AMERICAN ROBIN MIGRATION BETWEEN USA AND CANADA

Making Sense of Robin Migration

Many scientists say the American Robin’s spring migration follows the “line” formed by 36-degree average temperatures. (See  Spring Fever lesson.) Yet it’s clearly colder than that where many individuals and flocks spend the winter. Are the scientists wrong?

Robin migration can seem a bit confusing. Take a look at these facts and then think about the questions that follow.

• All robins are not the same: The vast majority of robins do move south in the winter. However, some stick around — and move around — in northern locations.

• Robins migrate more in response to food than to temperature. Fruit is the robin’s winter food source. As the ground thaws in the spring, they switch to earthworms and insects. While the robins may arrive when temperatures reach 37 degrees, this is because their food becomes available not because the robins themselves need warm temperatures.
• Robins wander in the winter: Temperatures get colder as winter progresses. Robins need more food when it’s cold and more and more of the fruit gets eaten. Robins move here and there in response to diminishing food supplies and harsh weather. If all robins wintered at their breeding latitude, there wouldn’t be enough fruit for them all. So robins tend to spread out in the winter in search of fruit. Most hang out where fruit is abundant, but some take the risk of staying farther north where smaller amounts of fruit remain.

• Robins sing when they arrive on territory: Robins sing when they arrive on their breeding territories. Sometimes robins even sing in winter flocks, due to surging hormones as the breeding season approaches. However, in the majority of cases, robins really do wait to sing until they have reached their territory.

February 13	March 20	May 8
First Robins

The three maps above are from the spring 2007 season’s robin migration. (Click for larger versions.) If some robins spend winter moving around northern locations, how can we possibly track their migration? When we look at a Journey North map, we have to pay attention to two things:

• Where the dots are (called “distribution”)
• How many dots are in each place (called “abundance”)

Ask these questions:

1. What patterns do you notice as the migration progresses?
2. What does it “tell” you about the progression of the migration?
3. In what ways is the migration’s pattern unclear?

The migration’s progress is definitely not clear or certain. However, following the real-time maps does reveal where most migrating robins are moving from week to week. You’ll find a step-by-step tutorial for interpreting robin migration maps here:

• For a step-by-step tutorial see: Analyzing Robin Maps

https://youtu.be/IEfosXtJmzI?t=104

February 13	March 20	May 8
First Singing Robins

Because robins sing when they arrive on their breeding territories, our map that shows the first singing robins is always the clearest migration pattern. Robins sometimes sing before and during migration, as they switch to spring migratory restlessness and territoriality from winter feeding and flocking behaviors. However, while some robins may produce their first songs on their wintering grounds, the vast majority wait until they are actually back on territory before singing. (After all, the reason songs work so well for defending territories is that male robins feel stressed when they hear other robins sing, so singing serves to break up winter and migratory flocks.)