Keen earing. The Yellow-Masked Ululare has advantage on Wisdom (Perception) checks that rely on hearing

Glide. The Yellow-Masked Ululare can use its skin membrane to glide. when falling it can move up to 5ft horizontally for every 1 foot it descends

Multiattack. The Yellow-Masked Ululare makes two attacks: one with its bite and one with it claws

Bite. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit: 6 (1d6 + 3) piercing damage. 

Claw. Melee Weapon Attack: +5 to hit, reach 5 ft., one target. Hit:  5 (1d4 + 3) slashing damage. 

Sonic Scream (Recharge 5-6. The Yellow-Masked Ululare emits a devastating scream in a 30-foot cone. Each creature in that area must make a DC 14 Constitution saving throw, taking 18 (4d8) thunder damage on a failed save, or half as much damage on a successful one. Creature that fail the save are deafened for 1 minute. Creatures failing the save by 5 or more are also stunned until the end of the Yellow-Masked Ululare's next turn. A creature can repeat the saving throw at the end of each of its turns, ending the deafened condition on itself on a success.

Enter the description for how bonus actions work for your monster here.

Sonic Disruption. When a Yellow-Masked Ululare or an ally within 30 feet of it is hit by an attack, it can emit a quick, disruptive scream as a reaction, causing the attacker to substract 1d4 from the attack roll. If another Yellow-Masked Ululare within 30 feet uses Sonic Disruption, it can join in, increasing the reduction by an additional 1d4

Choir. If another Yellow-Masked Ululare within 30 feet uses its Sonic Scram, the Ululare can use it reaction to also use Sonic Scream, if it is available. A creature affected by more than one sonic scream in a turn must make the Constitution saving throw with a DC increased by 1 for each additional scream beyond the first and the damage increases by 2d8 for each scream beyond the first.