Biology Reference
In-Depth Information
A
Migration in birds probably evolved many times, for many reasons. The northern tem-
perate and boreal zones are vast and rich with insects; they have fewer marauding insects
such as army ants, and fewer poisonous snakes and spiders than are found in the tropics.
The northern latitudes also have significantly longer summer days than tropical zones,
providing more hours for feeding young each day and significantly reducing the amount
of time vulnerable young remain in the nest.
BLOWN OFF COURSE
Although the mechanisms of bird orientation
and navigation are amazingly in-
tricate, some storm systems are powerful enough to send migrating birds far off course.
Individual North American songbirds, including several species of warblers, Dark-eyed
Juncos, White-throated Sparrows, Scarlet Tanagers, and Rose-breasted Grosbeaks, have
beencarriedallthewayacrosstheAtlantic,especiallytoEnglandandIreland,frequently
enough to be included in some field guides to the birds of Europe. A few Siberian birds
appear with some regularity in North America, especially Wheatears and Bramblings.
Andsomebirdsseemtohaveacompassthatcanbecomesodisrupteditentirelyreverses
their migration, such as Fork-tailed Flycatchers from South America that have turned
up dozens of times in North America. To see the migration routes of warblers and other
birds, go to
http://bna.birds.cornell.edu/bna/species/431/articles/migration.
Some birds breeding in even northernmost North America may well have originated
as tropical residents who, to escape competition and take advantage of huge insect pop-
ulation explosions in the north, started heading to the vast northern landscape to breed,
retreating “home” to the tropics again after the breeding season. A full 78 percent of
all northern migrant species, including flycatchers, swallows, vireos, wrens, and orioles,
have close relatives in the same genus or even the same species that are year-round Neo-
tropic residents.
Some non-migratory populations have probably become migratory as conditions
changedforthem.Oneexamplethatwe'vebeenabletoobservefirsthand:non-migratory
House Finches from the Southwest were released in New York in the early 1940s. Within
20years,someofthesebirdshadbegunmigratingtotheGulfStatesforthewinterandre-
turning north to breed. Other individuals remain year-round in the Northeast. In just two
decades this species has become a
partial migrant.
Q I've heard you can tell if birds are migrating on a particular day by looking at a
weather map! How does that work?
