Back in 1986, Amazon rancher Howard Voren of Loxahatchee, Fla., discovered in Honduras that the wing speculum (a group of feathers) of mature birds exhibited sexual characteristics in Red-loreds Amazons. It was the same with orange winged Amazons.
In one set of red-loreds, the last feather was bordered in maroon, with no yellow---a male. The last wing feather in the group of females was bordered yellow.
He was investigating Yellow-naped Amazons there. The majority of males had five red feathers in the speculum, the females had four. He had a dominant male in this collection which didn't mate, while #2 and others did. Then he checked the male. Only four red feathers in the speculum, and perhaps the reason he was unable to attract a mate.
In Paraguay, Blue-fronteds males had more red feathers in the speculum than females.
Voren postulates that sexual dimorphism in Amazons (other than White-fronted which have more demonstrated dimorphic characteristics, males have red alulas) might be exhibited in other ways in mature birds and save the time of buying birds to find one female by mechanical sexing procedures.
Males generally: larger, flat heads that begin tapering further back;
Females: round heads
Males sit upright on perches. Females often flatten themselves like ducks.
Females sit with feet further apart than males, probably because of the position of the hip bones.
Females more likely to bite. Males tend to be louder.
Females more likely to go to the floor of the cage.
Males spread their tail feathers more frequently; females keep their tail feathers closed.
Klaus Bosch & Ursula Wedde, Encyclopedia of Amazon Parrots, T.F.H. Publications, Neptune, N.J., 1981.
Others have stated that the males beaks and heads can be larger than females in the species, and Rosemary Low says males look bolder. I have seen that in the Ivy Aviary, but Low says that it is not an infallible guide and recommends (in 1980 in Parrots, Their Care and Breeding, Blandford Press, London) that surgical sexing be done for positive identification.
Mark Hagen* tells of Amazons (Yellow headed) who were thought to be a pair by apparent differences which when surgically sexed were found to be both males.
DNA non-invasive nail clipping blood samples only tells sex, he says.Sometimes there is a mix-up in paper work in mail-aways. Endoscoping (surgical sexing thru the air sac while the bird is anesthesized) provides sex determination at the hands of an experienced vet, but also health information. The example Hagen gave was "gonads are scarred" at the International Aviculturist society convention in Florida in 1993. He said that the costs of the various sexing is about the same, but surgical sexing provides more information for the money.
The Hagen Avicultural Research Institute, P. O. Box 490, Rigaud, Quebec, Canada JOP 1PO.