The Shabbat when we read the portion of "Beshalach" is called Shabbat Shira. This parasha has the shirat Hayam, the song the Jews sang when they crossed the Red Sea, which opens with the words "Az Yashir Moshe", tells of "Pharaoh's chariots and horsemen" being flung into the raging sea and ends with "Hashem will reign forever more.
Shabbat Shira falls before Tu Bishvat, or when Tu Bishvat falls on Shabbat. It is customary to scatter bread crumbs for the birds to commemorate the aggada in praise of them.
It is told that when Israel camped in the desert and ate of the Manna, Moshe told the people to gather a double portion on Friday, since no Manna would fall on Shabbat. Dattan and Aviram wanted to prove that Moshe was a liar, and rose early Shabbat morning to scatter Manna on the ground. Along came the birds and ate it all up and when the people went out to gather manna, they could not find any. In order to remember the birds, we scatter bread for them on Shabbat Shira.
