Must I double wrap bread before discarding it?
How big does the piece of bread have to be, to require double wrapping?
And is there any problem throwing lots of bread to the birds, so that some will stay on the ground for a while?
Single wrap is enough.
A kazayis of bread (or several small pieces that altogether add up to kazayis) should be wrapped (kazayis = a little less than an ounce). Less than a kazayis does not have to be wrapped but may not be stepped on.
Leaving bread on the ground for the animals to eat is fine if people won’t step on it.
See also Halacha2go article #28
ראה שו”ע או”ח סי’ קע”א, וסי’ ק”פ, ובמ”א שם ס”ק ג’, ובשע”ת שם, ובשו”ת אבן ישראל ח”ט סי’ ס”ב, ועוד.