Biology Reference
In-Depth Information
Formanybirds,thenameofthespeciesreallydescribesonlythemale.FemaleScarlet
Tanagers are never scarlet, female Red-winged Blackbirds do not have red wings, and
female Ruby-throated Hummingbirds do not have ruby throats.
Q Do male American Goldfinches change color in winter in the Midwest? Or do they
migrate and leave the females here?
A
Winter male American Goldfinches, like females, are dully colored. The difference
between winter and summer plumages is the most striking in its family, and the spring
moltisuniqueamongfinches.Acrossthecontinent,migratoryactivitypeaksinmid-April
to early June during the spring molt, and in late October to mid-December after the fall
molt. There is a great deal of variation in individual birds — some may migrate some
years and not others. Goldfinches associate in flocks that include both sexes, and indi-
viduals of both sexes migrate or remain residents in the Midwest as they please.
Location, Location, Location: Defending Territory
Q In my local park, I saw two mockingbirds fighting, but I couldn't tell what they
were fighting over. It seemed like there was plenty of space for both of them.
A
When it's time to nest, many birds establish and defend territories, and fight for mates.
To successfully raise young, birds need a safe place for their nest and adequate food to
feed their family. Many bird species defend an area around their nest site, chasing away
otherbirdsthatmightcompetewiththemforfood.Theymayalsochaseintrudersawayto
prevent rivals from mating with their partners. A Northern Mockingbird's territory may
range in size from about 1 to 6 acres (0.4-2.4 ha). Some songbirds have relatively tiny
territories — Ovenbirds and Song Sparrows may require less than half an acre. But song-
birds that require specialized food or food from higher up the food chain require more
space. Scarlet Tanagers can require 5 to 30 acres (2.0-12 ha) or more, depending on the
qualityofthehabitat.LoggerheadShrikesmayrequire75acres(30ha)ormore,andGray
Jays average about 250 acres (101 ha).
In nonsongbirds, too, the size of a territory is affected by the abundance and distribu-
tion of food and nest sites. Bald Eagles in sparsely vegetated Saskatchewan defend territ-
oriesatleast1½squaremiles(3.9squarekm)insize;theminimumsizeoftheirterritories
on Kruzof Island in Alaska, where food resources are more abundant, is only 1/5 square
mile (0.5 square km). Some eagles may fly quite a distance from their nest to fish, so if
thereareabundantfishingwatersinonearea,eaglescanbesomewhatconcentratedwhere
largetreesprovidegoodnesting.Onceaneaglecatchesafish,othereaglessometimesag-
gressively fight to take it, so even though they can share fishing areas, they need at least
some space between one another.Red-winged Blackbirds, bycontrast, vigorously defend
