Very interesting. Here's what BNA-online has to say about Tree Swallow feeding behavior--
"Pursue individual prey items in the air, using abrupt turns, sometimes converging in large numbers on concentrated insects. . .. Ability to find and use local concentrations of insects may be key for survival of returning birds in early spring when food is scarce (McCarty 1997). In flight, can pick prey items off surface of water. . .. Occasionally feed on ground or ice. . . and very rarely glean insects off vertical surfaces. . .. Rarely, may flutter against vegetation, putting swarms of insects into the air. . ..Also eat bayberries (wax myrtle, Myrica sp.) and other vegetable matter. . ., especially during the non-breeding season."
From what the OP has just said, the House Martins were apparently picking their prey directly off the leaves rather than flushing them and catching them in the air as the BNA account claims Tree Swallows sometimes do.