Mar 4:15 And these are they by the way side, where the word is sown; but when they have heard, Satan comes immediately, and takes away the word that was sown in their hearts

Alternative: But some are by the way side where the word is sown; and when they have heard, adversity begins immediately, and removes the idea that was sown in their hearts.

It is easy for the personalization of adversity, an artifact of incomplete translation, to get in the way of understanding Christ's thinking. The birds take the seeds on the wayside away because they get their first, but everyone listening knows that the seeds never had a chance on the walkway anyway. It gets too much traffic. The ground on the wayside is literally a path for adversity. Their traffic prevents any plants from growing up there because they "instantly" get trampled down.

Here, the birds are less a symbol for this adversity than they are for speed. In this parable, Christ explains different types of ground in terms of how long his ideas last on them. Christ doesn't use birds anywhere as a symbol for adversity (or evil or the devil, if you prefer to personalize adversity). Here, they are simply a rhetorical device for adversity to get to the seeds quickly. Even without this speed, these seed were doomed to adversity anyway because they were on the pathway where something was going to come along sooner or later to destroy them.

In terms of Christ's use of symbols, this is one of several where Christ connects the ground with the heart, which is his symbol for feeling and through feeling, understanding. Again, Christ teaches that we must not only see, hear, and feel, but that we must observe, listen, and understand. The term for "hear" used in this verse, is the same as the one used in Mar 4:12 to indicate physical hearing without mental listening.

"Word" is from the the Greek logos (logos), which means "word," "computation," "reckoning," and "value." It is also "an explanation," "an argument," or "a rule or principle of law."

"Sower" and "sow" are from speirô (speiro), which means "to sow a seed," "to beget offspring," "to scatter like a seed," and "to sow a field."

"Hear" and "hearing" are from akouô (akouo), which means "hear of," "hear tell of," "what one actually hears," "know by hearsay," "listen to," "give ear to," "hear and understand," and "understand."

"Satan" is from satan (satanus, satan), which means "adversary," "opponent," and
"accuser." These are Hebrew origin words, appearing in Greek only in the New Testament. The traditional Jewish view does not portray "satan" as an evil angel (explanation here). A case can be made that our personalization of Satan is largely an artifact of incomplete translation.

"Comes" is from erchomai, (erchomai) which means "to start," "to set out," "to arrive at," "to come" and "to go." It generally refers to any kind of motion. It is a little like we use the phrase "he is on his way," which can mean either that he is coming or going with no direct reference to coming to or going from the position of the speaker.

"Take away" is from airo, which primarily means "to lift," and also means "to raise up," "to take up," "to raise a child," "to exalt," "to lift and take away," and "to remove."

Heart" is from kardia (kardia), which means "heart" and which we discuss in a larger Greek context here.