Wyndham, after writing in several different genres under a different name in each, decided to write "realistic" science-fiction - and met with success.
In this book, one character expounds the view that in all science-fiction, aliens invade Earth by turning up with superior armaments and blasting away - until defeated, having underestimated humanity or overlooked some other factor of crucial importance (e.g. microbes in War of the Worlds). They are essentially doomed by their own hubris. During this lecture the character has clearly become a talking head for Wyndham: This is an alien invasion story - but the aliens do not turn up with all guns blazing.

Towards the end of the book, when relations between the Cuckoos and the British have turned ugly, a discussion of morality appears that reminded me of similar discussion in Starship Troopers. In summary the view taken is that when humans are attacked by a hostile species, there is no resort other than a fight to the death - and that this is essentially a biological imperative.

This may be so - but only on the assumption that neither party is amenable to any compromise at all. Wyndham mentions humans not tolerating dangerous species; Britain has no bears or wolves - they were inconvenient and potentially dangerous so they were all shot. But actually humans have compromised with "dangerous" species - otherwise, what are all those nature reserves and national parks all about?

The views of evolution and ecology presented by Wyndham in this novel (and by Heinlein in Starship Troopers) show a fundamental mis-understanding. Survival of the fittest does not mean "survival of the toughest, meanest, most agressive and intelligent" - it means survival of the most appropriate to the environment. This does not in any way imply the superiority of technological species or agressive ones. There is more co-operation in nature than competition. No mammalian species could survive without symbiosis with its stomach flora; lichen - something Wyndham really ought to have known about - is a symbiosis. Many plants and fungi are in direct symbiosis. Even parasitism is not about destroying your host - it might happen as an accidental by-product but not before at least one life cycle of the parasite is complete - even then, wiping out your host species amounts to suicide for the parasite species. Even predator species have no interest in wiping out their prey species.

Hence "intelligent" species are likely to be amenable to comprise if they find thenselves losing a war as they should be able to rationally recognise their own best interest as a species.

The crux of the matter is that whilst we hear the baby Cuckoos speak, we never hear a word from the adult Cuckoos: what are their motivations, methods, opinions? Their total absence from the book is what weakens the story.