What Causes Birds to Start to Migrate

Each spring and fall there are billions of birds that migrate. But, what is it that signals to this birds that it is time for them to move?

Sure, we all understand that birds migrate, but what exactly is it that makes them know that it is time to start their journeys. The answer, it turns out, depends on the species.


Some birds are just genetically predisposed to make a migration. Their instinct to migrate has been passed down generation after generation and they just know when they should go. These species also tend to just know where they should go as well.

What Causes Birds Start to Migrate©Rob Ripma
©Rob Ripma Blackpoll Warbler
Length of Day

As the days get shorter, many birds realize that it is time for them to begin their migrations. This has been documented in caged birds as well. Although they can’t go anywhere, in the fall, as the days become shorter they grow restless and move toward one side of their cages.

Diminished Food Source

As a bird’s food source begins to diminish, this will signal to the species that it is time to migrate. Many birds that we would consider to be irruptive species choose to migrate based on their food source.

What Causes Birds Start to Migrate©Rob Ripma
©Rob Ripma White-winged Crossbills are an irruptive species.
Dropping Temperatures

As temperatures begin to drop and the season change, some birds get the hint that it’s time for them to move south.

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Rob Ripma
Rob Ripma, a lifelong Indiana resident, has traveled and birded extensively throughout the Americas.