Adult Male:
The forehead and crown are ash gray. The nape, sides, and rear of the neck are gray-green. The back and lesser wing coverts are mahogany brown. The breast, belly, and upper tail coverts are yellowish-green. The greater wing coverts are blackish with a slight blue hue and bright yellow edging. The primary and secondary flight feathers are black, narrowly edged with yellow. The tertiary flight feathers are yellowish-green with yellow edging. Underwing feathers are gray. The central tail feathers are dark yellowish-green, and the outer tail feathers are gray with a yellowish-green outer edge and a broad black band across the middle of the feather. The undertail coverts are tawny-yellow. The lower abdomen, the feathers around the vent, and the thighs are yellowish-green, with white tips on the feathers.
Adult Female:
Lacks the mahogany brown coloration. The dorsal surface is entirely dark yellowish-green. The undertail is white with numerous pale yellowish-green streaks.
Ocular Features:
The outer eye ring is yellow, the inner eye ring is pale blue. The skin around the eye is pale blue. The base of the beak is red, and the tip is horn-gray with a bluish or yellowish tinge. The legs are red.
Measurements:
- Wing (male): 137 – 153 mm
- Wing (female): 135 – 152 mm
- Tail: 85 – 90 mm
- Tarsus: 18 – 23 mm
- Bill: 15 – 18 mm
Distribution:
The Collared Falconet (Microhierax caerulescens) is distributed in Nepal, Assam, Myanmar to Tenasserim, Thailand, Laos, Cambodia, Southeastern China, and Vietnam.
In Vietnam, this species is present throughout most dense forests in the summer, and during the winter, it is also found in the lowlands where there are many large trees, especially banyan and fig trees.