Migration season is here at last! Birds you see migrating right now began their migration in late fall, and will migrate back through beginning late winter. Migration is essential for birds, and for birders. Many birders and nature enthusiasts love to view bird migration in action: the Cornell Lab of Ornithology provides resources such as migration forecasts and live migration maps so people can observe bird migration in real time throughout the season.
The term “migration” describes periodic, large scale movements of populations of animals. Migration is a very essential event, especially for bird species. In this post, I will cover the exciting topics of why birds migrate, how they migrate, and what unfortunate hazards birds may face when migrating.
Why do birds migrate?
Birds migrate for two primary reasons: food and nesting locations. As the end of fall draws near, both food and nesting resources become scarce. To combat this, birds will move from areas of low or decreasing resources to areas of high or increasing resources, according to the Cornell Lab of Ornithology. For example, in the spring, birds in the northern hemisphere will migrate northward to refuel on budding plants, abundant insects, and nesting locations. As winter approaches and these resources drop, birds will move back south again. According to the Cornell Lab of Ornithology, there are 4 types of migration:
- Permanent Residents: birds that do not migrate and are able to find adequate supplies of food year around. Examples include Spotted Towhee (Pipilo maculatus), Downy Woodpeckers (Picoides pubescens), Peregrine Falcon (Faclo peregrinus), and American Kestrel (Falco sparverius).
- Short Distance Migrants: birds that migrate only a short distance from lower to higher elevations. Examples include Cedar Waxwings (Bombycilla cedrorum) and American Robin (Turdus migratorius).
- Medium Distance Migrants: birds that cover distances from one to several states. Examples include Western Bluebirds (Sialia mexicana), Long-Eared Owl (Asio otus), and Killdeer (Charadrius vociferus).
- Long Distance Migrants: birds that move from breeding ranges in the United States and Canada to wintering grounds in Central and South America. The Arctic Tern (Sterna paradisaea) is the world’schampion long distance migrant with a round trip journey of about 18,641 miles!
The Use of Navigational Cues
Birds use navigational cues, such as landmarks during the day and the stars at night, to help them migrate. In fact, most birds have their migration genetically coded in what’s known as internal maps. This means that their paths of migration and how to migrate are already within their DNA.
Navigation by Day
Some birds will use the sun as a navigational tool during the day. However, when the sun is not visible, such as on a cloudy day, birds may stray off and end up traveling in the wrong direction. Other birds rely on landmarks to help guide them. Hawks and Eagles flying high in the sky might use mountain ridges, while some landbirds might use the coastline.
Navigation by Night
Most songbirds will choose to travel at night with the stars serving as their navigational tool. One experiment using Indigo Buntings (Passerina cyanea) observed their behaviors when it came to navigating by stars by using a planetarium. The study found that birds inside the planetarium would orient themselves in different directions based on the locations of the stars in the sky. When the map of the stars was rotated to reverse the north and south directions, the birds continued to follow the stars. Although they were following the right directions as indicated by the stars, they would be traveling the wrong direction of earth’s magnetic field – indicating that stars were the guide. Birds such as pigeons use the earth’s magnetic field to navigate.
Navigation by Senses
Interestingly enough, while most birds have a poor sense of smell, there are some birds who will use their sense of smell to navigate. For example, Homing Pidgeon’s (Columba livia domestica) and some sea birds like the Leach’s Storm-Petrel (Oceanodroma leucorhoa) have the ability to locate their nest burrows by smell.
Very often, migration can be a tiring as well as dangerous task for birds due to various migrating hazards. The two most lethal barriers to bird migration are wind turbines and building collisions. Urbanization effects are also raising concerns.
Windmills are highly favored and are used as renewable energy resources,however, according to the Biophilia Foundation, these can be a lethal barrier to birds migrationing. Data from the United States Fish and Wildlife Service (USFWS) reported that turbine collisions were responsible for 140,000 to 500,000 bird deaths per year. Birds tend to collide with turbines due to their placement and design. However, a recent study from the Norweigan Institute for Nature Research found that painting a single wind turbine blade black could reduce bird fatalities by 72%.
There have been some suggestions that urban areas may lack some specific factors, like habitat and sound management, that some migratory species of birds need in order to thrive.
Nocturnally migrating birds move through increasingly polluted skies due to light pollution and urbanization, which can impair navigation cues. In one study, avian behavioral responses to the National September 11 Memorial and Museum’s “Tribute in Light” in New York were monitored by using radar and acoustic sensors. When the memorial was illuminated, birds aggregated in high densities, decreased their flight speed, circled, and increased their vocalizations. This also revealed a high probability of disorientation. When the lights went out, these behaviors disappeared.
One of the most common hazards that birds face are collisions with buildings and other man-made structures. In fact, one study proposed a rough estimate between 100 million and 1 billion birds being killed annually in the United States due to collisions with buildings and windows.
Along with data from collisions with lighted structures, one study that used 40 years of collision records from Chicago, Illinois and Cleveland, Ohio found that nocturnal flight calls were a predictor of collisions with lighted buildings. Species who produced flight calls during nocturnal migration tended to collide with buildings more than expected given their local abundance, whereas those that do not use such communication collided much less frequently. This suggested that a stronger attraction response to artificial light at night in species that produce flight calls may mediate these differences in collision rates. It was predicted that these nocturnal flight calls evolved in order to mediate collective decision making when it came to navigation.
With bird migration being a huge part of a bird’s life cycle, studies are continuing to look for new solutions to improve bird collisions and other migratory hazards. However, it is absolutely amazing how birds are able to travel these far distances and be able to withstand such unpredictable conditions. As fall continues, look to the sky for species such as Cackling Goose, Prairie Falcon, Sandhill Cranes, Northern Shrike, and Ruby-Crowned Kinglet migrating through the Willamette Valley on their way north!
View the Cornell Lab of Ornithology’s migration tool “Birdcast”.