All right WW, wwh, eta....be prepared for a treat of links! The first is a brief overview from JHU on the research into the capability of birds and humans of picking one particular sound amidst many others, as in say, a loud room or a forest.

http://www.jhu.edu/~gazette/janmar97/mar1097/hulse.html

And this one has an interview with a gentleman who records similar experiences to yours, WW, with ravens, mockingbirds and blue jays! Nice interview too! I think he is a musician; he makes a reference to a CD in the end of the interview and the last url is a link to a CD by someone of the same name, (David Dunn - I went a-googling for him), called Angels and Insects. There is a brief audio clip there; listen...you will love it.

http://artscilab.org/~david/writings/Interv2.pdf

http://www.earthear.com/catalog/angels.html

WW, am convinced he is trying to learn what you are playing. Try playing the same thing everyday at the same time, for a week or so and maybe you will wake up one morning to sweet violin-like bird notes! Wouldn't that be simply marvellous!!