Biology Reference
In-Depth Information
nesting season. If you have a nesting female nearby, she will visit your feeder only peri-
odically, spending most of her time incubating her eggs.
After the eggs hatch, she usually concentrates her feeding at flowers that supply tiny
insects as well as nectar. Insects contain the protein that her nestlings need in order to
grow, and she's regurgitating a slurry of the nectar and insects she's eaten to feed them.
Once the young have fledged, she continues feeding them for several days until the
fledglings have mastered getting their own food.
At this time, she may bring them to your feeders to teach them how to take advantage
ofthiseasyfoodsupply,too.Thisisalsowhenmalesstopdefendingaterritoryandbegin
migrating, with adult females soon following. Many of the hummingbirds that suddenly
appear are actually migrants from farther north, just passing through.
Taking the Night Flight
Q I've heard that many songbirds migrate at night. Why?
A
There are several major advantages to nocturnal travel.
Temperatures are cooler andthe air is moister,protecting them fromoverheating orde-
hydrating.
Winds are often lighter, helping them to conserve energy.
Songbird-hunting hawks are active by day and nocturnal owls don't chase prey in
midair, so migrants are safer from predation at night.
Since songbirds require daylight to see and capture their food, nighttime travel allows
them to spend their days fueling up for the next leg of their journey.
Q How do birds see where they're going when they migrate at night?
A
Theynavigatebysensingmagneticfieldsandbyusingthestarstoorientthemselvesin
therightdirection. Birds,likeus,have“rod”cellsintheirretinas,providingsomelimited
nocturnalvision.And,alsolikeus,theyhaveaneasytimeseeingthestars.Buttheycan't
seeotherobjectsintheskyorbelowthemverywell,particularlyonmoonlessandcloudy
nights.
Researchers have long suspected that some migratory birds can use the earth's mag-
netic field to orient. One classic study in the 1960s involved homing pigeons wearing
tiny metal helmets. Half the helmets were magnetic; the other half were the same weight
and size but not magnetic. On sunny days, all the birds found their way home easily. On
cloudy days, the birds sporting magnetic helmets didn't orient or find their way back, at
least not until the sun came out again.
More recently, researchers have discovered deposits of magnetite in the nasal tissues
ofseveralmigratoryspecies,includingBobolinksandWhite-throatedSparrows,andhave
