Birds of Paradise

The birds of paradise are some of the most fascinating birds in the world. This is due to the striking coloration of the males of most species, and the wide range of behaviors demonstrated in the group. Researchers of animal behavior are particularly interested in the elaborate mating displays performed by male birds of paradise.

Birds of paradise are members of the family Paradisaeidae, which probably evolved on the island of New Guinea. The family is comprised of 43 species, 38 of which are found mainly or entirely on New Guinea. Two species are found only in the Moluccan Islands to the west of New Guinea, and four others are found mainly or entirely in northeastern Australia. Included within the family are such birds as astrapias, manucodes, paradisaeas, parotias, riflebirds, and sicklebills.

Polygynous birds of paradise

As mentioned above, many species of birds of paradise are sexually dimorphic, meaning the males and females have different appearances. The males have elaborate plumage patterns, which are used in their mating displays. The females of these species are drab and cryptic. Sexually dimorphic species are usually also polygynous, meaning the males may mate with more than one female. To attract a female, a male may perform a mating dance on the ground while conspicuously displaying its bright plumage and calling loudly, or it may display while perched on a shrub, or while hanging upside down on a tree branch. Males may perform these displays alone, or in competitive groups in a place called a lek. The females watch the displays and choose which male to mate with. The female choice appears to be based on the vigor of the display of the male, and the condition and color of his feathers. By choosing a vigorous mate, the female presumably ensures that her offspring will also be relatively healthy. Therefore, the strongest, most brightly-feathered males have a better chance of being An Emperor of Germany's bird of paradise in Baiyer River Sanctuary, New Guinea. Photograph by Tom McHugh. National Audubon Society Collection/Photo Researchers, Inc. Reproduced by permission. chosen as a mate by females, while less attractive males may be passed over. The elaborate plumage of the males is thought to have evolved through this evolutionary process of sexual selection (i.e., females choosing mates on the basis of their desirable behavioral and anatomic traits, including color). After mating, the female returns to her nest and raises her offspring alone.

Researchers have noticed a relationship between the mating system and diet in the birds of paradise. Polygynous bird of paradise species that display in leks (such as the raggiana bird of paradise) tend also to eat mainly complex fruits. This is thought to be because females searching for these fruits fly long distances in the forest, and thus are likely to encounter groups of males displaying together. Polygynous species in which solitary males display (such as the magnificent bird of paradise) tend to eat insects plus complex fruits. To get insects, females need not fly long distances, so a male is more likely to be seen and chosen as a mate if he displays alone near the small home range of the female.

Interestingly, the polygynous birds of paradise also show sexual bimaturism. This means that males and females become sexually mature at different ages. Females of these species are thought to begin to breed when 2-3 years old, while males do not acquire mature plumage (and do not breed) until age 4-7 years. However, males of these species will grow adult plumage at a younger age when kept alone in captivity. This suggests that the delay in male maturation in the wild is due to hormonal suppression related to the presence of already-mature adult males.

Monogamous birds of paradise

Nine species of birds of paradise, including the manucodes, are sexually monomorphic. The males and females have coloring that is the same or nearly so (they tend to be brown or black), and both lack the elaborate plumage that characterizes most other birds of paradise. These species are monogamous, meaning the males and females mate with only one partner at a time, and in some species pair for life. As in most monogamous species, the males help the females raise the young.

These species typically feed mainly on simple fruits, such as figs. This type of fruit is relatively low in nutrients, compared to complex fruits and insects. Scientists think that the monogamous mating system may have developed in these species because two parents are necessary to provide enough nutrition to raise the young. Thus, in the birds of paradise, it appears that the diet of a species has influenced the evolution of its social system.