Monitor lizards are common near rivers and open water, and can often be found basking on sandbanks. They can grow extremely large, we have seen individuals which were more than 3 feet long (nose to tail).
They feed on carrion, birds, eggs and small mammals and will rob nests.
However we have watched as a Grey-headed Kingfisher (which isn't the world's largest bird) chased a large Monitor away from its nest.
It did this by dive-bombing its head and pecking it. They are quite popular in areas which rely on fishing as a source of income because they dig up and eat Crocodile eggs (and crocodiles are happy to eat fish, thus depleting the stocks when water levels are low).