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Biodiversity Blog

 

Fall Migration of Birds, Texas and Beyond

Prairie warbler Setophaga discolor paludicola male J
 Prairie Warbler (Photo: Charles J. Sharp -CC BY-SA 4.0)

Signs of autumn are all around. The weather is cooling and the days are getting shorter. It’s also a time of migration for some birds. Of Texas’ 615 documented species of birds, about half will migrate. Through the course of the season, millions of birds will pass through the Lone Star State on their way to warmer southern climates, or some will stay here for a while.

Why do birds migrate? First, not all birds do. The reasons why and where they go are largely about nesting resources and food. Birds need abundant food supplies because of their high metabolic rate. In the Northern Hemisphere when the weather changes, birds move north in the spring and south for the winter. Milder regions provide more food options, but also more daylight hours to find the food they and their nestlings need. Fall migration spans a longer time range, late August into mid-November. Spring migration is shorter, about four weeks from mid-April to mid-May. Areas with more severe climate see a larger percentage of bird migration. For example, many nesting species migrate from Canada to the Gulf Coast States for winter.

As with many things about bird migration, the mechanisms that trigger it are not completely understood, but include temperature changes, alteration in food supply and day length, as well as genetic predisposition. Migratory birds have some adaptations that make them different from birds that do not migrate. They can build fat stores as energy sources. They also weigh less and have longer and more pointed wings.

CentralFlyway 
From "Migration and the Migratory Birds of Texas: Who They Are and Where They Are Going" Texas Parks and Wildlife

Migratory birds follow specific routes defined by geography, altitude, and they can alter their course depending on weather conditions they meet during their journeys. Some birds make long distances, sometimes crossing oceans, while others may go down a mountain just a few miles. There are several primary routes birds typically use, and one is right over Texas: the Central Flyway.

Migration can be particularly dangerous for birds, as it involves challenging journeys that can be negatively affected by weather, predators, and human interference. There are also the marathon lengths some birds must go. For some examples, the Ruby-throated (Archilochus colubris) Hummingbird flies about 600 miles across the Gulf of Mexico nonstop. Quite a trip for a tiny bird. The American Golden-Plover (Pluvialis dominica) will assemble in southeastern Canada, then begins its nonstop flight over the Atlantic Ocean to Brazil, traveling about 2400 miles at the incredible speed of 60 MPH. But perhaps most impressive is the Arctic Tern (Sterna paradisaea). It breeds in the northern most regions of Asia, Europe, and North America, then for winter, will head to the Antarctic ice pack 11,000 miles away.

During flight, sudden changes in weather that cause fog, heavy rains or winds will often stop migrating birds in their path. If the birds happen to be flying over a body of water, like the Gulf of Mexico, this will drown many. In fact, millions drown at sea during migration.

Birds will group during their migration, even when they are normally solitary during other times. Traveling in groups provides additional protection as it’s difficult for a predator to hone in on one individual. How birds navigate is still quite a mystery, but researchers believe they use different abilities to do it. They can orient themselves by using landmarks, or detect when airmasses have a temperature shift. Birds also can fly in a constant direction, and the assumption is they have a sensitivity to the earth’s magnetic field. They also use the sun and stars for orientation.

Goose
Bar-headed Goose (Photo by DAVID ILIFF. License: CC BY-SA 3.0) 

There are other ways they attempt to safely reach their destinations, and this includes flying at higher altitudes. The record for that would be held by the Bar-headed Goose (Anser indicus), which flies at 29,500 feet above sea level over the Himalayas in India. However, most migratory birds usually fly below 7400 feet.

Some birds also migrate nocturnally. The cover of darkness provides extra protection from diurnal predators. The weather also is typically more stable and cooler, which minimizes overheating and dehydration. As most birds need daylight to find for their food, they will forge during the day, gain the energy they need, then travel at night. However, as these birds use the stars for navigation, light pollution from human activity can disorient them, making the normally-risky trip even more perilous.

Humans also present other challenges to migratory birds, the largest being clearing of land for agriculture or construction. Without their normal stopover habitat, the food resources they desperately need are not available. Tall structures like cell phone towers and buildings with reflective glass also kill birds. Other urban hazards are domestic cats that roam outdoors. With all of these factors, it is amazing birds survive migration at all.

As part of the Central Flyway, Texas is valuable for migrating birds. Researchers at the Caesar Kleberg Wildlife Research Institute at Texas A&M University-Kingsville conducted a three-year study evaluating the movements of migratory birds along the lower Texas coast. This area is relatively undeveloped and provides a stopover for birds, so it sees very high rates of passage. However, this area is also heavily targeted for wind turbine development, which would pose risks to birds on the move.

This three-year study also revealed an interesting difference between the migrations happening in fall, versus the one in the spring: the migration rates are 157% higher than in the spring. This has three probable causes: there are new fledglings in the population, winter and the higher bird mortality it brings has not yet come, and the fall route is different than the spring one.

This means that if you are interested in seeing what birds are passing through and want to track bird migration in Texas, this is a great time of year to do it. Luckily, the internet has helped researchers and citizens learn about migration, and create new tools. Just a few resources include:

LIGHTS OUT TO HELP BIRDS

Of all forms of pollution, we typically don’t think about how light is one. By turning off our outdoor lights, we each can help birds complete their dangerous journeys during migration. Learn more here.

Wallace TeX

Image: Tim Wallace

eBird: A joint project by the Cornell Lab of Ornithology and Audubon, eBird is a free online program that allows birders to track their sightings, while other birders watch and search in real-time. https://ebird.org/home

Birdcast: BirdCast provides realtime info on when, where, and how far birds will migrate in the continental US. https://birdcast.info/ There are forecast maps. You can also follow Birdcast on Twitter to learn about the latest @DrBirdCast https://twitter.com/drbirdcast?lang=en

Merlin Bird ID app: need help trying to identify a bird? Use this app and answer three simple questions and it will come up with a list of possible matches. https://merlin.allaboutbirds.org/

Texas Parks and Wildlife: Resources abound here, including maps, birding locations, events and activities, and info on bird behavior and conservation. https://tpwd.texas.gov/huntwild/wild/birding/index.phtml

U.S. Fish and Wildlife Service: They offer a variety of information from surveys and data, to resources for educators, to info for those just getting started with birding. https://www.fws.gov/birds/bird-enthusiasts.php

SOURCES

McAllister, Caroline “Why migrating birds need Texas so badly.” July 5, 2020. The Dallas Morning News.

Shackelford, C. E., E. R. Rozenburg, W. C. Hunter and M. W. Lockwood. 2005. “Migration and the Migratory Birds of Texas: Who They Are and Where They Are Going.” Texas Parks and Wildlife PWD BK W7000-511 (11/05). Booklet, 34pp.

“The Basics of Bird Migration: How, Why, and Where” January 1, 2007. All About Birds: Cornell Lab of Ornithology. (accessed online: https://www.allaboutbirds.org/news/the-basics-how-why-and-where-of-bird-migration/)

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