Why Mob a Martin?

At all but the hottest hours this time of year, Purple Martins are among the most conspicuous birds of our desert suburb. They seem to fly and sing all night–as the owners of a new puppy can attest with only too much certainty–and gather on the wires in the yard at dawn and at dusk.

The perched birds invariably attract a gang of House Finches, which sidle along the wires to crowd the big swallows, even chasing them when the martins find the finches too close for comfort.

I’ve seen this behavior repeatedly here in southeast Arizona, and am puzzled every time. What possible reason could House Finches have for mobbing Purple Martins? There is, no doubt, some slight potential for competition for nest sites–House Finches will happily tuck a nest into a saguaro cavity–but that seems a bit too abstract a motivation for this strange behavior.

Anybody know the answer to this mystery? I’m sure it’s obvious, just not to me.