Fawn-breasted Brilliant typically forages in understory and and midstory of humid forest, and usually in the forest interior (Hilty and Brown 1986, Ridgely and Greenfield 2001b, Schulenberg et al. 2010). Otherwise there is very little information on the foraging behavior of this hummingbird.
Social and interspecific behavior
Fawn-breasted Brilliant usually is solitary (Hilty and Brown 1986), as is typical of hummingbirds, although sometimes "a few may congregate at flowering trees, sometimes even out in clearings" (Ridgely and Greenfield 2001b).
Dennis Arendt made the following observations of agonistic behavior: "On 5 October, 2010, at the hummingbird feeding station at Cabanas San Isidro east of Quito, Ecuador, two male Fawn-breasted Brilliants (Heliodoxa rubinoides) fell to the ground together. They appeared to have their feet intertwined and their bills gabbing each other. It was 15:31 and a light rain had begun. They stayed quiet in the wet grass with one bird on top of the other. Both birds has their wings extended fully. The bird on top had its long bill stuck into the neck feathers of the bird underneath. The bird dominating the other would move its bill from the neck to the wing, probing and sometimes grabbing feathers. After about ten minutes, the bird underneath struggled to release himself from the bird above. They flapped their wings against each other, flopping sideways and moving a few centimeters, but the dominating bird stayed on top. They would stay quietly in the grass for several minutes, then the struggle would be repeated with the same results. The rain grew heavy. This struggle continued for one hour and forty minutes. At 17:11 the dominating bird flew up to a sugar water feeder and sat on top, not on the perch, and drank for a half minute. Then it flew away. The other hummingbird was picked up from the wet grass. It was thoroughly wet, but alive, and showed no indications that its flesh had been pierced by the other bird's bill. The defeated bird would surely have died without help. It was taken back to the kitchen at Cabanas San Isidro where it was warmed and fed sugar water. It, too, flew off after about fifteen minutes."