ID a bird egg i found?

My nephew found a little birds egg. no nest near by and theres a lot of kids in the area that could possibly trample it if left where it was (middle of there front yard) so i took it home. i'm no bird expert. i like cats! but i know that you need to keep it warm.

he found it friday night and i've been keeping it warm sense then (today is tuesday) yesterday i noticed something in the egg when i back lit it. (using my phone flash light) this morning when i checked again you can see little bit of veins and what i think is the heart beating!

even though i was keeping it warm i just assumed it was dead but didn't have the heart to toss it until i was sure. now i have a growing egg and i'm not kidding myself, i know i can take care of a baby bird.

i emailed a bird place for help. but til then i wanna know what kinda bird it is. i'll attach a link for the photo. its all white.

http://desolatekitten.tumblr.com/image/48716631797