This answer expands on the answer above given by ab2, and is in response to her comment that she could not find the factors that are measured for the morphological model. The five factors are indeed given in the article she linked to.
"The five features measured were; body mass, wing chord length, tip-cere length, tarsus length and head-bill length."
And then later...
"These variables were selected because they are frequently measured by field researchers (e.g. Olendorf 1972; Arroyo et al. 2000; Balbontin et al. 2001) and are likely to differ between sexes (Baker-Gabb 1984; Olsen 1995)."