The Natal Pterylosis of Hummingbirds
Of the hummingbirds examined, most have a common pattern of natal down distribution: a single paired row of filamentous mid-dorsal neossoptiles. One exception, Glands hirsuta, typically has 30 short conglomerate downs on eight tracts, including the dorsal cervical region. The taxonomic usefulness of natal pterylosis seems to be at the intrafamilial level in the Apodiformes.
Collins, Charles T.
"The Natal Pterylosis of Hummingbirds,"
Bulletin of the Southern California Academy of Sciences:
Available at: https://scholar.oxy.edu/scas/vol77/iss1/5