Sexing American Bitterns, Botaurus lentiginosus, using morphometric characteristics

Morphometric measures from 1995-1998 were used to develop a discriminant function that provides investigators with a practical, non-destructive technique for sexing American Bitterns (Botaurus lentiginosus). Thirty-two males were lured into mirror traps and mist nets using tape-recorded territorial vocalizations and 17 females were captured at nest-sites using long-handled dip nets. Sex of captured birds was known because only males respond aggressively to territorial vocalizations and only females incubate nests. Average morphometric measures were greater for male than female American Bitterns with overlap between the sexes. Tarsus length was the single most useful measurement in discriminating between sexes, correctly identifying 100% of individuals used to construct the function and 71.4% of birds that were not used in model development (hold-out test data set). The addition of short bill length measurements increased the proportion of correctly classified individuals in the hold-out test data set to 76.2% for males and 85.7% for females. This technique will enable field ecologists to separate population and behavioral data according to sex.