At whatever moment you read these words, day or night, there are birds aloft in the skies of the Western Hemisphere, migrating. If it is spring or fall, the great pivot points of the year, then the continents are swarming with billions of traveling birds…
– Living on the Wind: Across the Hemisphere with Migratory Birds
Among birds, migration is the regular, endogenously controlled, seasonal movement of birds between breeding and non-breeding areas (Salewski and Bruderer 2007). Spring migration is on, so where did all the birds on the Flyway cam go? In short: migrating waterfowl often follow ice melt north, stacking up at the edge of the iceline until the ice breaks. The huge numbers of waterfowl we saw the week of March 17 are now stacked up somewhere north, waiting for more ice to break.
What does migration look like? Let’s start by categorizing migrants into two groups: obligate (I have to migrate) and facultative (I’ll migrate if I need to or feel the urge to do so). Obligate migrators are ‘programmed’ to leave their winter and summer ranges at a certain time each year, regardless of weather and habitat conditions, food availability, and other factors. Facultative migrants migrate along a general schedule, but they can change their schedule by days or even weeks, depending on the circumstances. Many of the birds we’re seeing on the Flyway cam right now seem to have some characteristics of both. For example, American White Pelicans are classified as obligate migrators because they have to migrate – they can’t fish when ice has sealed the lakes! But they tend to migrate short distances and have been observed migrating just ahead and behind the ice line – two characteristics of facultative migrators. Like so much else in the natural world, they escape our neatly bounded categories as we learn more about them.
However a bird is classified, its circannual cycle of molt, migration, and reproduction is synchronized by photoperiod, or daylight length. As we know, daylight length pulls a lot of strings in a bird’s body. We’ve talked about how increasing daylight length causes non-migratory bald eagle gonads to swell, sex hormones to flow, and egg production to begin. But something similar is happening to migratory birds spending the winter in the southern US, Mexico, and Central and South America. While the photoperiod is quite consistent at tropical latitudes, something – slight changes in photoperiod, daylight intensity, changes in spectral composition, the lack of photoperiod change, or even circannual clocks set by photoperiod changes at higher and lower latitudes – causes birds to begin producing hormones on their wintering grounds. Depending on the species, birds might respond by gorging on food, storing fat, completing molt, and shrinking their livers, kidneys, and alimentary canals to reduce weight in preparation for migration. Body condition, temperature, prevailing winds, and food availability can all induce migration once a facultative migrant is ready to go. Obligate migrants are more influenced by circannual clocks than local conditions and they tend to make more extreme changes to their bodies prior to their marathon migrations, but they are driven by the same general suite of hormones.
Although birds like the Bar-Tailed Godwit and the tiny Blackpoll Warbler migrate thousands of miles, a lot of birds migrate relatively short distances. We saw American White Pelicans, Sandhill Cranes, and Common Goldeneye ducks on the Flyway cam last week. These species all breed in Canada south to the central United States and winter in the central United States south to northern Mexico. This allows them to sense local conditions and seize opportunities to move closer to their breeding grounds. They are being driven by hormonal changes in response to photoperiodism, but arriving too soon might mean starving and/or freezing if everything is still sealed under ice and snow – especially for birds that eat primarily fish, amphibians, reptiles, or insects.
In the spring, many of the birds we see at the Flyway are trying to hit the sweet spot on their breeding grounds! What does that look like? The ice is out, food is plentiful, everyone wants to pair off (or at least reproduce), and the best nesting spots are still available! This means that spring migration is shorter, more intense, includes a higher percentage of adult birds, and is heavily influenced by reproductive timing. Fall migration is longer, less intense, and includes a higher percentage of juvenile birds transitioning from dispersal into migration. It also includes birds like Sandhill Cranes that migrate to their wintering grounds in family groups. These birds may spend a lot of staging time at places like the Mississippi river as they eat, gain weight, molt, and prepare for migration – probably similar to their lives on their winter ranges, but quite a bit different than the rapid rush north that occurs in spring.
So how do birds navigate? Migration studies have found four major methods:
- Geographic mapping: When I’m in Minneapolis, I use a number of tall buildings to help me orient the city. It turns out that birds do the same thing, using landforms and geographic features such as rivers, coastlines, and mountain ranges to guide them. The Mississippi river provides a road through the United States almost all the way to Canada. Birds that don’t follow it north all the way can skip from Lake Pepin to the south shore of Lake Superior in just 150 miles or so. From there, they can follow the shore north-northeast all the way into Canada.
• Celestial navigation: migratory birds use the position of the sun (during the day) or the rotation of stars (at night) to orient themselves. Experiments done by Dr. Emlen in 1967 indicate that celestial navigation is learned: http://www.birds.cornell.edu/AllAboutBirds/studying/migration/navigation
- Learned Routes: Some bird species, such as sandhill cranes and snow geese, learn migration routes from their parents and other adult birds in the flock. Once learned, younger birds can travel the route successfully themselves.
- Magnetic sensing: Some birds, including pigeons, are able to use the direction and strength of Earth’s magnetic fields to orient themselves. https://www.sciencedaily.com/releases/2018/04/180406091756.htm
What does this mean for spring migration on the Flyway cam? While we are seeing a lot of birds, they aren’t staying very long. Many of them tend to fly long distances at night. During the day, they feed, rest, and take shorter hops along the river. So:
- Try checking in at sunset or shortly after day break to see who’s coming and going.
- Take a look at Explore’s gallery or our forum to see what people are posting. You can scroll back three hours in the video to get a look at the recent past.
• Make sure you have the audio on! Flocks of migrating birds are often noisy. Take a look when you hear them coming in!
Obligate versus facultative migration doesn’t easily divide into neat categories. In his paper ‘Obligate and facultative migration in birds: Ecological aspects (2011)’, Ian Newton lists the following features for obligate and facultative migrants.
- Have a consistent scarcity/absence of food in their breeding areas outside of the breeding season.
- Individuals leave before food supplies collapse, which means their exodus is anticipatory.
- Timing, directions and distances of outward movements are relatively consistent from year to year.
- Individuals behave in the same way every year, usually returning to the same nesting locality each year and often also to the same wintering locality.
- Many species migrate long distances and often travel at night.
- Migration is viewed as being under fairly tight genetic control, usually with limited variation between age and sex groups.
- Migration is not a flexible response to prevailing conditions.
- Have inconsistent food availability in breeding areas outside of the breeding season. Food may be plentiful or scare, but it seldom vanishes completely.
- The proportions of birds that migrate vary from year to year.
- Distances and sometimes timing and directions are highly variable from year to year.
- Individuals vary in behavior from year to year, remaining in some years, migrating short or long distances in others, and often wintering in widely separated areas in different years.
- Birds migrate mainly short distances, depending on year, and often travel by day.
- Migration occurs as a direct response to declining food supplies.
- Migration is under much less rigid genetic control and there are often marked differences in distances between age and sex groups.
- Migration is a ﬂexible response to prevailing conditions.
American White Pelicans are defined as obligate migrators. In the north, they have to migrate because their food is sealed under ice and snow in the winter. However, at least some pelicans migrate short distances, pelicans have been seen on Lakes in the central United States as late as mid-December, and some populations in Texas and Mexico don’t migrate at all. This would tend to indicate more plasticity than obligate migrants are supposed to have. Peregrine falcons at the latitudes we work in (roughly 47 to 40 degrees) are partial facultative migrants, but arctic populations look a little more like obligate migrators. They tend to have longer migrations than their southern cousins, leave before food supplies collapse, and may time their molt around their marathon migrations. But they also migrate during the day, are flexible in routes and timing, and have marked differences in age and sex groups – all characteristics of facultative migrants. One study of European and Siberian Stonechats found that responses to environmental change, range expansion, and novel migration patterns may depend on the particulars of a species’ underlying circannual programming. Given how strongly latitude affects circannual programming, I don’t think it is surprising that at least some species could defined either way depending on the latitudes they nest at.
Things that helped me learn about this topic