A couple of recent threads compare crocodile and lace monitors, making inferences about diets and natural histories. These two monitors are closely related, and form a distinct lineage with Komodos according to recent DNA-sequencing work by Jennifer Ast (2001, Cladistics 17: 211-226 – lacies and Komodos form a pair, with crocs as the outlying member). It's appropriate, then, to compare them.
We know a lot about Komodos, and there is some good new work on lacies (F. Guarino, 2001, Australian Wildlife Research 28: 627-663 on diet; and Guarino, 2002, J. Zool. London 258: 449-457, on home ranges and movements), but there is basically no information on the ecology of wild croc monitors. All three species are highly arboreal as juveniles, but Komodos become wholly terrestrial as adults, and adult lacies forage both in trees and on the ground, though they typically use tall hollow trees as overnight and overwinter refuges. There are some indications that croc monitors also use tall hollow trees as refuges, but there are also a fair number of reports of encounters with them on the ground.
Dietary studies on wild lacies indicate that they now get about half of their total food intake from carrion (principally roadkills, and dead hoofstock), and young rabbits during spring in the southern part of their range. This is all stuff introduced by Europeans; native prey includes a variety of small mammals and reptiles, and particularly eggs and young of birds. Most of these birds nest off the ground, and many of the native mammals eaten are arboreal as well. There are quite a few observations of even quite large lace monitors pursuing possums and so on in trees, and going after hole-nesting birds such as cockatoos, so size and weight evidently does not prevent them from foraging well off the ground.
Who knows what wild croc monitors do? Nobody, yet, but there are some things that suggest that they behave a bit differently from lacies. One of these factors is that there are surprisingly few small- and medium-sized mammal species in lowland New Guinea, and the people who work on mammals there always complain that there aren't many individuals, either. Mammals (whether as carrion or live prey) are just not common where crocs occur, and of course there aren't any introduced rabbits, sheep and so on.
What lowland New Guinea does have in spades is fruit-eating birds (pigeons and parrots, mostly), and large nectar- and fruit-eating bats ('flying foxes'), and crocs certainly have the long teeth that go along with bird-catching in other herps. Crocs also have some behavioral traits that differ from, say, lacies in captivity, and this suggests a little more about their lifestyle in the wild.
Carrion, rabbits, bird nests, etc. are scattered across the landscape, and to find each one of them a lace monitor has to cover a lot of ground. By contrast, fruit-eating animals gather themselves in trees with ripe crops, so the strategy for crocs may well be to know where such trees are, and to go hang out in them. Also, no monitor is going to be able to chase and catch birds or bats, so it seems much more likely that crocs are ambush hunters, lying in the ferns and orchids on large branches of fruiting trees and waiting for a pigeon or bat to come too close.
In my experience (and here I'd be happy to hear what others have seen), captive crocs don't have much of a "feeding response", such as lacies and most other large monitors seem to show. By this I mean that you can open or enter the cage while carrying food, and you are not going to have a flying chainsaw experience – instead, crocs will wait until you toss them the food or hold it out on tongs. Again, this is what ambush hunters do, versus the pursue-and-grab methods of animals that have to actively search for every prey item.
What I'm suggesting here is that crocs, even large ones, are likely to do a lot more foraging in trees than even lacies, and certainly Komodos. This is both because that's where almost all the food is in lowland New Guinea, and it's what the feeding behavior of captives suggests. And sorry, Mark, I am not hugely impressed by travellers' tales of crocs dropping out of trees on their prey, if only because there's darn little in the way of suitable prey running around on the ground in New Guinea lowland forests. Those stories may be in the same category as the famous Australian drop-bears, eh.
