Ashy Drongo
Dicrurus leucophaeus
The adult ashy drongo is mainly dark grey, and the tail is long and deeply forked, There are a number of subspecies varying in the shade of the grey plumage. Some subspecies have white markings on the head. Young birds are dull brownish grey. Subspecies longicaudatus of India (which includes beavani of the Himalayas that winters on the peninsula, with one breeding population in central India that Vaurie separates as longicaudatus in the restricted sense) is very dark and almost like the black drongo although this bird is slimmer and has a somewhat longer and less-splayed tail.
ID text from Wikipedia, CC BY-SA 4.0

Black Drongo
Dicrurus macrocercus
This bird is glossy black with a wide fork to the tail. Adults usually have a small white spot at the base of the gape. The iris is dark brown (not crimson as in the similar ashy drongo). The sexes cannot be told apart in the field. Juveniles are brownish and may have some white barring or speckling towards the belly and vent, and can be mistaken for the white-bellied drongo. First-year birds have white tips to the feathers of the belly, while second-years have these white-tipped feathers restricted to the vent.
ID text from Wikipedia, CC BY-SA 4.0

Where you might meet both
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Play today's Daily BirdPhotos and recordings are credited to their authors under the licence shown. Full profiles: Ashy Drongo and Black Drongo.