In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: English jeweller and naturalist William Bullock and his son travelled to Mexico soon after its independence, spending six months in 1823 collecting archeological artifacts and many bird and fish species new to science. The bird specimens were given to naturalist William John Swainson, their countryman, to formally describe, which he did in 1827. Among these was the red warbler, which was assigned to the genus Setophaga, as Setophaga rubra. Over the next half century, other authorities moved it to Cardellina, with the red-faced warbler, and to the widespread tropical warbler genus Basileuterus, as well as to the Old World warbler genus Sylvia and the Old World tit genus Parus. In 1873, English naturalists Philip Lutley Sclater and Osbert Salvin moved the species to the genus Ergaticus, where it remained for more than a century.The red warbler forms a superspecies with the pink-headed warbler of Chiapas and Guatemala. Despite their disjunct ranges and considerably different plumages, the two have sometimes been considered conspecific. Conversely, it has also been suggested that the red warbler should be split into a northern gray-eared species (C. melanauris) and a southern white-eared species (C. rubra). A comprehensive 2010 paper by Irby Lovette and colleagues analysing mitochondrial and nuclear DNA of the New World warblers found that the red and pink-headed warblers are each others' closest relative and that their common ancestor diverged from a lineage that gave rise to the red-faced warbler. The authors recommended moving the red and pink-headed warblers to the genus Cardellina, a suggestion which has been adopted by the International Ornithologists' Union (IOC).There are three subspecies, which differ slightly in appearance:
C. r. rubra, described by Swainson in 1827, has white ear patches and is found from southern Jalisco and southern Hidalgo to Oaxaca.
What is the last name of the person who formally described bird specimens in 1827?