Biology Reference
In-Depth Information
CHAPTER
8
The Birds and the Bees: How Birds Reproduce
Since earliest history, eggs have served as evocative symbols of new life and rebirth.
We humans turn to the avian world to describe our own “broodiness,” our urge to
save up a “nest egg,” and how, when our children leave, we have “empty-nest syn-
drome.” When June, and eggs, are bustin' out all over, we're filled with joy and won-
der. And questions.
The Mating Ritual
Q The song says, “Birds do it. Bees do it. Even educated fleas do it.” But what exactly
is it that birds “do”?
A
Before mating, birds often court one another with ritualized displays. For example, a
pair of Whooping Cranes will face each other and dance, leaping into the air, bowing, and
flapping their wings. Red-tailed Hawks may grasp one another's talons in flight and spiral
together toward the ground. Male songbirds sometimes present their mate with a choice
insect or berry. Mallards bob their heads up and down, the pair moving their heads in op-
posite directions so that one is bobbing up while the other bobs down.
Whenreadytomate,inmostcases,themalebirdalightsonthefemale'sback.Thebirds
twist their tails so that their
cloacae
(the common chamber where the urinary, intestinal,
and genital tracts open) meet. The sperm pass from the male's cloaca to the female's. As
the fertilized egg works its way down the female's oviduct, the cells along the way secrete
the proteins that make up the albumen, and then secrete the calcium that will form the
shell. Usually by early the next morning, the female lays the egg. Females have one func-
tional ovary and in most species ovulate once every day or two, so the laying period lasts
at least as many days as there are eggs in a clutch.
Duringthedaysorweeksbetweenstartingnestbuildingandfinishinglayingaclutchof
eggs,apairofbirdsmaymateseveralormanytimes,andspermcanremainviableinsidea
female often for more than a week. If her mate is killed, she may be able to raise young on
her own, though normally she replaces her lost mate with another male within a day or so.
In recent decades, ornithologists have been discovering that many species have “extra pair
copulations.” Even when female birds are socially bonded to their mate, they sometimes
mate with another male once or twice, and often the young birds in a single brood have
more than one father.
