sentences:  Barn swallow nestlings have prominent red gapes, a feature shown to induce feeding by parent birds.An experiment in manipulating brood size and immune system showed the vividness of the gape was positively correlated with T-cell–mediated immunocompetence, and that larger brood size and injection with an antigen led to a less vivid gape.The barn swallow has been recorded as hybridising with the cliff swallow ("Petrochelidon pyrrhonota") and the cave swallow ("P. fulva") in North America, and the house martin ("Delichon urbicum") in Eurasia, the cross with the latter being one of the most common passerine hybrids.