[12] Using nuclear and mitochondrial DNA, the species has been placed within the subfamily Threskiornithinae in a clade that includes both New and Old World members.

[15] The red-naped ibis uses lakes, marshes, riverbeds, irrigated farmlands, dry fallow fields, villages, towns and cities.

It extends into southern India but is not found in the forested regions or the arid zone of the extreme southeast of the peninsula or Sri Lanka.

[18] Red-naped ibis commonly use villages, towns and mega-cities seeking food, nesting and roosting.

[4][15][17][19][20][21] The red-naped ibis is largely diurnal in its foraging and other activities,[22] at night roosting communally on trees or on islands.

[9] In semi-arid areas, they avoid potential competitive interactions with other sympatric ibis species by foraging largely in upland habitats.

[23] Early observers and investigations of stomach contents showed their diet to include crustaceans (prawns, crabs), insects (beetles, grasshoppers, crickets), scorpions, carrion and frogs.

[24][25] Subsequent additional observations have shown the red-naped ibis to be more omnivorous, feeding on carrion,[26] insects, frogs, other small vertebrates, and grain.

[8][19] In small towns, red-naped ibis hunt adult Rock Pigeons and predate their eggs from nests on buildings.

[19] Near towns, ibises ate road kills and pulled out marrow of bones of cattle dead from collisions with traffic.

[28][25] Adult and juveniles birds in Delhi dig into flowers of Bombax ceiba seemingly drinking nectar.

[30] A small but expanding colony of 20 nests spread over two large trees in a village is being monitored in Gujarat.

[15] After successful fledging of chicks, Red-naped ibis nests were taken over by pairs of Red-necked Falcons (Falco chicquera) in Surendranagar district, Gujarat.

Madhaviah Krishnan identified the bird positively as the black ibis and ruled out contemporary suggestions that this was a sarus crane.

He based his identification on a line that mentions the arrival of anrils at dusk and calling from atop palmyra palms.

Sangam poetry also mentions that the birds mated for life and always walked about in pairs, one of the leading reasons for others to assume that this was the sarus crane, a species that is not found in southern India.

[49] Cultivators referred to the species as "planter's friend" based on the large number of crickets the ibises consumed on indigo plantations.

[50][15] DDT levels in blood plasma was 19 ng/mL in red-naped ibis - the lowest among 13 bird species sampled in a project in India.

[16] Studies in urban areas are showing the ability of Red-naped ibis to exploit city-based foods, and nest on artificial structures such as lighting poles even if these are located in busy marketplaces.

[19][20] Red-naped ibises may be benefitting from expanding urban areas as long as urban greening continues to provide them with nesting trees, especially in arid and semi-arid areas where tall trees for nesting are sparse on the countryside, and people continue not to persecute the birds.

[31] The studies collectively suggest that the red-naped ibis may be increasing in numbers due to their behavioural plasticity, especially their ability to use a range of human-modified habitats and areas.