Assuming that the swamp was the Green Place (as mentioned by the many mothers), who/what are the crows? We did see normal crows, but also some creatures on stilts. What are those? They didn't seem hostile. What was their role in the Green Place becoming a swamp? And why didn't they venture further?
They are humans (judging from their shape) disguised. It might be because the soil is moist and there are chances of forming quicksand (quote from Wikipedia):
Quicksand forms in saturated loose sand when the sand is suddenly agitated. When water in the sand cannot escape, it creates a liquefied soil that loses strength and cannot support weight.
The soil there was pretty moist remember the war rig was struck for most part in the soil. I am thinking they walk on sticks to avoid quickands. Since the many mothers didn't say if some people stayed back, I am assuming some of them might have been afraid to move and stayed back and adapted to these new forms, to avoid sinking and dying of suffocation.
Neither the crows nor those people on stilts seemed to have anything to do with the Green Place turning bad. If we recall their exact dialogue:
The soil...We had to get out...We had no water...The water was filth...It was poisoned...It was sour...And then the crows came. We couldn't grow anything.
It seems the place simply went bad due to some natural desaster that turned the soil and the water bad. Of course that natural desaster might have been caused by pollution or any other human-made problem (since the world seems pretty run-down in general), but it's clear the crows didn't have anything to do with it, they came later.
In fact crows, in their common image as scavengers, have long served as symbols of devastation and abandonment. Think of crows scavenging a battlefield for corpses, or the crows sitting in the tree in front of an abandoned house. The arrival of the crows is basically a symbol that the place was lost and nothing but death was to get there. It's no coincidence one of the breeders immediately says:
The crows. The creepy place with all the crows.
since it was creepy and to a large part because of the crows. Now the way that the Vuvalini say "And then the crows came. We couldn't grow anything." might point to the crows also being responsible for a difficulty in growing things, be that by eating the seeds maybe, but the primary reason they couldn't grow things still seems to be because the place went sour, which the crows have nothing to do with. Their arrival was merely the last sign that the Green Place was lost, which is their significance.
The people on stilts in turn just seem to be a few left humans who tried to adapt to the swampiness of the place by walking on stilts. It's not clear if they came after the Vuvalini left or if they once were Vuvalini who separated and decided to stay there. However, they seem to be as independent and just trying to survive on their own as many of the other groups.
According to the Mad Max wiki:
Matriarchal system & Crow Fishers
According to production designer - Colin Gibson, the Vuvalini displayed a broken matriarchal system. It manifested itself in banishing all boy-children to the outer swamps of The Green Place and when they reached puberty they would be visited by the Vuvalini for sperm milking and the odd ritual of family. As such, those banished men would learn to survive by stilt-walking the ooze and fishing for crows on the muddy mangroves between The Green Place and the desert.
In literature, crows symbolize a lot of things, such as the mysteries of life, power of insight, fearlessness and determination. Crows were in those scenes to foreshadow Furiosa's determination to find home, that hope and patience will take her to the green place. Those "stilt people" has lost hope. They wander in the ruins. They failed to rise up from the ashes. They looked UP to Furiosa, who is superior with hope and determination. Furiosa will rise, because she is determined, because she will find home, which is exactly what happens in the end.