Complex sentence: Young birds moult into a subdued version of the adult plumage, lacking the crest, in autumn and acquire the adult plumage when they are nearly one year old in females, and nearly two years in males.
Split into simpler sentences: Young birds moult into a subdued version of the adult plumage in autumn, yet these lack the crest. They do not acquire their adult plumage until they are nearly one year old in females, and nearly two years in males.