Common in the coastal regions of Surinam preferring the edges of forests where flooded. The species also soars high above the trees in contrast to the Snail Kite.

Slender-billed Kite populations appear to be highly localized and restricted to flooded gallery forest or semiforest habitats

The Slender-billed courtship behavior consists of some aerial display and offering snails by the male to the perched female.
Slender-billed Kite nests are usually were built in the crotches of slanting limbs in large trees in the flooded gallery forest. Nest height averages 14 m (9-20 m) above ground, and some sites are reused in subsequent years. Minimum distances between known active pairs ranges from 1.0-l .2 km. All nests are in habitat that flooded during the wet season, with standing water 5-60 cm in depth, but dries nearly completely during the dry season. Most nests are positioned over heavy brush, although sometimes nests are located over open water (pools or borrow-pit pools). The nest is a small platform approximately 50-60 cm in diameter, constructed of bulky dead twigs, and most are placed in bifurcating crotches. Often the nest is surrounded by green leafy twigs that stick into the air at an angle, perhaps to assist in camouflage. Clutch size is 1-4 eggs, usually 2 in one brood. Both parents incubate the eggs for about 30 days. The young are quiet in the nest and fledge after 35 days, but can fly after 40 days.

The diet consists for 92% of snails and the remaining 8% of freshwater crabs. The Slender-billed Kite feeds on smaller snails than the Snail Kite. the snails are grasped in one or both talon(s) and held against a limb, the kite inserted its bill, presumably between the shell and operculum, and twisted its neck back and forth to pull off the operculum. The remaining snail body is than consumed.

This species has a large range, with an estimated global Extent of Occurrence of 2,200,000 km2. The global population size has not been quantified, but the species is not believed to approach the thresholds for the population size criterion of the IUCN Red List (i.e. less than 10,000 mature individuals in conjunction with appropriate decline rates and subpopulation qualifiers). Global population trends have not been quantified, but the species is not believed to approach the thresholds for the population decline criterion of the IUCN Red List (i.e. declining more than 30% in ten years or three generations). For these reasons, the species is evaluated as Least Concern.

Probably sedentary in all of its range.