Most, but not all, species are rather plain, with various hues of brown, gray and white commonplace. Obvious exceptions include the bright red vermilion flycatcher, blue, black, white and yellow many-colored rush-tyrant and some species of tody-flycatchers or tyrants, which are often yellow, black, white and/or rufous, from the "Todirostrum", "Hemitriccus" and "Poecilotriccus" genera. Several species have bright yellow underparts, from the ornate flycatcher to the great kiskadee. Some species have erectile crests. Several of the large genera (i.e. "Elaenia", "Myiarchus" or "Empidonax") are quite difficult to tell apart in the field due to similar plumage and some are best distinguished by their voices. Behaviorally they can vary from species such as spadebills which are tiny, shy and live in dense forest interiors to kingbirds, which are relatively large, bold, inquisitive and often inhabit open areas near human habitations. As the name implies, a great majority of tyrant flycatchers are entirely insectivorous (though not necessarily specialized in flies). Tyrant flycatchers are largely opportunistic feeders and often catch any flying or arboreal insect they encounter. However, food can vary greatly and some (like the large great kiskadee) will eat fruit or small vertebrates (e.g. small frogs). In North America, most species are associated with a "sallying" feeding style, where they fly up to catch an insect directly from their perch and then immediately return to the same perch. Most tropical species however do not feed in this fashion and several types prefer to glean insects from leaves and bark. Tropical species are sometimes found in mixed-species foraging flocks, where various types of passerines and other smallish birds are found feeding in proximity.