What did Virgil's swallows eat?

Juturna drives Turnus’ chariot now here now there, hoping to throw off Aeneas’ pursuit, but he follows the twisted circles (tortos orbes, 12. 481) of her course. Virgil compares her to a black hirundo flying through a rich man's house out into the colonnades and then round the pools or fishtanks. Hirundo can mean swallow, martin, or even swift. All these birds eat insects and air-borne spiders; they do not eat human food. The common swallow chiefly eats flies, and feeds the nestlings on flies; it also eats wasps and bees. Its average prey size is much greater than the house martin's. Virgil's hirundo gathers pabula parua for the nestlings. W F. J. Knight in the Penguin translation writes ‘tiny scraps of food’; C. Day Lewis translates ‘crumbs of food’. If Virgil meant scraps of meat or crumbs of bread, stolen from the rich man's dinner table, then Virgil did not know what these birds eat.