The most basic definition of a species is a group of biological individuals who can interbreed. Using DNA, and morphological and behavioral studies, birds are placed into categories based upon the closeness of their relationships. Carl Linnaeus was a Swedish botanist, physician, and zoologist, who formalized the modern system of naming organisms called binomial nomenclature. And from there were created various groups such as Kingdom, Phylum, Class, Order, Genus, and Species and many subgroups such as tribes and subspecies to indicate organisms’ lineages and relationships. The species is the lowest category and distinguishes the group of interbreeding individuals.
Hypothetically, two different species cannot interbreed, but the reality is different. “Many birds occasionally mate with members of other bird species, producing hybrid offspring,” said Irby J. Lovette, director of the Fuller Evolutionary Biology Program at the Cornell Lab of Ornithology. Dr. Lovette notes that perhaps 10 percent of the world’s 10,000 bird species have interbred with other species at least once, either in the wild or in captivity. Actually, the Avian Hybrids Project estimates that at least 1714 out of 10, 446 bird species (16.4%) have hybridized. If one includes hybrids that occurred in captivity, the figure rises to 21.1%. There is actually a Handbook of Avian Hybrids of the World.
The reasons two different species cannot interbreed are varied: mechanical differences in sex organs, different courtship behavior, genetic incompatibility, infertile offspring, etc. Some of these barriers break down in captivity and give us Ligers (lion and tiger hybrids) or mules (female horse and male donkey). Among birds there are chicken/pheasant and pheasant/turkey hybrids, swan/goose hybrids (a “swoose”), and Mallard/Northern Pintail crosses.
Recently it was discovered that the Golden-crowned Manakin of the Amazon rainforest is a new species that arose from the hybridization of the Snow-capped Manakin and Opal-crowned Manakin. DNA studies showed that about 20 percent of the Golden-crowned Manakin’s genome came f rom the Snow-capped Manakin and 80 percent from the Opal-crowned Manakin. Apparently the hybridization first occurred about 180,000 years ago. The Italian Sparrow, Audubon’s Warbler, and a ground finch of the Galapagos were also results of hybrids of two different species.
Hybrids are common in the plant world, but fairly rare, ornithologists thought, in the bird world. Turns out they are more common than we realized.
Joel Cracraft, an evolutionary biologist and ornithologist at the American Museum of Natural History in New York recently headed a study to define bird species as any group that exhibited a unique set of straits, regardless of whether it could mate with any other ‘species’. They ended up identifying more than 18,000 bird taxa, nearly twice as many as we recognize now. This ought to make birdwatchers happy but should it come to fruition, it would really tear all sort of studies and conservation laws asunder.