Skull evolution in woodpeckers via articular innovation and allometric decoupling facilitates pecking performance

Woodpeckers exhibit a unique pecking behaviour that subjects their skulls to extreme mechanical stresses, requiring specialised morphology. Several adaptations enabling them to withstand these impacts have been identified, yet the interaction between osteological features that mitigate biomechanical stress, craniofacial architecture, and macroevolutionary patterns remain underexplored. Here, we examine the quadrato-mandibular joint and analyse cranial shape in woodpeckers compared with other insectivorous non-picid birds using geometric morphometrics and phylogenetic comparative methods. We document previously undescribed reinforcing articular contacts within the cranioquadrate-mandibular system, including an expanded articular surface on the condylus medialis of the quadrate forming a trochlea lateralis that contacts the caudal facet of the mandibular crista intercotylaris, as well as multiple stabilising interfaces between the quadrate and the neurocranium involving the suprameatic, zygomatic, and basicranial regions. These features are present across Picidae but vary markedly in their degree of development, being weakly expressed in Picumninae (Picumnus cirratus) and most pronounced in Picinae, particularly in taxa specialised for forceful pecking such as Campephilus. Shape analyses reveal among woodpeckers a decoupling from the craniofacial evolutionary allometry seen in other birds, replaced by a significant influence of relative brain size on skull shape, and uncover distinctive cranial configurations with important biomechanical implications (e.g., reduced maxillary base, altered temporal fossa position and quadrate orientation). This skull-brain association, together with woodpeckers' unique anatomy, likely enhances mandibular stability and quadrate articulation, providing additional support and increasing resistance to impact forces during pecking. The selection of this alternative morphological transformation trend in woodpeckers optimised a demanding ecological performance while allowing morphological diversity without compromising biomechanical stability. Our findings provide new insights into the biomechanical strategies underlying woodpeckers' pecking and highlight the role of skull macroevolution in their ecological specialisation.