That depends on what Harvest Moon Wii game you are playing.
Tree of Tranquility:
Once you get a marriage candidate to eight hearts you will be told the Legend of the Blue Bird.
A few days after that you will see a Blue Bird flying. Go to the Waterfall area to find a fallen Blue Feather.
Animal Parade:
Once a marriage candidate reaches nine hearts leave your house on a Sunny morning and Mira will be on your farm. She will tell you she was following a Blue Bird and then tells you about the Legend behind the Blue Feather.
After that scene go to the summit of the Mountain between 5pm to 7pm (17:00 to 19:00 on the Animal Parade clock) on a sunny day to get a Blue Feather.
Once you have the blue feather you can purpose to a Girl.
You can get the Blue feather from Jeff's General Store. The Blue feather will cost 1000 G.
The Blue Feather costs 1,000G.
i think its 1000
well to get the blue feather one girl of your choice has to like you at the MAXIMUM level then you can buy the blue feather from the supermarket (for harvest moon ds)
A year after you get married with the blue feather.
You give them the blue feather
To get married on harvest moon you need to give the them their favorite gift and talk to them everyday. After you get all of the hearts filled by their picture, there will be a blue feather lying around somewhere. Give the blue feather to them and you will be married. p.s wait which harvest moon are you talking about?
In all Harvest Moon games the Blue Feather is used to propose to the person you want to marry. However, there are other requirements besides just proposing with the feather. The person will not accept if you haven't completed all the requirements. The requirements are different for almost all Harvest Moon games.
You can Purchase a Blue Feather From Karen's Shop after you have reached an Orange heart Level with your Lover
When the Harvest Sprites give you the blue feather it means that they have realised you have four red hearts. You then give it to that girl and you get married on Spring 1 the next year.
In Summer of your first year Nik, Nak, and Flak will give you the Blue Feather.