1. The descent of man, and selection in relation to sex;C Darwin,1871
2. Animal Behaviour: an evolutive approach;J Alcock,2001
3. Vertebrate Life;FH Pough,2002
4. Sexual dimorphism and differential niche utilization in birds;RK Selander;Condor,1966
5. Der Sexuelle Grossendimorphismus der Anatiden als Anpassung an des Hohlenbruten [The sexual dimorphism of the great Anatidae as an adaptation to the caves broods];G Bergmann;Biol Soc Scient Fenn,1965