Immature Yellow-crowned Night-herons and immature Black-crowned Night-herons look very similar.
Both are brown with large white spots on the wings and blurry streaks on the underparts.
However, immature Yellow-crowned Night-herons have dark bills while the bill of immature Black-crowned Night-herons are dark on top and yellowish on the bottom.
Furthermore, immature Black-crowned Night-herons are chunky and squat like the adults and have short necks. 
Immature Yellow-crowned Night-herons tend to stand straighter and have heavier bills and longer legs, and the back and wings have finer spots and streaks than those of immature Black-crowned Night-herons.

First 4 photos: An adult. Playa Tamarindo, Guánica State Forest, Puerto Rico, 120 January 2026
5th photo: A juvenile. Bosque de Pinones Nature Reserve, Puerto Rico, 23 January 2026