The bird of the day is… the White Bellied Drongo. I believe.
There’s the White Bellied Drongo (Dicrurus caerulescens), also known as Kauda in Sinhala, and then there’s a sub-species, also called the White Bellied Drongo (Dicrurus caerulescens leucopygialis). I think the second is my bird based on the image at the link.
The main difference? Dicrurus caerulescens has "dark grey upperparts, and the tail is long and deeply forked" while Dicrurus caerulescens leucopygialis has "the white on the underparts restricted to the vent area and undertail coverts", according to Wikipedia. Which means I’ll have to figure out what’s meant by the vent area and undertail coverts.