Puerto Rican Tody is a heterotherm, and exhibits climatic adaptation in basal metabolic rate and body size in different areas of its range (Merola-Zwartjes and Ligon 2000). Birds from xeric shrub are smaller in all measures of body size than those from cooler montane forests, and birds in the hot xeric shrub also have significantly lower basal metabolic rates (Merola-Zwartjes and Ligon 2000). Birds in the montane forest forage far below the canopy, potentially to keep cool, whereas the smaller todies in drier climates do not forage far below the low and open canopy (Merola-Zwartjes and Ligon 2000).
Todies are in the Order Coraciiformes, which includes todies, kingfishers, motmots, hornbills, and hoopoes. They are part of the suborder Alcedini, family Todidae. The genus Todus is endemic to the Greater Antilles, and contains five species (Raffaele 1989). Egg white protein electrophoresis indicates todies are more closely related to kingfishers (Family Alcendinidae) than to motmots (Family Motmotidae), but maximum likelihood and maximum parsimony estimates using data from mitochondrial genes strongly support that todies are more closely related to motmots than kingfishers (Overton and Rhoads 2004).
Todus mexicanus is most closely related to Jamaican Tody Todus todus; genetic evidence suggests that an ancestor of Broad-billed Tody from Hispaniola (Todus subulatus) colonized Jamaica and Puerto Rico and those two species diverged due to reduced genetic exchanges and geographic separation (Overton and Rhoads 2004). All tody species were considered part of a single species, Todus viridis, described in 1805 and Todus mexicanus was later described as a separate species in 1838 (Ridgway 1914, Kepler 1972). The species also was known as Todus hypochondriacus in some literature from 1866 to 1892.