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.

Top: An adult. Biological Station Las Guacamayas, Parque Naturel Laguna del Tigre, Petén, Guatemala, 21 January 2020
Left: An immature. San Benito, Petén, Guatemala, 20 January 2020
Middle & Right: San Benito, Petén, Guatemala, 20 January 2020