Sibley's taxonomy has been a major influence on the sequences adoped by ornithological organizations, especially the American Ornithologists' Union.
He is of no known family relation to renowned bird artist David Sibley, although the families knew each other (one of Charles' daughters babysat David, for instance) and David's father Fred worked for Charles at Yale. There is some family resemblance, and Charles did a fair amount of genealogical research but could only establish that any relationship was no closer than fourth cousin.
Sibley developed an interest in hybridisation and its implications for evolution and taxonomy and, in the early 1960s he began to focus on molecular studies: of blood proteins, and then the electrophoresis of egg-white proteins.
By the early 1970s Sibley was pioneering DNA-DNA hybridisation studies, with the aim of discovering, once and for all, the true relationships between the modern orders of birds. These were highly controversial to begin with, and regarded by colleagues as anything from snake-oil salesmanship on the one hand to Holy Writ on the other.(Ahlquist 1999) With the passage of time and ever-improving laboratory methods, the balance of scientific opinion has shifted closer to the latter interpretation, though the picture is by no means clear-cut and simple. Some of Sibley's results - such as the close relationship of galliform birds and waterfowl and their distinctness from other neognaths, have been verified, others, such as the inclusion of diverse groups into the Ciconiiformes have turned out to be very much in error.
Sibley became estranged from his American co-workers for a time and corresponded with overseas colleagues extensively. But by the mid to late 1980s, Sibley's ongoing work had reversed the trend. His revised phylogeny of living birds in the light of DNA analysis, published in various forms between 1986 and 1993 was both controversial and highly influential.
In 1986 he was elected a Member, National Academy of Sciences, U.S.A. In 1990 Sibley was elected President of the International Ornithological Congress. His landmark publications, Phylogeny and Classification of Birds (written with Jon Ahlquist) and Distribution and Taxonomy of Birds of the World (with Burt Monroe) are among the most-cited of all ornithological works, the former setting out the influential Sibley-Ahlquist taxonomy.
... a rebel with a cause. In argument he would bulldoze through, brooking no contradiction. Critics were baited with an acid tongue, and, in fits of temper, he could be a cruel mimic. In short, lesser mortals were not tolerated easily and, as has been said by others, collegiate friends were few. ... I never found him malicious or vindictive, even against those who had tried to bring him down. Nor was he particularly sophisticated or cultured, just a big, up-front Yank possessed by 'the big picture' in avian phylogeny and convinced of the righteousness of his cause and invincibility of his intellect. (Schodde 2000)
Partly due to personality conflicts, Sibley had few long-term collaborations with other scientists, with the notable exception of Jon Ahlquist. However, he was effective in persuading others to provide him with the blood, tissue, and egg white samples which were the key to his work.(Ahlquist 1999, p. 856)