Slate-throated Redstart feeds primarily on flying insects, particularly those in the orders Homoptera (planthoppers and relatives), Diptera (flies), and Lepidoptera (butterflies and moths) (Collins and Watson 1983, Mumme 2002, Mumme et al. 2006). Non-flying invertebrates and insect larvae are consumed occasionally (Shopland 1985), as are the Müllerian bodies produced by Cecropia plants (Shopland 1985, Stiles and Skutch 1989, Naoki and Toapanta 2001). Of 55 prey items delivered to nestlings in Costa Rica and sampled by the neck ligature method (R. L. Mumme, unpublished data), 51% were Homoptera, 35% were Diptera, and 9% were representatives of other flying insect orders (Psocoptera, Coleoptera, and Hymenoptera); the remaining 5% were small snails and jumping spiders (family Salticidae).
Slate-throated Redstart, like other members of the genus Myioborus, is an active forager, most noted for its flush-pursuit foraging behavior. It uses animated displays of the white patches on the outer tail feathers to startle flying insects, which are then attacked and captured in frequently intricate and acrobatic pursuit flights. In a field experiment performed in Costa Rica, birds with experimentally darkened tail feathers were less successful at attacking prey and delivering food to their nestlings than were unmanipulated control birds. Although flush-pursuit foraging is the most conspicuous and characteristic of their foraging behaviors, flycatching is more widely employed, especially for birds not feeding nestlings or dependent fledglings (Mumme 2002).
Slate-throated Redstart typically forages actively in the middle and lower canopy and upper understory, occasionally foraging on the ground. It frequently joins mixed species foraging flocks (Shopland 1985, Poulson 1996, Pomara et al. 2003) and often follows army ant swarms to feed off the insects that are stirred up by the ants (Vallely 2001, Rios et al. 2008).