Terrestrial and generally inconspicuous. Gate is slow and deliberate, somewhat like a rail (Haverschmidt 1968), and involves extending the neck forward then following with the body, creating a head-bobbing motion (Slud 1964). May also hop agilely along rocks when looking for prey (Stiles and Skutch 1989), often utilizing their wings in the process (Slud 1964). Hunts by looking intensely and patiently for prey and then lunging with a rapid thrust, similar to a heron (Haverschmidt 1969), but will also search for prey by gleaning rocks or streamside vegetation (Slud 1964). Only rarely wades through water (Schulenberg et al. 2007). Haverschmidt (1969) reported that the Sunbittern is capable of swimming, while Edwards (1998) explicitly states that the species is not a swimmer. Birds sometimes perch in overhanging branches, especially when disturbed (Ridgely and Gwynne 1989). Flight is a series of 1-2 deep wing beats followed by a long glide (Stiles and Skutch 1989).
Sunbitterns display a characteristic tail-waving behavior in which the back side of the body is swayed side to side while the head remains motionless (Lyon and Fogden 1989, Peterson and Chalif 1973, Slud 1964). Resting birds sometimes open and close beaks, as if breathing (Slud 1964).
Birds are fairly unsuspicious and can be easily tamed (Ridgely and Gwynne 1989).
Found alone or in pairs (Haverschmidt 1968, GEPOG 2003). Birds are most likely territorial, at least during the breeding season, with nest located at least 500 m apart. The common whistle used by the Sunbittern (see Vocalizations) probably has a territorial function as birds respond to human imitations by approaching the area and answering with with their own whistles. Birds may also return to the same nesting sites in subsequent years (Thomas and Strahl 1990).
There is little evidence of the Sunbittern using the Frontal Display (see Social and Interspecific Behavior) for courtship (Frith 1978). There is, however, a flight display which takes place in the dry season and early in the breeding season. Birds fly up about 10-15 m, then glide downward with wings hung in a way that showcases the sunburst pattern from a side view. This visual display is combined with the "kak-kak-kak" call followed by a trill (Thomas and Strahl 1990).
Thomas and Strahl (1990) also reported some other unusual behaviors demonstrated by nesting birds that probably are related to pair-bond formation. The brooding adult (probably the female) may greet its returning mate by begging and even grasping at its mate's bill. On another occasion, the male flew to the ground and opened his wings slightly at the wrist while giving a rattling call and bending his head over his back. The female joined him and the pair repeatedly gave this "head-tilting display" in synchrony while rattling in a duet. After about a minute the birds began dipping their heads, then parted, walking in opposite directions around a pool and giving rattles and trills. When they met up again they resumed rattling and dipping their heads.
Courtship begins as early as January (Haye 1989).
Social and interspecific behavior
The spectacular Frontal Display of the Sunbittern has been documented only in threat and defensive situations, with little evidence supporting any role in courtship behavior (Frith 1978). The wings are spread and the upper surface tilted forward to a vertical angle, revealing the chestnut, black, and gold pattern. The tail is lifted and fanned out to fill the gap between the wings (Stiles and Skutch 1989). This posture dramatically increases the apparent size of the displaying bird while exposing what may be interpreted as fake eyes. The defensive role of this display also is supported by the fact that the colorful wing patterns are obtained by both sexes with no intermediate plumage types, as well as by the fact that young Sunbitterns begin practicing the display at an early age (see "chick development" under Reproduction) and will make display attempts when threatened (Frith 1978). If a nest is threatened, both parents may engage in a Frontal Display in unison, but also will demonstrate intermediate displays such as only partially opening the wings or opening only one wing. Sunbitterns on a nest also may exhibit a less-aggressive bittern-like freeze with the bill vertical and the body oriented towards the intruder (Thomas and Strahl 1990).
Adult Sunbitterns, which are fairly large birds that feed predominately in open river habitats, probably are vulnerable to predation by large forest raptors (Lyon and Fogden 1989). Feeding in open river habitats also may make Sunbitterns especially vulnerable to nest predation, which could explain the extended incubation cycles practiced by adults (Lyon and Fogden 1989).