I apologize that I have not posted in the last couple months, but we are back in business. I hate to say it, but fall is here which means winter is just around the corner. The geese are gathering and flying together. Blackbirds are forming those flowing black clouds that are so cool to watch (Figure 1).
How do they do it? How do they know when to migrate and where to go?
First, let’s answer one big question. What is a migration? A migration is when animals from one location move in a seasonal pattern to another location. Migratory animals need to move to different location throughout the year to find the resources they need like food and breeding sites.
Out of the more than 650 breeding bird species in North America, more than 50% of those are migratory. Some species only move short distances like changing elevation along a mountain according to the season. Others travel a little further across several states. However, some birds travel far into South America for winter1. It all depends on their needs and finding a place that meets them. (Please see the video below.) The Mississippi River alone acts as a corridor for over 40% of those migratory species. Bird watchers from around the country come to the Mississippi River to see the huge variety of birds that travel the flyway (Figure 2). So how do the birds know when and where to move?
This is a more difficult question to answer than you might think. There are entire fields of science that are dedicated to understanding the evolution and genetics of migration. How a bird knows it’s time to migrate depends on the species, and not all mechanisms are known or understood. More research needs to be done, but some migrations are triggered by a combination of day length, changes in temperature, and/or changes in food supply. Populations of the same species may migrate at different times1. For example, birds further north are going to start noticing changes in daylight and temperature sooner, and therefore begin their migration sooner than their cohorts that may be further south.
Once a bird begins to migrate, it has to be able to navigate across thousands of miles and find places to rest and refuel throughout the trip (Figure 3). That’s no easy feat! Birds use a number of different cues and senses to navigate long distances. Click here to watch the journey of birds during their migration.
Birds can use information from the sun, stare positions, the earth magnetic field and landmarks along their journey1 (Figure 4). A bird’s eye and brain work together along with tiny bits of iron in the bird’s inner ear to determine which way points north.
This magnetic north can also be sensed by other nerves that help the bird determine the strength of magnetic fields which are stronger at the poles and weaker at the Equator. There is also evidence that the nerve connections between the bird’s beak and brain help it pinpoint its location2. Incredible! To learn more about bird migrations, I encourage you to watch the following videos:
On this journey, birds face a number of challenges. Imagine a little bird only 5 inches long weighing about as much as a pack of gum flying 5,000 miles from Minnesota across the Gulf of Mexico and down to Venezuela3. That’s what the Prothonotary Warbler (Figure 5) does every year! I struggle doing a 5K. This little bird puts me to shame. As this bird travels though it must be able to find sufficient food and shelter for the trip. It has to deal with dangerous weather and increased predation risk. Recently, migratory birds face a growing threat from an unlikely source, buildings and communication towers. Many birds are attracted to the lights and can become disoriented by the tower lights that may look like strange, new stars and disrupt their navigation. Millions are killed each year by collisions with structures like glass windows1. Read more on this problem.
Other threats to our migratory birds are deforestation, loss of wetlands and coastal zones, wind turbines, and house cats of all things. Migratory animals are particularly susceptible to habitat loss because they require multiple sites that can be far away and under various government entities. Songbirds’ homes may be protected in Illinois, but their winter site in the Amazon may not be or their stop-over sites might disappear. Thankfully, they do have some protection under the Migratory Bird Treaty Act. Read more about it.
If you would like to help migratory birds that may nest or move through your yard or town, there is plenty you can do to make a difference. You can join the American Bird Conservancy and/or become an Audubon Society member. You can even set up your own celebration of World Migratory Bird Day Saturday, May 13, 2017. Learn how you can participate and spread the word and action to help migratory birds.
I hope you are as inspired by our fearless, feathered travelers as I am. If the Prothonotary Warbler can fly 10,000 miles a year, then we can surely do something to help them get where they are going!