Sometimes it feels like spring comes overnight. As if it suddenly turns from the last, cold, snowy day of winter to a beautiful 60 degree day with the first blooms appearing, trees leafing out, and scattered rain showers. As you probably know, spring doesn’t actually arrive overnight; it takes trees months of preparing their buds to open and flowers remain dormant through the winter or sprout from seeds. However, one part of spring does happen overnight; the arrival of migrant songbirds. While many larger birds such as hawks migrate during the day (sometimes forming groups of hundreds to thousands of individuals called kettles) and are easy to study at hawk watches, many of our songbirds migrate at night instead of during the day.
Migration is a term encompassing any seasonal movement of birds – usually from their breeding grounds to an area where they spend their winter. These movements are largely due to lack of food resources, not stress from cold temperatures. In fact, most of our North American songbirds evolved to migrate north in the spring to access the greater food abundance (largely insects for songbirds). Regardless, migration is a stressful activity, requiring a number of adaptations, including altering metabolism, storing energy as fat, and replacing high-use feathers by molting.
Length of migration is highly variable and the route migrants take is often species specific. Some migration movements are small – hardly detectable by the untrained observer. Other species move the distance of a few states during the winter, returning north to breed in the summer. Long-distance migrants may migrate from the boreal forest to the tropics, and sometimes even further! For example, Arctic Terns travel as far as 24,000 miles round trip traveling from northern Canada, where they breed, to Antarctica where they spend their winters.
Many of the birds migrating these longer distances (during the day or at night) are referred to as neotropical migrants. The prefix “neo” roughly means new in Greek – often used to denote their occurrence in the “New World” (the Americas) – while “tropical” refers to the region between the Tropic of Cancer and the Tropic of Capricorn. Neotropical migrants spend their summer breeding in North America and their winter in Central America, South America, and/or the Caribbean. Most warblers, hummingbirds, and shorebirds are neotropical migrants, as are some hawks and other songbirds. A total of 386 bird species are defined as neotropical migrants by the Neotropical Migratory Bird Conservation Act. One of the most extreme examples of migration is the Blackpoll Warbler (Setophaga striata), a neotropical migrant which weighs approximately 12 grams – about the weight of 12 paper clips! In the fall, this species accomplishes a non-stop flight ranging from 72 to 88 hours, while they travel from the northeastern United States directly to Puerto Rico, the Lesser Antilles, or South America! To accomplish this feat, these warblers frequently double their normal body mass.
Nocturnal migration requires a species to feed during the day and give up their sleep at night. This is in exchange for safer passage to their breeding grounds. Migrating at night helps minimize their exposure to hot temperatures, preventing overheating, as well as limiting their exposure to daytime predators (which are more abundant than nocturnal predators) during their flight. Birds use a variety of cues during migration to orient themselves, including magnetic fields, visual landmarks, olfactory cues (scent), and sun direction during the day. There’s also been evidence that birds can use stars and the moon to help navigate at night. It’s been shown through satellite tracking that older, more experienced birds tend to navigate better and correct for factors like strong winds with greater success.
Studying the migration of neotropical migrants can be a difficult task, especially for nocturnal migrants. To accomplish this, scientists use a variety of techniques. The most prevalent method is ringing or bird banding, which was developed in 1899 by Hans Christian Cornelius Mortensen. Since the establishment of the procedure, migration stations have been set up worldwide to catch and mark migrant songbirds. Each bird is marked with a band representing a unique number before releasing the bird so individuals can be tracked. During this process the banding stations collect data on the captured birds including information such as species, age, sex, size, and condition. Recovery rates (the numbers of birds recaptured the following years) of banded birds tend to be low at migration stations. This is because most birds don’t follow the exact same route annually. However, there are many notable recoveries of birds on wintering grounds, at other banding sites, and on breeding territories. Techniques such as satellite tracking have also provided direct understanding of routes birds take during migration and are often combined with banding practices.
Other techniques that don’t allow for tracking of individual birds, but allow for a better understanding of migration includes isotope analysis. This method looks at the ratios of stable isotopes such as hydrogen, oxygen, and nitrogen in different parts of a bird (including feathers, muscles, and blood). By looking at these ratios it is possible to “trace” where a bird has been and connect wintering sites to breeding grounds.
In recent years, recordings of nocturnal calls have helped assist scientists in better understanding density and direction of migration, but this technique doesn’t allow for tracking of individual birds. Birders and scientists can go outside, on a calm night, and hear these calls, but full analysis is often not possible without recordings and prior knowledge.
There are a few other practices the average observer can use to track migration. Recording arrival dates of spring migrants is one of the oldest techniques, tracing its roots to Johannes Leche of Finland in 1749. Observers can record this information in databases maintained by the Cornell Lab of Ornithology (eBird) or in local bird records. Another technique anyone can use to track migrations is analyzing reflectivity measurements on Doppler radar. Reflectivity measurements are designed to use radio waves to reflect off water particles at cloud height and bounce off of essentially anything. When migrating, most birds travel at cloud height so these radio waves actually detect birds! With a little background in reading Doppler imaging, anyone can use these reflectivity measurements to see when birds are migrating. Check out this 10 minute tutorial here for more information on using Doppler radar to track birds. Similar to nocturnal flight calls, radar imaging is limited as it can’t track individual birds. Unlike nocturnal flight note studies, radar can't provide identification of migrating bird species. Perhaps the easiest and most beginner-friendly way to observe nocturnal migration is watching birds fly by the moon (basic magnification is needed). On good nights, one can see dozens of birds flying by in streams in just a few minutes!
- Lauren Kras received a M.S. in Plant Biology from the University of New Hampshire where she studied rare, threatened, and endangered plant species. She is the President of the Seacoast Chapter of New Hampshire Audubon and the co-founder of the Odiorne Point Banding Station (OPBS) where she is conduction research on bird migration. Lauren continues to teach general and marine biology at the University of New Hampshire as well works as a conservation easement steward at the Society for the Protection of New Hampshire Forests.
Check out these sources for more information:
All About Birds. 2014. Web: http://www.birds.cornell.edu/netcommunity/page.aspx?pid=1636.
Cochran, W. W. 1987. Orientation and other migratory behaviors of a Swainson’s thrush followed for 1500 km. Anim. Behav. 35: 927-929.
Farnsworth, A., Gauthreaux, S.A., and D. van Blaricom. 2004. A comparison of nocturnal call counts of migrating birds and reflectivity measurements on Doppler radar. Journal of Avian Biology 35(4): 365–369.
Graber, R. R. and Cochran, W. W. 1959. An audio technique for the study of the nocturnal migration of birds. Wilson Bulletin. 71: 220-236.
Greenwood, Jeremy J. D. 2007. "Citizens, science and bird conservation". Journal of Ornithology 148: S77–S124.
La Puma, D. 2014. Web: www.woodcreeper.com.
Sibley, D.A., Elphick, C., and J.B. Dunning Jr. 2009. The Sibley Guide to Bird Life and Behavior. Knopf Doubleday Publishing Group. 608 pp.
Thorup, Kasper, Thomas Alerstam, Mikael Hake and Nils Kjelle. 2003. Bird orientation: compensation for wind drift in migrating raptors is age dependent. Proceedings of the Royal Society Bulletin 270: S8–S11.
Weidensaul, S. 1999. Living On the Wind: Across the Hemisphere With Migratory Birds. Douglas & McIntyre. 420 pp.