyar T. (2018), published in Frontiers in Zoology, offers a significant breakthrough in understanding corvid communication. Deciphering crow calls has been a notoriously difficult challenge. However, this study focused on ravens, revealing that their “haa” food calls encode information about the caller’s age and sex. This is groundbreaking progress in the study of corvid vocalizations. Previous research had identified long “haa” calls as a means to recruit other individuals to food sources, but the specific information encoded within these calls remained unknown.
The researchers recorded hundreds of “haa” calls from wild ravens whose age and sex were already known through previous banding efforts. Using acoustic software, they meticulously analyzed the vocalizations, looking for patterns in elements such as frequency and inflection rate.
The key finding, as suggested by the study’s title, is that ravens appear to embed information about their age and sex within their “haa” food calls. In “fission-fussion” social systems, like those of ravens, where individuals frequently interact with both familiar and unfamiliar conspecifics, class-level information conveyed through calls is crucial for rapid assessment of a caller’s identity. This information can influence an individual’s decision to join a feeding event, a critical choice given the potential for severe injury from aggression at such gatherings.
Critically Endangered Mariana Crow
An update of the breeding population status of the critically endangered Mariana Crow (Corvus kubaryi) on Rota, Northern Mariana Islands 2013–2014 by Kroner A, and Ha R. (2018), published in Bird Conservation International, presents a sobering update on the status of the Mariana Crow, also known as the Aga. This native species to Guam and Rota faced extinction on Guam following the introduction of the brown tree snake in the 1940s, leaving only the Rota population to sustain the species. While the population was around 1,300 individuals in 1982, a continuous decline led to its endangered status in 1984 and critically endangered status today. Unlike on Guam, the reasons for the continued decline on Rota are less clear, but likely include habitat loss, human persecution, natural disasters, and introduced predators such as cats.
The study involved extensive surveys of known island territories by researchers during 2013-2014. These surveys, spanning 845 hours of labor and covering 1,485 hectares, aimed to count breeding pairs while also documenting unpaired or subadult birds. To estimate the total population size, models were employed to account for missed detections across the island.
The findings were deeply concerning. Only 46 breeding pairs were detected across all surveys. After accounting for unpaired birds and potential detection failures, the estimated population of Aga was around 178 individuals. This figure represents a significant decline of 10-23% since 2007 and a stark 46-53% decrease since 1998. Crucially, researchers estimate that at least 75 pairs are necessary to maintain a viable population. The study concludes that without intensive predator management and widespread community advocacy, the future of the Mariana Crow remains increasingly precarious.
Ravens and Reintroduced Wolves
Population responses of common ravens to reintroduced wolves by Walker LE, Marzluff JM, Metz MC, Wirsing AJ, Moskal ML, Stahler DR, and Smith DW. (2018), published in Ecology and Evolution, examines the dynamic relationship between common ravens and reintroduced wolves in Yellowstone National Park. A persistent myth suggests a symbiotic relationship where ravens intentionally lead wolves to carcasses, but the reality is more complex, with ravens often being “unwelcome dinner guests.” Nevertheless, the two species significantly influence each other. The reintroduction of wolves to Yellowstone in 1995 provided a unique opportunity to study how wolf presence affects the spatial distribution and feeding behaviors of park ravens.
This collaborative study involved avian and spatial ecologists from the University of Washington and Yellowstone wolf biologists. Utilizing data from 2009-2017 on wolf abundance and prey kills, alongside raven surveys conducted both within the park’s interior and at anthropogenic food sources in surrounding areas (like the Gardner town dump), the researchers developed models to estimate raven abundance during the study period and before the wolf reintroduction. These models aimed to identify the key predictors of raven abundance.
Contrary to the hypothesis that ravens would rely more heavily on wolf kills during severe winters (when wolves make more kills due to difficult prey evasion), the study found different patterns. Yellowstone ravens tended to utilize consistent anthropogenic food sources during harsh winters. However, they leaned more on wolf-provided carrion during milder winters. Nonetheless, the presence of wolves has demonstrably increased and stabilized the raven population in the park by providing a consistent year-round food source, unlike the seasonally limited kills from human hunters. These findings underscore the critical role of apex predators in stabilizing food webs and supporting a diverse array of other species.
As a bonus, the blog post also references the highly significant 2018 study, “Occurrence and variability of tactile interactions between wild American crows and dead conspecifics,” highlighting the continued exploration of complex corvid behaviors.
