 Members of both sexes possessed a large tuberous knob of bone exostosis situated at the base of the carpometacarpus of each wrist . Other wing bones also sometimes show similar structures . The knob was cauliflower @-@ like in appearance , and consisted of up to two or three lobes . The knobs were about half the length of the metacarpus , were larger in males than females , and described as the size of a musket ball . One study measured the largest knob to be 32 @.@ 9 millimetres ( 1 @.@ 30 in ) in diameter . The knobs vary in size across individuals , and were entirely absent from 58 % of specimens examined for the study . These are thought to be immature birds , or birds without territory . The carpometacarpi of males without the knobs were smaller on average than those with it , but there was little difference between the females . In life , the knobs would have been covered by tough cartilaginous or keratinous integument , which would have made them appear even larger . Carpal spurs and knobs are also known from other extant as well as extinct birds . Within Columbidae , the crowned pigeons and the Viti Levu giant pigeon have outgrowths on the carpometacarpus which are similar to those of the female Rodrigues solitaire . Other well known examples are the steamer ducks , the torrent duck , sheathbills , screamers , the spur @-@ winged goose , and the extinct Xenicibis xympithecus . 
