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A new parameter,the Web Development Index (WDI),is proposed to quantify the extent of web development in bird tracks.The WDI is expressed as a ratio of distance between the proximal part of a track and a line passing through distal points and neighboring digits and that between the proximal part of a track and the innermost point of web impressions.WDIs of 19 modern bird tracks with web impressions and 34 fossil bird tracks with web impressions were measured and grouped into five categories.They are proximal (0.20-0.34),proxi-mesial (0.35-0.49),mesial (0.50-0.64),disto-mesial (0.65-0.79),and distal (0.80-1.00) web.WDI,instead of presence or absence of web,is regarded to be useful criteria for ichnotaxonomy of bird tracks and it is thought to be useful in recongnizing bird tracks with proximal or proxi-mesial web impressions which may be difficult to see in some fossil wading bird tracks.