Cleon, sound good to you, too? I wanted to go with something reasonably CR appropriate.
I was going to use the standard barrier
detect spells use "The spell can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt blocks it."
The
protection from evil effect blocking the drain was also something I was considering. Should we have a higher level abjuration spell break the link? Probably
magic circle against evil?
How's this for a revision:
Life Leeching (Su): Wastrels have the uncanny ability to slowly drain the life-force from a creature they have wounded. To use life leeching, a wastrel must first establish a "blood-link" with the victim. It does this by hitting an opponent with its bite attack and drawing blood. A wastrel can only maintain one blood-link at a time, and can unerringly sense the direction and distance of the creature it has a blood-link with. A blood-link is broken if the wastrel and its victim are even more than a mile apart, the wastrel can also cancel a blood-link as a free action.
The blood-link can penetrate barriers, but 1 foot of stone, 1 inch of common metal, a thin sheet of lead, or 3 feet of wood or dirt is enough to break the link. A magic circle against evil or similar spell will break a blood-link.
Once it establishes a blood-link, the wastrel can leech its victim's life force. After every 24 hours of life leeching the victim takes 1 point of Constitution damage. Life leeching has a 300 foot range, but the bird doesn't have to stay within 100 yards the entire day provided it can average 300 feet or less from its victim throughout the course of a 24-hour period. Life leeching does not require line of sight or line of effect, but any barrier that breaks the blood-link will also stop the Con damage.
A protection from evil spell prevents the Con damage but does not break the blood-link.
If more than one wastrels life leeches the same victim, the rate of the leeching is multiplied by the number of birds, i.e. 4 wastrels life leeching a single creature would leech 1 point of Constitution every 6 hours.