Biology Reference
In-Depth Information
may travel as far as 1,800 miles (2,897 km) between resting and feeding stops during mi-
gration. Some Pacific Golden-Plovers migrate between western Alaska and the Hawaiian
Islands, an overwater journey of more than 3,000 miles (4,827 km). Sooty Shearwaters
have the longest migration on record: they travel about 40,000 miles (64,360 km) each
year, traversing the Pacific Ocean in figure eights.
Q How do birds know when to migrate?
A
From the time they hatch, migratory birds apparently respond instinctively, growing
restless as the day length and angle in the sun change in spring and fall. Ornithologists
have termed this migratory restlessness
Zugunruhe.
Even individual migratory birds that
have been hatched and reared in captivity experience this urge.
Byrespondingtodaylength,birdsarriveontheirbreedinggroundsatthebestaverage
time, regardless of weather conditions wherever the bird has wintered. In fall, this rest-
lessness helps ensure that the journey will take place while rich food resources are most
likelyavailable,or,inthecaseofbirdsthatmustcrosslargebodiesofwaterwithoutstop-
ping, that they consume rich food supplies before starting out. Many people assume that
their feeders will entice birds to remain too long, but
Zugunruhe
ensures they do not.
Althoughthismigratoryrestlessnessprovidestheurgetomove,conditionsfromdayto
day may provide the impetus to send birds packing. In spring, Neotropical migrants usu-
ally wait for northerly storms to pass over rather than flying into headwinds. Birds win-
tering in the southern United States often begin moving north with good weather. Robins
and geese seem to follow the 37-degree
isotherm
(the “line” visible on a weather map
where temperatures average 37°F [3°C]), from which they can backtrack when a sudden
deep freeze puts them in danger.
Spring arrival dates for these species can vary widely from year to year. Birds winter-
ing in the tropics can't predict what weather will be like in the north, so their flights are
morepreciselytiedtodaylength.Oneconcernforthemwithcontinuedwarmingtrendsis
that insect emergences are now beginning earlier, coinciding with warm weather. Mean-
while, the Neotropical migrants arrive and begin nesting and raise their young at closer
