It does not sound good that the birds eat the seed—and it is not. The "birds of the air" are a negative symbol. In verse 19, instead of saying "birds of the air," Jesus interprets them as "the wicked one" devouring the seed. In his version, Mark uses the specific word "Satan," and similarly, Luke has "the Devil."
Notice Genesis 15:11, where Abraham is making a covenant with God: "And when the vultures came down on the carcasses, Abram drove them away." What were the vultures doing? They were interfering between God and man, trying to defile Abraham's sacrifice.
Deuteronomy 28:26, a somewhat different context being within the Blessings and Cursings chapter, has this as a curse: "Your carcasses shall be food for all the birds of the air and the beasts of the earth, and no one shall frighten them away." God promises that there would be no Abraham around, as it were, to fend off the birds. "Birds of the air" is a negative symbol.
Revelation 18:2 really nails it down:
And [an angel] cried mightily with a loud voice, saying Babylon the great is fallen, is fallen, and has become a habitation of demons, a prison for every foul spirit, and a cage for every unclean and hated bird!
What are the "birds of the air"? Demons! They try to get us when we are young in the church. Like the lion of I Peter 5, they go for the stragglers, the weak, and the newborn, because they are the easy pickings. What is step one of Satan's plan against the church? Attack God's people early in their calling. Distract them. Persecute them. Crush them—before they are strong enough to resist.
Richard T. Ritenbaugh
Parables of Matthew 13 (Part 1): The Mustard Seed