BIRDS NUMBER IN THE THOUSANDS
by Whit Gibbons
April 30, 2006
Someone wrote to Marilyn vos Savant asking whether crossing a blue jay
with a cardinal was genetically feasible. The inquirer was interested
in producing a new national bird, though whether he wanted a purple bird
or one with blue wings and a red body, I don't know. But whatever color
a blue jay/cardinal mix might be, Marilyn's answer was that the birds
"belong to different species, so they won't crossbreed." She
is right about blue jays and cardinals--no crossbred specimens are known.
But she is wrong about species not crossbreeding simply because they are
different species. The phenomenon is called hybridization, and literally
thousands of documented records exist for birds.
of Avian Hybrids of the World (2006; Oxford University Press) by Eugene
M. McCarthy is my source for asserting that Marilyn's statement that different
species cannot crossbreed is just plain wrong. In fact, "crossbreeding"
between two species is included among the definitions for the term "hybridizing."
Almost everyone is familiar with one of the most common animal hybrids,
mules, the result of matings between horses and donkeys, two distinct
species. A goal of the Handbook is an attempt to list all the crosses
between different species of birds ever reported, whether in the scientific
literature or on the Internet. The crosses include ones occurring naturally
in the wild as well as the numerous ones between captive species.
states that one of his primary interests in hybridization is its "role
in evolutionary processes." Evolution is of course occurring constantly,
as genetic combinations change from one generation to the next. But can
hybridization be responsible for the formation of offspring that are sufficiently
different from either parent that they constitute a viable new species?
Mules are sterile and cannot propagate more mules. Only horses and donkeys
can do that. But research within the last three years on sunflowers has
provided scientific proof that hybrids can result in offspring that become
their own species, not only genetically and functionally different from
either parent but also able to reproduce.
hybrid birds, McCarthy identifies three primary causes of hybridization.
One is imprinting, a common trait among birds in which newborn and young
juveniles fixate on the first object they see, normally a parent, and
ultimately are programmed to mate with that species (which under normal
circumstances would be their own). But their own species is not always
the first one a young bird sees, particularly in captive situations where
one species of bird might serve as the foster parent of another. Because
of mixed signals during mating, a bird that has imprinted on another species
might attempt to mate with one of that species rather than one of its
own. If the two species are genetically similar enough to successfully
produce offspring, but genetically dissimilar enough to be distinct species,
the young will be hybrids.
situation is one in which the urge to mate overrides the restriction of
selecting one's own species. Captive birds in which only one individual
of a species is present will sometimes mate with other species, producing
hybrids. Crossbreeding because of the lack of same-species mates can even
occur in natural situations, such as when storm-blown birds end up as
the only one of their kind in an isolated area.
A third commonly
proposed cause of hybridization among birds and other animals is that
human disturbance of natural habitats places individuals of different
species in contact with each other when they would not be so naturally.
However, McCarthy does not consider habitat disruption to be a valid cause
of hybridization among birds. Instead, he finds that hybridization in
birds occurs much more commonly in natural settings with no human disruption
than previously believed. Therefore, invoking human alteration of the
environment is not necessary for explaining hybridization.
that produce hybrids are generally between closely related species; for
example, wood ducks have successfully interbred with more than three dozen
species of waterfowl, including mergansers and geese. But the phenomenon
of hybridization among birds is worthy of consideration by biologists
as a natural phenomenon that can affect ecology and evolution. Whether
we will ever end up with a new national bird as a result of crossbreeding
is doubtful, though not impossible.
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