That birds travel seasonally is perhaps one of the most familiar facts about the natural world. Whether it’s the arrival of technicolor spring migrants, or the din of waterfowl above our heads in the fall, it takes no formal training to recognize that something novel and beautiful has suddenly erupted into our lives. Their ephemeral presence offers an opportunity to ground ourselves in time and place and reflect on the shape of our lives since our last meeting. Or, if you don’t want to get that deep with it, there’s always Looney Tunes or any number of other children’s cartoons to remind us that some birds come and go with the seasons.
At the same time, for most of us living in the continental United States, that crows will be nearby to accompany us throughout our year is something we take for granted. Their predictability on our telephone poles and near our garbage cans is one of those quiet details not everyones thinks of often, but whose consistency surely calms us as so many other things feel unsteady. But at the intersection of these truths is an interesting question: if migration seems such an essential part of bird life, why don’t crows do it? Or do they?
Of the world’s ~ten thousand birds species, only 18% actually undertake annual long distance migrations.1 But there are other types of migration including short distance migration, altitudinal migration (short migrations from shorter to higher altitudes) and partial migration. Partial migration is when only certain individuals within a population migrate, while others are sedentary, and it’s this one that applies to crows. Because while crows in temperate Seattle may be quite comfortable year round, those that call higher latitudes “home”, say central Canada, would have a tougher time making it through the winter unscathed. So much like a wealthy aging relative, they snowbird it to more welcoming climates for the winter.
Until the last decade or so that’s really all we knew about about crow migration. Some did it, some didn’t, and that was that. But given the value of better understanding this behavior, as well as the technological advances that make it possible, western science has finally turned its eye to inspecting these patterns more closely. Because there are so many questions one could ask about this phenomenon. Why do some crows migrate and others do not? Do the same crows migrate each year or can they opt out? Do they return to the same place? How far do they fly? How do they survive the journey? How do they sleep?
In some cases the answers to these questions are surprisingly nuanced. For instance, birds living in the Central and Southern Canadian provinces will nearly always migrate hundreds of miles south into the US, which makes sense because Canadian winters can be especially harsh. But studies have shown that other crows make even shorter migrations, only about 350 miles between contiguous US states with similar climates.2 What motivates these shorter-distance trips remains to be seen. Likewise, whether migrating is a discretional activity is still in question. From a climate change perspective, that individual birds might be able to choose whether or not to migrate each year would have important implications in their ability to adjust to changing conditions. But in the studies conducted so far there’s been no evidence that crows pick and choose each year, rather it seems that you’re either a migratory individual or not, albeit this is based on low sample sizes.3 As a result, migrating crows show extremely high site fidelity; returning to the same breeding and winter sites each year.
As far as distances go, there’s huge variability there as well. In a study that looked at crows from both sides of the country, the average distance traveled by east coast crows was 287 miles, while west coast crows traveled an average of 366 miles.3 The longest migration on record is 1740 miles.4 One thing that’s for certain is that migration is a daytime affair. While most birds migrate at night, crows are among the minority that stick to business hours for their travels. The most compelling explanation for strategy is the “fly-and-forage” hypothesis. Look, even for the arguably most efficient fliers in the animal kingdom, flying is hard work and crows need a lot of calories to sustain flight speeds up to 37mph covering as much as 186 miles in a single day.2 To accomplish this, crows will make up to several pit stops during a marathon flight session, allowing them to refuel and prepare for the next leg. Importantly, although crows migrate both in groups and alone, when stopped they almost always make sure to dine with others. Traveling in the daytime probably helps facilitate finding other crows, and by extension, the local feeding grounds.
When it comes to sleeping, crows leave the fancy mid-flight naps to the swifts and the frigatebirds, opting instead to sleep at night in communal groups. In fact crows will sometimes abruptly change direction just to follow local birds to the nearest roosting grounds. In other cases though, crows have been know to travel routes that allow for stops at the same roosts year after year, underscoring why roost locations should be considered key pieces of habitat.2
So as it stands, crow migration can be summarized the same way as most other parts of their lives: we know some stuff, not everything, we’re probably wrong about a few things, and our best bet is to accept our role as eager pupil and celebrate the gifts of knowledge as they are granted to us.
1. Sekercioglu C.H. 2010. Partial migration in tropical birds: The frontier of movement ecology. Journal of Animal Ecology: https://doi.org/10.1111/j.1365-2656.2010.01739.x
2. Ward M.P. and Raim A. 2011. The fly-and-social foraging hypothesis for diurnal migration: Why American crows migrate during the day. Behavioral Ecology and Sociobiology 65: 1411-1418
3. Townsend A.K., Frett B., McGarvey A. and Taff C.C. 2018. Where do winer crows go? Characterizing partial migration of American crows with satellite telemetry, stable isotopes, and molecular markers. The Auk 135: 964-974
4. Brewer D., Diamond A.W, Woodsworth E.J, Collins B.T.,, and Dunn E.H. 2000. Canadian Atlas of Bird Banding, vol 1: Doves, Cuckoos, and Hummingbirds through Passerines, 1921-1995. Canadian Wildlife Service, Ottawa, ON, Canada