Sexing juvenile Greenfinches Carduelis chloris by the extent of black on the tail

There is increasing interest in the study of the evolution of sex ratios in birds. Correct assignment of sex to fledgling and juvenile birds is critical for studies that deal with differential sex allocation at nests or sexual variation in survival or dispersion rates. Here we describe a non-intrusive method based on plumage colouration by which juvenile and fully developed nestling greenfinches can be sexed reliably. By measuring the length of the black area on the fifth tail feather rachis, we were able to sex 100% of birds reliably; males had lengths less than 31.44 mm, whereas females were longer. The Greenfinch may, therefore, be a valuable species for the study of sex ratio variation.

Marta Arenas and Juan Carlos Senar, Ringing & Migration (2004) 22, 1-3