Biology Reference
In-Depth Information
What made the birds so unafraid of predators, nonhuman or human? Bruce's field notes
from the expedition indicated relatively few hawks, falcons, and eagles during his bird sur-
veys. Perhaps their densities were too low to provide much of a threat to the displaying birds.
Then, too, there were none of the midsized and smaller wild cats that frequently prey upon
birds west of Wallace's Line. As for the human influence, one might expect much more se-
cretive behavior if hunters regularly passed through. Finally, if such breeding displays are
genetically hardwired, the birds can't help themselves from becoming momentarily oblivious
to everything else. In the desire to breed with a female, what Darwin termed “Nature's urge,”
being a shy wallflower conferred no selective advantage. A male had to step out on the dance
floor if he wanted to mate and pass on a copy of his genes to future generations.
Birds of paradise have fascinated indigenous peoples and scientists alike, and those on
Bruce's team were no exception. The sixteenth-century Spanish explorers who named the
species they encountered believed that these birds were emissaries from heaven. Some
Europeans in the nineteenth century even assumed that birds of paradise were ethereal, leg-
less creatures that never touched ground. Birds of paradise are native to New Guinea and sur-
rounding islands, with only a few species resident in northern Australia. Many in this family
of forty species dazzle biologists with their bright, metallic wirelike feathers, brilliant gor-
gets (throat patches), and elaborate tails. Their mating dances, full of shakes and shimmies,
put those barbs and bristles in best light to advertise their fitness to interested females. Today
most species are not threatened and some are widespread, while others fit the definition of
rarity by occupying slivers of altitudinal ranges along the flanks of the steep mountains.
Back at camp, Bruce thumbed through his own field guide to explain the evolution of this
group to the several Papuan students gathered round. There are good reasons why the story
of these spectacular birds illustrates so well the links between evolution, narrow ranges, and
rarity.
Birds of New Guinea
indicates the tremendous variation in the most obvious traits of
plumage color and size, as well as in less prominent features such as beak length and size of
feet.
Evolution is not sorcery, but the transformation of a single bird of paradise ancestor into
these forty wondrous variations is magical just the same. Biologists, though, use a different
term for “abracadabra”—adaptive radiation. This is the evolutionary phenomenon by which
a number of species evolve from a single ancestor, often diverging to occupy different eco-
logical niches and isolated geographic spaces. Genetic evidence shows that birds of paradise
probably descended from a crow-like bird about 28 million years ago when the bird of para-
dise line split off from the ancestors of the crows (the so-called corvine assemblage that arose
in the Australian region). In the absence of primates and other bird groups as predators, birds
of paradise spread their wings, so to speak. They flew to new places and then changed in
certain traits, often in the presence of and in competition with other resident species. Once
reproductive isolation from members of their species they had left behind had been estab-
