Mangrove Cuckoo is variable in size and plumage color. At one point, the species was split into 14 subspecies based primarily on variation in plumage color and pattern – especially extent of ventral coloration and the color and width of the loral and aurical patch – and secondarily on size (length and depth of the bill and wing length). A largely qualitative review by Banks and Hole (1991) argued against any geographic pattern to this variation and as a consequence most avian taxonomies now treat the species as monotypic.
Lloyd (2016) provided a quantitative analysis of phenotypic variation in Mangrove Cuckoo and, while finding no support for treating different populations as subspecies, noted a pronounced cline in bill size. At one extreme were individuals from populations in Florida, the Bahamas, and the Greater Antilles, which had relatively small bills; at the other extreme were birds of the Lesser Antilles, which had relatively large bills. Geographically separating these two extremes were phenotypically intermediate indivduals on the islands of Antigua and Barbuda. Bill size was also intermediate in populations from South and Middle America.
Ventral color showed a similar geographic pattern (Lloyd 2016), although variation in ventral color was not entirely concordant with variation in bill size. Populations in Florida and the Bahamas comprised mostly pale individuals. Ventral color became progressively darker among populations in the Greater Antilles and reached a maximum in the Lesser Antilles on Dominica. Ventral color then faded again in populations on St. Vincent, Grenada, and South and Middle America, many of which were indistinguishable in color from specimens collected in Florida and the Bahamas.