excessive eating

Question
I have a 3 year old female quaker who has begun excessive eating. She is eating twice the amount of food she used to a week ago. Any ideas? There have been no other changes, feathers and skin look good, beaks in good shape.

Answer
hi. thanks for your question. kudos to you by the way for being so observant of your bird's behavior...behavioral changes are usually the first indicators of illness and so many people overlook the early warning signs of illness and are unable to save their birds in time!
anyway, that is a very interesting situation...usually i am only alarmed when birds UNDER eat versus overeat.  undereating is a sign of illness in birds.  also, you say you have not noticed any changes in her appearance which would be indicative of illness.
however, there is one possibility...given that your bird is a female, is 3 years old, in good health, and it is now springtime (depending on what side of the equator you are on), she may be preparing to lay an egg.  most parrots come in to sexual maturity at about age 2 and hens will begin laying their first clutches of eggs around that time, give or take a year.  female birds will lay eggs regardless of whether or not a male is present in the cage. obviously, if she is solitary, the eggs will not be fertilized...she however will not be aware of this and sit on the eggs anyway, eventually abandoning them when they do not hatch. they will sometimes lay multiple eggs in a laying period, usually laying one egg each morning until their "clutch" of eggs is complete.  
thus, my guess is that she may be preparing to lay...my female birds get ravenous around laying time...akin to a pregnant woman's food cravings. their bodies require more nutrition and more sustenance to grow the egg and eventually push it out and producing an egg is very draining on her body...it uses up about 25% of a hens body reserves.  
i would continue to feed her as much as she needs to eat and if she does lay an egg, you may want to supplement her diet further with calcium fortified snacks like hardboiled egg whites, egg shells, calcium/oyster shell grit. there is even special "egg food" you can buy at a pet supply for laying hens.
if she does not end up laying and continues this behavior for a prolonged period of time (longer than another couple of weeks) it may be a good idea to take her to a vet just to make sure she is getting the nutrition she needs and is not sick.
good luck and i hope this helps!  alicia