Making
Sense of Robin Migration
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American
robins wintering in Canada and the northernmost states? How can that be?
Don't they all go south?
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. |
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A
Few Facts About Robin Migration |
- 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.
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"First"
Robins: Making Sense of Confusing Maps |
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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:
- What patterns
do you notice as the migration progresses?
- What does
it "tell" you about the progression of the migration?
- 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:
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"First
Singing" Robins: A
Clearer Pattern |
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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.)
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