| Summary: | The taxonomy of the Order Passeriformes, at the family level and above, is still in considerable flux. An historical review of the taxonomy, including traditional studies, and more recent works (particularly DNA-DNA hybridization), is given. The present study attempted to analyse little used characters, i.e. those on the plantar surface of the foot, of over five hundred species of bird. Large amounts of variation and homoplasy were found. Compatibility analysis (by a LeQuesne test computer program) was used to find cliques of compatible characters for families, and other groups of Passeriformes. The polarities of character state changes were hypothesized by outgroup comparison, with non-passerines from a number of orders. Some characters were found to be linked to the scansorial habit, others of reasonable use taxonomically, and yet others, not obviously adaptive, showed so much homoplasy as to be almost worthless taxonomically. Useful characters were then used to describe and define families, and in some cases other taxonomic groups, attempts were made, with varying degrees of success, to allocate 'problem genera'. The same characters were used to construct cladograms, and then other informative characters (i.e. those which define subgroups within the cladogram, but show homoplasy) were added, to produce one or more trees of each group investigated. The results were compared to those of earlier studies, based on morphological, anatomical and biochemical characters. The degree of congruency varied considerably. It was particularly interesting to find that in some cases there was a reasonable fit with the results of DNA-DNA hybridization.
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