Trumpeters are frugivorous, eating mostly fallen fruit on the forest floor. As trumpeters are unable to dehusk fruits, their preferred fruits are soft. Larger fruits are pecked at, while smaller fruits (under 20 mm) are swallowed whole. The seed is defecated with little apparent scarification (Erard et al. 1991). Fruit that is eaten includes species of the families Moraceae, Lauraceae, Euphorbiaceae, Arecaceae, Myrtaceae, and Cucurbitaceae (Erard et al. 1991).
Although trumpeters will pluck fruits from small understory plant species, the majority of the fruit they eat comes from medium to large canopy trees that produce copious amounts of fruit. Since trumpeters forage on the ground, they depend on arboreal frugivores (especially primates but to a less extent birds) to knock down fruit. The fruit that trumpeters eat are abundant, and persists for a number of days on the forest floor, allowing trumpeters to frequent the same foraging areas multiple times. There is little competition for their fallen fruit, and so trumpeters do not need to follow the primates that knock down the fruit.
Gray-winged Trumpeters also consume insects and small animals, but these consist of only 10% of their diet (Erard et al. 1991). The Gray-winged Trumpeter feeds on a wide variety of arthropods, such as millipedes, ants and their larvae, winged (alate) termites and their larvae, and various orthopterans such as katydids and grasshoppers. The closely related Pale-winged Trumpeter (Psophia leucoptera) has been observed feeding beetles and their larvae, caterpillars, centipedes in addition to the aforementioned species, and Gray-winged Trumpeters probably also eat these.
In addition, both Gray-winged and Pale-winged trumpeters have been observed wiping large millipedes that defensively exude hydrogen cyanide on their contour feathers before eating them. In Gray-winged Trumpeters, a pair will often take turns wiping their feathers and skin with an exuding millipede, before two birds eventually consume the millipede (Sherman 1996). This behavior is thought to repel ectoparasites, and has been reported in few other species (Clunie 1976, Parkes et al. 2003).
Trumpeters forage for arthropods by turning over leaves with their bill or by pursuing them rapidly across the ground. Arthropods that sting or bite are pecked persistently before being consumed.
Trumpeter groups often follow groups of foraging army ants, darting in snatching insects as they are flushed (Willis 1983). Sometimes trumpeters forage on the arthropods that are dropped or flushed to the ground by groups of squirrel monkeys, and will even follow monkey troops to eat the insects (usually katydids and grasshoppers) that the monkeys flush.
Small vertebrates are rarely consumed. Trumpeters do kill and eat small snakes, usually under 250 mm, and scavenge extremely rarely on dead lizards, frogs, and even once a dead mouse.
Although adults are largely frugivorous, the young chicks are fed twice as many insects as fruit, but the proportion of insects gradually declines to become a minor part of the adult bird's diet.