Brazilian moorhens - a laughing matter

When I first visited Brazil over five years ago, I soon became aware of how different the long 'cackling' calls of the local race of Common Moorhen Gallinula chloropus galeata are compared to the familiar short calls of nominate G. c. chloropus back in the UK. A closer look at these birds also revealed that they look a little different too, being slightly bigger with a larger red shield with a broad square top edge, giving the head profile a peak at the front of the crown. With the recent taxonomic scrutiny of moorhens focusing on the variation between Old World and New World forms, much has been written about the morphological differences between the North American race G. c. cachinnans and Old World chloropus, that might allow a transatlantic vagrant to be identified. Adult cachinnans, in addition to the features above, tend to show a different bill pattern with less extensive yellow on the lower mandible and a more clearly defined red/yellow border, a duller red iris and richer reddish-brown upperparts, particularly on the upper mantle where there is a more clearly demarcated contrast with the dark grey neck (see here and here). Immature birds are much more similar and possibly indistinguishable in the field?

In July, the AOU (but so far not the BOU) decided to split all New World forms from Old World chloropus as Common Gallinule (in favour of the name Laughing Moorhen suggested by The Sound Approach). However, it is galeata, the most widespread form across South America, that has been designated the nominate race rather than cachinnans. I'd like to know what features distinguish these two races? As far as I can tell they are vocally inseparable (compare the calls of galeata and cachinnans here and chloropus here), but I've observed that galeata does appear to be much greyer on the upperparts than both Old World chloropus and North American cachinnans, with brown tones restricted to the lower mantle, rump, greater coverts and flight feathers (compare below), and perhaps they are also a little darker overall. Are there other differences?

Former race G. c. galeata, now split by the AOU as Common Gallinule Gallinula
galeata, REGUA, Brazil, November 2011. Note the large, square-topped shield, the
well defined lower mandible pattern and the dark eye. galeata seem to have much
greyer upperparts than both North American cachinnans and Old World chloropus.

Common Moorhen Gallinula chloropus, London Wetland Centre, UK, December
2011. Note the smaller, narrower shield with a rounded top, slightly more extensive
yellow on the lower mandible with a slightly less demarcated pattern, brighter red
iris and browner upperparts (click to enlarge). North American cachinnans is
apparently even browner above.

At REGUA, galeata also exhibit some differences in behaviour compared to the Old World Common Moorhen. Here the birds seem to be more social, forming large flocks throughout the year, often containing 40 or so individuals that feed and rest together. They also appear to be more aquatic in their feeding habits, preferring to forage in open and often quite deep water, picking plant material from the surface, and feed much less frequently on land than Common Moorhen do in the UK, where bird seem to prefer to graze on grass. Has anyone noticed a similar feeding behaviour in cachinnans, or in any other race? For some excellent footage by Ron Jackson showing the behaviour, calls and features of galeata at REGUA, click here.

A typical feeding group of Common Gallinule Gallinula galeata, REGUA, July
2011. Based on observations at REGUA, Brazilian birds are seemingly much
less terrestrial in their feeding habits than their Old World counterparts?

Common Moorhen Gallinula chloropus London Wetland Centre, UK, December
2011. Birds in the UK seem to prefer grazing on land.

So is the situation as straightforward as separating New World from Old World moorhens? Looking at photos, meridionalis from sub-Saharan Africa and St. Helena in the mid South Atlantic (here), orientalis of south-east Asia (here), and pyrrhorrhoa from Madagascar and surrounding islands (here), all look very similar to galeata to me, with perhaps a very slightly more rounded top to the shield. Have these been allied to Common Moorhen or Common Gallinule, and are they also different species?