David M. Larson
Winter crow roosts are interesting phenomena. Thousands or tens of thousands of American Crows (Corvus brachyrhynchos) will congregate in winter roosts. Recently, Dana Duxbury-Fox (2018) characterized such a roost of American and Fish (C. ossifragus) crows in Lawrence, Massachusetts, consisting of as many as 16,000 birds. Crows are intelligent social animals that often maintain family groups even during the breeding season. Winter crow roosts provide social interactions, safety in numbers, and possibly information exchange about food resources (Verbeek, 2002). Often, as in Lawrence, roosts are in urban areas, which may provide ample food, few predators, and warmth on cold winter nights.
When you visit a large crow roost in winter, you may consider where these noisy, gregarious birds come from. American Crows are partial migrants—some individuals in a population migrate and some are resident—so winter roosts are augmented by migrants from harsher climates. The phenomenon of partial migration is poorly understood, and some authors have suggested that it may be an intermediate stage in the evolution of complete migration.
Townsend et al. (2018) reported on a study of partial migration on American Crows from winter roosts in Davis, California (C. b. hesperis), and Utica, New York (C. b. brachyrhynchos). They captured crows and banded them with USGS bands and unique color bands. They collected blood samples for genetic analysis, tail feathers for isotopic analysis, and affixed satellite tags using backpack harnesses.
Using satellite telemetry data from 18 crows, the authors determined that 78% were migratory (8 of 11 on the West Coast and 6 of 7 on the East Coast). Resident birds did not stray more than 25 km from the center of their breeding territory during the year, while migratory birds traveled 177–1095 km from roosts to breeding territories. Migrants maintained the 25 km radius from the center of their breeding territory during the breeding season of late March or early April to September. Retention of satellite tags for up to 4 years in small numbers of resident and migrant birds showed high fidelity to breeding sites but less fidelity to wintering roosts, especially for long-range migrants. Using satellite telemetry is a gold standard in assessing movements of migratory birds, but it is expensive. Finding less expensive means of determining the migratory dynamics of birds was another goal of this project. Therefore, the authors also used isotopic analyses and genotyping to see if these measures could provide clear distinctions between migratory and resident crows.
Isotopic analyses measured the ratio of deuterium to hydrogen in samples from known migrant and resident birds. Since deuterium in precipitation decreases with increasing latitude, the level of deuterium in feathers grown on the breeding grounds has been used as an indicator of breeding latitude. As expected, deuterium levels from feather samples were generally negatively correlated with breeding latitude in this study. The authors found that the reliability of assigning birds to the migrant category was generally much better with longer-range migrants than with short-range migrants.
The authors also genotyped birds at 33 loci and tested if this technique could reliably determine known resident versus known migratory birds. Genetic analysis of migrant versus resident crow samples showed clear differentiation between these birds on the West Coast and a slightly lower differentiation for the East Coast birds. Again, long-range migrants were more distinct from resident birds than were short-range migrants.
Comparing the results from telemetry, isotope, and microsatellite tests suggests congruence between these methods, though estimates of the proportion of migratory birds in the winter roosts varied among methods: 73–86% migrant by telemetry, 48–66% by genetic, and 27–28% by isotopes. Migrants that traveled more than 3.5o N from roost to breeding sites were successfully classified using all three techniques. Use of the isotopic and genetic tests could provide means to gather more data on larger populations at considerably lower cost than the use of telemetry. And validation of this integrated approach would allow studies on other species that are too small for telemetry packs. This integrative approach provides a baseline for assessing population adaptations to changing climate, a pressing issue in ornithology.
Based on data from crows fitted with satellite telemetry packs, approximately 80% of crows in these roosts were migrants. If this proportion holds true for the Lawrence roost, perhaps as many as 13,000 of these birds are long-range migrants. It would be interesting to find out if smaller roosts contain a lower proportion of migrants, if crows move between local roosts, and what causes shifts in roost locations. Determining the migratory status of birds in urban crow roosts and the dynamics of these assemblies could be useful in studies of disease transmission, since crows can carry known human and wildlife pathogens, including West Nile virus.
David M. Larson, PhD, is the Science and Education Coordinator at Mass Audubon's Joppa Flats Education Center in Newburyport, the Director of Mass Audubon's Birder's Certificate Program and the Certificate Program in Bird Ecology (a course for naturalist guides in Belize), a domestic and international tour leader, President of the Nuttall Ornithological Club, and a member of the editorial staff of Bird Observer.
- Duxbury-Fox, D. 2018. A History of Winter Crow Roosts and a Visit to a Roost in Lawrence, Massachusetts. Bird Observer 46 (1): 22-31.
- Townsend, A. K., B. Frett, A. McGarvey, and C. C. Taff. 2018. Where do winter crows go? Characterizing partial migration of American Crows with satellite telemetry, stable isotopes, and molecular markers. The Auk 135(4): 964-74.
- Verbeek, N. A. and C. Caffrey. 2002. American Crow (Corvus brachyrhynchos), version 2.0. In The Birds of North America, A. F. Poole and F. B. Gill, Eds. Ithaca: Cornell Lab of Ornithology: NY, USA. https://doi.org/10.2173/bna.647