A review paper has found that recent moves to split great Egret and Eastern Great Egret are not robust, and that at present the species should remain 'lumped'.
The cosmopolitan large egret species Great Egret Ardea alba (formerly placed in the genus Casmerodius), has long been a candidate for a Nearctic/Palearctic split in many birders' and ornithologists' eyes. There are four subspecies: nominate Eurasian, African A a melanorhyncha, Indo-Pacific modesta and the form egretta from the Americas.
The International Ornithological Congress (IOC) currently splits modesta from the others as Eastern Great Egret, but Pratt (2011) has shown that this split is derived from a misinterpreted 1987 DNA-DNA hybridisation study, which found that modesta differed from egretta as much as it differed from Egretta intermedia. Then only isolating mechanism cited was an 'aerial stretch' display – allegedly unique to modesta – which may actually be present in melanorhyncha and alba as well, though observations of all forms are incomplete.
The main physical difference between the forms involve their rapidly appearing and disappearing breeding colours, which appear to be distinct but variable in each, though there is a degree of overlap. This breeding coloration produces three groups among the subspecies: alba/modesta, melanorhyncha and egretta.
If there were to be a split within the Great Egret superspecies, Pratt concludes that one between alba and modesta is untenable, owing to the similarities between breeding coloration and the existence of possible intergradation. As the genetics of alba and melanorhyncha have yet to be studied, the genetic limits and divisions of the group are still unknown, and the species should either be retained as one containing four subspecies, or split as Great Egret and American Egret based on breeding colours and their order of development and an apparent lack of the 'aerial stretch' display.
Pratt, H D. 2011. Observations on species limits in the Great Egret (Ardea alba) complex. Heron Conservation www.HeronConservation.org/vol1/art5.