The Chilli de árbol is a small and potent Mexican chilli pepper also known as bird’s beak chilli and rat’s tail chilli. These chillis are about 5 to 7.5 cm long, and 0.65 to 1 cm in diameter. Their heat index is between 15,000 – 30,000 Scoville units. The peppers start out green and turn a bright red colour as they mature.