India, Sept. 29 -- You've probably seen one today. A small brown bird hops around a cafe patio, pecking at crumbs as if it owns the place. It's easy to ignore, but this bird has an interesting North American backstory. According to a 2010 study in Biological Invasions by Michael Moulton and colleagues, the first house sparrows in the country were released in Brooklyn, New York, in 1851. The authors, from the University of Florida and USDA Wildlife Services, say just 16 birds were probably enough to get it all started.

The study contains an account by Nicholas Pike, who was directing the Brooklyn Institute at the time. He said eight pairs of sparrows were imported from England in 1850. That makes 16 birds altogether. They were kept in cag...