Now we know that 9 percent of bird species breed cooperatively and share parental care, more than nine hundred species, including acorn woodpeckers, Florida scrub jays, western bluebirds, and many of Australia’s little bands of fairy-wrens. Most cooperative breeders are “facultative,” meaning they can breed successfully without helpers. Relatively few, like the ani, are “obligate”—they require helpers at the nest in order to successfully fledge young.
