Bird-eating tarantulas like all arachnids undergo a process called ecdysis, or moulting, to shed their exoskeleton as they grow

This process is essential because their rigid exoskeleton does not expand. As they outgrow their skin, they secrete enzymes to loosen the old exoskeleton and absorb the fluid to soften it further. They then carefully split the exoskeleton along predetermined lines and emerge, often taking hours to fully free themselves.⁠
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After moulting, the tarantula's new exoskeleton is soft and pliable, allowing it to expand and harden over time. This vulnerable phase is critical for growth, enabling them to increase in size and develop reproductive maturity. The entire process is energy-intensive and risky, as they are defenseless until their new exoskeleton hardens.⁠