Male Scarlet Ibises (Eudocimus ruber) and White Ibises (Eudocimus albus) have bills that are on average 22% longer than females, yet only half of this difference can be attributed to body-size differences. In this work it is suggested that the sexual dimorphism in bill length and bill shape is based in large part upon the role of bill length in determining the outcome of male bill-sparring contests. Sexual differences in bill morphology and behavior were investigated in a captive flock of 350 Scarlet Ibises. In 29 instances of males sparring with males in contests prior to breeding and in 11 instances of nest take-over during breeding, no significant differences in age, body size, or body condition between winning and losing males were found. However, males winning bill-sparring contests and nest take-overs had significantly longer bills than losing males. Longer billed males also bred significantly earlier than shorter billed males in captivity. Sexual differences in the relationship between bill curvature, bill chord and bill length suggest that male Scarlet and White Ibises are under selection for increased bill length. In addition, among 16 of 23 ibis species for which information was available, sexual dimorphism in bill length was positively associated with colonial nesting but was not associated with group foraging. This result supports the theory that close proximity during nesting is one feature that may facilitate bill sparring between males, resulting in selection for longer bills.