Cranial bones. At 2–3 days after hatching, cartilaginous templates of the supraoccipital, laterosphenoid, orbitosphenoid, mesethmoid, and otic elements are present, while the basioccipital, squamosal, parasphenoid rostrum, and parasphenoid lamina are already ossified (Fig. 1A, B). The parietal and frontal display small intramembranous ossification centres, weakly stained and surrounded by mesenchymal tissue (Fig. 1A). An ossification centre is visible in the exoccipital (Fig. 1B). No evidence of ectethmoid or lacrimal is observed. By 9–10 days after hatching, the supraoccipital, laterosphenoid, mesethmoid, otic elements are ossified, and the parietal and frontal show a marked expansion of their ossified areas (Fig. 1C, D, G). The exoccipital and basioccipital also enlarge their ossified regions but retain cartilaginous portions in the paroccipital process and occipital condyle (Fig. 1G), except for the dorsolateral portion of the paroccipital process, which lacks a cartilaginous precursor. At this stage, the opisthotic and prootic fuse with each other and with the exoccipital, the epiotic fuses to the internal lateral margin of the supraoccipital and extends dorsally to reach the external surface of the cranial vault (Fig. 1D), and the parasphenoid lamina is fully fused to the parasphenoid rostrum and partially to the basioccipital (Fig. 1G). At 13–14 days after hatching, ossification expands throughout all elements. The squamosal is fully ossified but unfused, contacting the exoccipital caudoventrally, the parietal dorsally, and the laterosphenoid rostrally (Fig. 2A, B). The orbitosphenoid begins with ossification (Fig. 2B). The cartilaginous precursor of the dorsolateral paroccipital process appears (Fig. 2A), extending rostroventrally from the lateral margin of the exoccipital and contacting the squamosal dorsally. The exoccipitals are completely fused with the basioccipital, supraoccipital, and epiotic (Fig. 2A). From 15–16 days after hatching, ossification intensifies in the parietal and frontal (Fig. 2C, D), with progressive ossification of the ventromedial paroccipital process and parasphenoid ala. In dorsal view, the rostral ends of the frontals extend laterally, with the circular dorsal lamina of the mesethmoid visible between them (Fig. 2C). At 20–21 days after hatching, an ossification centre appears in the cartilage that will form the dorsolateral portion of the paroccipital process (Fig. 2F). At this stage, the occipital condyle, the ventromedial portion of the paroccipital process, and the parasphenoid ala are fully ossified (Fig. 2E). The parietal and frontal appear thinner and more translucent compared with earlier stages.

Maxillary and palatal bones. At 2–3 days after hatching, ossification centres are present in the quadrate (corpus and proximal otic process) and nasal, while the pterygoid, palatine, jugal, quadratojugal, maxilla, and premaxilla are ossified (Fig. 1A, B). The quadratojugal is weakly stained, indicating incipient ossification. The jugal and maxilla are fused (Fig. 1A), and the paired premaxillae are fused rostromedially (Fig. 1B). At 9–10 days after hatching, the quadrate shows complete ossification of the mandibular and orbital processes (Fig. 1G). By 13–14 days after hatching, ossification expands, and the quadrate develops ossification in the orbital process, mandibular process, and zygomatic articular tubercle. Ossification of the orbital process of the quadrate progresses at 15–16 days after hatching, and all maxillary bones show more intense staining (Fig. 2C). By 17–18 days after hatching, the quadrate is fully ossified (Fig. 3C, D). At 20–21 days after hatching, all elements appear thinner and more translucent than at the preceding stage (Fig. 2E).

Mandibular bones. At 2–3 days after hatching, the dentary, splenial, angular, supraangular, and prearticular are ossified (Figs 1A, 3A), with an ossification centre in the articular at the medial mandibular process. In the mounted specimens (Figs 1A, 2B, D), where the upper and lower jaws could be examined in articulation—the lower beak consistently projected slightly beyond the upper beak. The dentary displays an elongate fenestra. At 9–10 days after hatching, the articular, splenial, and dentary remain unfused, while the angular fuses with the supraangular, and the prearticular is partially fused to the articular (Fig. 4A). At 13–14 days after hatching, the paired dentaries are partially fused rostrally (Fig. 2A). Ossification of the dentary increases at 15–16 days after hatching, and all mandibular bones show more intense staining (Fig. 2D). By 17–18 days after hatching, caudal elements are fully ossified and fused, while the splenial remains separate, and the dentaries are fused to each other at the mandibular symphysis (Fig. 4B, C). At 20–21 days after hatching, the mandible appears thinner and less fused than in the preceding stage (Fig. 4D).

Hyoid apparatus. At 2–3 days after hatching, the entoglossum and basihyal are ossified, while the urohyal, ceratobranchial, and epibranchial remain cartilaginous (Figs 1A, 3A). By 15–16 days after hatching, the morphology of the entoglossum is clearly distinguishable, being arrow-shaped with paired foramina; the basihyal is ossified in its caudal two-thirds; the ceratobranchial is completely ossified; the urohyal remains cartilaginous; and the right epibranchial shows a proximal ossification centre (Fig. 5).

Cervical vertebrae. At 2–3 days after hatching, ossification centres are visible in the vertebral bodies of all cervical vertebrae except the atlas, which remains cartilaginous (Figs 1A, B, 6A). Bilateral ossification centres occur in the neural arches of all cervical vertebrae, including the atlas (Fig. 6A). At 9–10 days after hatching, an ossification centre appears in the body of the atlas, and the neural arches of the atlas are ossified but unfused dorsomedially (Fig. 6B). Other cervical vertebrae show fusion of arches and bodies, except for the rostroventral junction, which remains cartilaginous between the costotransverse ansa and cranial articular surface (Fig. 6B). The dens of the axis is cartilaginous, and cranial and caudal articular facets, zygapophyses, ventral processes, neural spines, costal processes, dorsal tubercles, transverse processes, carotid processes, and hypapophyses of the cervical series are incompletely ossified. At 13–14 days after hatching, ossification expands in all structures except the condylar fossa of the atlas, which remains cartilaginous (Fig. 6C). No further changes occur at 15–16 days after hatching. By 20–21 days after hatching, the ventral and lateral portions of the condylar fossa of the atlas are ossified, the dens of the axis is fully ossified, and the ventral processes and transverse processes are completely ossified. Ossification also expands in the neural spines and costal processes (Fig. 6D).

Cranial bones. At 11–12 days after hatching, all neurocranial elements are ossified except the orbitosphenoid and ectethmoid, with varying degrees of fusion in the ventral and occipital regions (Fig. 7A), but no fusion between the frontals and parietals dorsally (Fig. 8A). The exoccipital is partially fused with the supraoccipital and the epiotic, and the latter two begin to fuse externally on the cranial vault (Fig. 7B). The paroccipital process of the exoccipital is absent. The basioccipital is completely fused to the parasphenoid lamina, and the base of the occipital condyle is ossified (Fig. 7A). Ossification begins in the parasphenoid ala (Fig. 7A). On each side of the mesethmoid septum, cartilage precursors of the ectethmoid are visible (Fig. 7C), while no recognizable cartilaginous tissue is present where the lacrimal typically develops. By 27–28 days after hatching, ossification appears at the base of the dorsolateral portion of the paroccipital process (Fig. 7D). The squamosal is fused with the basioccipital and laterosphenoid. The frontals and parietals are fully ossified but remain unfused. The mesethmoid shows expanded ossification contributing to both its medial septum and dorsal lamina, while the ectethmoid develops laterally through ossification of its cartilaginous precursors (Fig. 7E).

Maxillary and palatal bones. At 11–12 days after hatching, the pterygoid is ossified except for its dorsal process (Fig. 8A), which remains cartilaginous. In the quadrate, the orbital process, mandibular process, and zygomatic articular tubercle remain cartilaginous (Fig. 8A). Elements of the maxilla and part of the palatine were lost during preparation. By 27–28 days after hatching, the base of the pterygoids’ dorsal process is ossified (Fig. 8D). The maxillary elements are completely ossified, and the premaxillae are fused rostromedially at the base of their frontal processes (Fig. 8C). The palatine, jugal, and quadratojugal are completely ossified but remain unfused from other elements.

Mandibular bones. At 11–12 days after hatching, all mandibular elements are ossified. Partial fusion is observed among caudal elements, while the dentary and splenial remain unfused (Fig. 8B). By 27–28 days after hatching, all mandibular elements are ossified and the splenial is fused with the dentary.

Cervical vertebrae. At 11–12 days after hatching, the atlas corpus is ossified, except for the condylar fossa, and fused to the neural arch. The two centres of the neural arch are fused dorsally. The proximal portion of the dens of the axis is ossified. In the remaining cervical vertebrae, corpus and neural arches are completely ossified and fused (Fig. 9A). Cranial and caudal articular facets, zygapophyses, neural spines, ventral processes, costal processes, carotid processes, transverse processes, dorsal tubercles, and hypapophyses show early or incomplete ossification (Fig. 9A). By 27–28 days after hatching, all cervical vertebrae are completely ossified (Fig. 9C), except for the atlas, in which the dorsal part of the condylar fossa remains cartilaginous (Fig. 9B).

At comparable stages, no major differences were observed in the general pattern of ossification between the two species. The C. melanochloros specimen at 9–10 days after hatching and the C. campestris specimen at 11–12 days exhibit very similar developmental features. However, due to the loss of maxillary and palatal elements during preparation in the C. campestris specimen, their degree of ossification could not be evaluated. In C. melanochloros, these bones remain largely cartilaginous at this stage, suggesting that in C. campestris they may have been in a similar condition and thus more prone to loss during preparation. In contrast, comparison of the C. campestris specimen at 27–28 days after hatching with the C. melanochloros specimen at 20–21 days after hatching reveals a more advanced degree of development in the former, consistent with the age difference. Beyond this chronological offset, no marked differences were detected in the sequence of ossification or in the fusion of skeletal elements between the two species.

Cranial bones. The elements forming the cranial bones and occiput are completely ossified and fused (Fig. 10C). In the calvaria, the frontals and parietals remain unfused, both to each other and to adjacent bones (Fig. 10A, B). In the temporal region, the squamosal is not fused with the exoccipital or parietal and it barely contacts the frontal due to the rostral extension of the lateral margin of the parietal and the caudal development of the laterosphenoid between them. The squamosal contributes to the rostral process of the paroccipital process, although only the base of this structure is ossified. The laterosphenoid entirely forms the postorbital process, which remains unfused with the frontal. The interorbital septum is incompletely ossified at the level of the interorbital fonticulus (Fig. 10B).

Maxillary and palatal bones. In the maxilla, the frontal processes are not fully fused. The quadrate and pterygoid are fully ossified (Fig. 10B), while the other palatal elements are absent in the specimen and therefore could not be described.

Mandibular bones. In the mandible, both the articular and the dentary show incomplete ossification. Mandibular elements, including the angular, prearticular, surangular, and splenial, remain incompletely fused (Fig. 10D, F).

Cervical vertebrae. In cervical vertebrae C6 and C10, the vertebrae are fully developed except for the dorsal lamina of the neural arch, which remains incompletely ossified in its rostral half (Fig. 10E, G). The other vertebrae were absent in the available specimen and therefore could not be analysed.

Cranial bones. The frontals remain unfused, except rostrally where instead they are fused to the dorsal lamina of the mesethmoid, which appears as a porous unpaired ossified structure (Fig. 11A). The mesethmoid is externally visible on the forehead between the frontals, whose rostral ends project laterally around it. It is also fused to adjacent cranial bones, including the caudal and ventrocaudal surfaces of the frontal processes of the premaxillary and the nasals (Fig. 11A). The frontals remain unfused and show no fusion with the parietals (Fig. 11A, C, D). However, the frontals still are fused with the surrounding cranial elements. In the temporal region, the rostral processes of the paroccipital process are markedly thinner compared with adult specimens of the genus, suggesting incomplete ossification (Fig. 11G).

Maxillary and palatal bones. In the maxilla, the nasal is not completely fused with the maxilla or with the frontal process of the premaxilla (Fig. 11B). The frontal processes remain unfused caudally. The jugal and quadratojugal are also unfused (Fig. 11A).

Mandibular bones. In the mandible, the articular is fully ossified and fused with the prearticular, angular, and supraangular. However, the angular has not completed its fusion with these elements. The splenial and dentary are fused to each other but remain unfused to the supraangular and angular.

Cervical vertebrae. The atlas shows the condylar fossa partially ossified dorsally. The ansa costotransversaria exhibits incomplete development in its ventral region and remains free, without fusion to the vertebral body (Fig. 11F). The remaining cervical vertebrae are completely ossified.

Woodpeckers displayed a delayed onset of cranial and cervical ossification, with many elements still cartilaginous in the first post-hatching days, a condition typical of altricial birds (Genbrugge et al. 2011; Mitgutsch et al. 2011; Carril and Tambussi 2017; Skawiński et al. 2021). In contrast, precocial species ossify most skeletal elements before hatching (Hogg 1978; Starck 1993). The timing of cranial fusions parallels other highly altricial taxa, such as Myiopsitta monachus (Psittacidae, Psittaciformes) (Carril et al. 2021) and Geospiza fortis (Thraupidae, Passeriformes) (Genbrugge et al. 2011), where basicranium and occipital integration begins early and progresses rapidly throughout the postnatal period. In both M. monachus and G. fortis, some mandibular fusions occur prenatally, whereas in woodpeckers these unions take place exclusively postnatally. In all three altricial taxa, however, the articular ossifies during the first post-hatching days and remains separate from the prearticular until late nestling stages, reflecting a conserved altricial pattern (Genbrugge et al. 2011; Carril and Tambussi 2017).

Variation was also detected within the woodpeckers examined in this study, with the specimen of 20–21 days showing lower ossification than that of 17–18 days. Similar intraspecific variability is documented in Taeniopygia guttata (Estrildidae, Passeriformes) indicating natural developmental heterogeneity rather than methodological artifacts (Mitgutsch et al. 2011). Such differences among individuals of comparable age are common in avian ontogenetic series (Starck and Ricklefs 1998; Maxwell 2008) and may reflect environmental influences or intrinsic growth plasticity (Blom and Lilja 2004). Recognizing this variability is essential for interpreting developmental timing across taxa and for distinguishing genuine interspecific patterns from individual variation (Morris et al. 2024).

Cervical vertebrae development followed the altricial pattern: the atlas ossified later, and neural arches remained unfused to centra at early stages (Carril and Tambussi 2017; Skawiński et al. 2021), contrasting with Acrocephalus scirpaceus (Acrocephalidae, Passeriformes), where fusion from C3 is visible early (Skawiński et al. 2021). This highlights considerable heterogeneity in cervical skeletal development among altricial birds, suggesting that neck ossification may evolve under lineage-specific functional demands rather than following a uniform altricial pattern (Carril and Tambussi 2017; Skawiński et al. 2021). In woodpeckers, this delayed cervical maturation could reflect the late onset of behaviours—particularly pecking— that require head stabilisation (Jung et al. 2016).

The ossification sequence of the basicranium and occipital bones followed the ventrodorsal and caudorostral fusion model (Jollie 1957; Hogg 1978; Plateau and Foth 2020). In juveniles, these regions were fully ossified and fused, whereas frontals and parietals showed variable states: in Campephilus leucopogon, both remained unfused; in Melanerpes candidus, parietals were fused to each other but not to frontals, suggesting a more advanced developmental stage despite unknown age. Additional evidence supporting this interpretation includes the complete ossification of the rostral process of the paroccipital process in M. candidus (only incipient in C. leucopogon) and a more mature mandible, with the articular fully ossified and fused to adjacent elements. Together, these features indicate that the M. candidus specimen likely represents a later juvenile stage than the C. leucopogon individual.

A distinctive feature of woodpeckers is the late ossification of the orbitosphenoid around days 13–14, delayed relative to G. fortis (Genbrugge et al. 2011) but earlier than in precocial Gallus gallus (days 77–84, Hogg 1978). This reflects the broader trend that postnatal ossification progresses slowly in precocial birds but accelerates in altricials during the first three weeks (Hogg 1978; Blom and Lilja 2004). Elements essential for pecking performance, such as the dorsal process of the pterygoid, the rostral process of the processus paroccipitalis, and the ectethmoid contacting the jugal bar (Spring 1965; Donatelli 1996; Manegold and Töpfer 2013), ossify later, supporting delayed emergence of clade-specific traits (Starck and Ricklefs 1998).

The ossification sequence of the hyoid apparatus in woodpeckers shows that by days 2–3 the entoglossum and basihyal are ossified, while the ceratobranchiale and epibranchiale ossify only after days 15–16 and the urohyal remains unossified. The pronounced elongation of the epibranchial horns (Bock 1999) may reflect an extended cartilaginous stage prior to ossification (Ortega et al. 2004). This ossification pattern contrasts with the monk parakeet M. monachus, where these elements are already ossified by days 0–5 (Carril and Tambussi 2017), reflecting an earlier functional readiness of the tongue in psittacids. In woodpeckers, despite the tongue’s extreme specialization (Bock 1999; Villard and Cuisin 2004), full functionality is achieved only post-fledging (Blume 1971). Such delayed ossification challenges the “functional hypothesis” which predicts early ossification of performance-critical elements (Mabee et al. 2000; Maxwell 2008), and aligns woodpeckers more closely with other altricial taxa, such as the Darwin’s finch Geospiza fortis, where the urohyal and epibranchiale also ossify late (Genbrugge et al. 2011). However, because the woodpecker tongue operates as a functionally complex system whose mechanics do not straightforwardly depend on the degree of skeletal ossification (Jung et al. 2016), the implications of delayed hyoid ossification should be interpreted with caution. Additionally, we confirm the presence of a urohyal in woodpeckers (Donatelli 1996), contradicting earlier claims of its absence (Jung et al. 2016).

Overall, woodpeckers fit within the altricial developmental spectrum, with early ossification of broadly functional regions but delayed maturation of lineage-specific specialisations.

A further cranial feature observed in the earliest post-hatching stages is a slight projection of the lower beak relative to the upper one. This condition is shared with other members of Picocoraciae—a clade uniting Piciformes, Coraciiformes, and Bucerotiformes (Prum et al. 2015)—as coraciiform and bucerotiform neonates also hatch with a prognathic lower beak that persists for several days (Gamble 2014). Interestingly, woodpeckers retain this trait into adulthood: although externally masked by the longer rhamphotheca of the maxilla, the bony mandible is slightly longer than the premaxilla (Wang et al. 2011). This suggests that the neonatal prognathous condition may be paedomorphically conserved in woodpeckers. Although Wang et al. (2011) suggested that this retention might contribute to absorbing impact stresses, its functional relevance for pecking or cranial loading dynamics remains to be tested.

Postnatal skeletal development is central to shaping avian skull morphology and its evolutionary trajectory. Delayed ossification and variable bone fusion timing can influence adult form, either constraining or facilitating evolutionary change (Bhullar et al. 2016; Smith-Paredes et al. 2018; Picasso 2025). In this context, analyses of woodpecker postnatal sequences provide insights into the developmental origins of their cranial specialisations.

One of the most striking findings of this study is the complete absence of the lacrimal bone across all examined stages. Since this element normally ossifies early in avian ontogeny (Carril and Tambussi 2017; Arnaout et al. 2021), its absence in Picinae supports a true secondary loss rather than fusion with the ectethmoid, as previously suggested by Cracraft (1968), and later by Simpson and Cracraft (1981). Rather than weakening the skull, the loss of the lacrimal likely reflects a functional reorganisation. In other amniote lineages, particularly during the evolution from premammalian cynodonts to early mammals, simplification of cranial elements has been shown to redistribute stresses and enhance performance (Lautenschlager et al. 2023). More specifically in birds, the loss of the lacrimal bar has been associated with the evolution of cranial kinesis, facilitating the transfer of quadrate motion to the upper beak via the jugal bar and palate (Bout and Zweers 2001).

Classical studies emphasised the role of the lacrimal–ectethmoid complex as a retractor stop, preventing excessive depression of the upper jaw during kinesis (Fisher 1955; Bock 1964; Cracraft 1968). These stops were thought to buffer abnormal external forces acting on the bill, such as accidental impacts against hard objects, which could otherwise disrupt the kinetic hinge. Interestingly, woodpeckers habitually subject their skulls to extreme external forces during pecking, yet they entirely lack this structural stop. In woodpeckers, the cranial system has been observed to reorganise in a different way to accommodate such forces (Lyons 2025).

A key element of this reorganization is the unusual prokinesis in Picidae: Rather than the typical elevation of the upper beak coupled with mandibular depression, woodpeckers exhibit a consistent downward displacement of the upper beak (Lyons et al. 2023). In this context, the lacrimal—located more rostrally than the ectethmoid—could represent a structural obstacle to upper beak depression, a motion that woodpeckers actively use during the rapid retraction of a beak that becomes lodged in wood (Van Wassenbergh et al. 2022b). Its absence may thus remove kinetic constraints, while the ectethmoid, positioned more caudally, can still maintain contact with the jugal bar and preserve functional connectivity. The loss of the lacrimal, therefore, could represent an adaptive modification enhancing both cranial kinesis and the redistribution of stresses.

Altogether, these observations suggest that the secondary loss of the lacrimal in woodpeckers is not a byproduct of cranial simplification, but an evolutionary adaptation aligned with their distinctive kinetic regime.

Beyond the absence of the lacrimal, additional modifications in the frontonasal hinge may contribute to the distinctive kinetic system of woodpeckers. In some species, bony projection known as the frontal overhang restricts movements of the upper beak (Bock 1999; Lyons et al. 2023). This structure has been interpreted as an apomorphy of the clade including Picumninae and Picinae (Manegold and Töpfer 2013), although its degree of development varies among lineages: it reaches its greatest development within the tribe Melanerpini, while it is absent in Picini and strongly reduced in Campephilus species (Donatelli 1996; Manegold and Töpfer 2013; Lyons 2025). Our observations suggest that in juveniles, the highly porous, unpaired bone located rostrally between the frontal bones correspond to the dorsal lamina of the mesethmoid, rather than the frontals, which are usually fused to each other until reaching the frontal processes of the premaxilla or those of the nasals (e.g., Livezey and Zusi 2006; Plateau et al. 2024). Consequently, what is usually referred to as the frontonasal hinge is, to a large extent, formed by the junction between the mesethmoid and the premaxilla, with only a relatively small contribution from the contact between the frontals and nasals at the lateral margins of the hinge (Fig. 11A). Thus, the dorsal lamina of the mesethmoid appears to constitute the main component of the so-called frontal overhang, a structure likely to play a key role from a functional perspective.

Functionally, this configuration has two major implications: first, it places the mesethmoid as a key structural element reinforcing both the interorbital septum and the cranial roof immediately posterior to the base of the upper beak (Van Wassenbergh et al. 2022a); second, by forming the main component of the frontal overhang, it restricts the elevation of the upper beak during cranial kinesis, as originally suggested by Bock (1999) and further supported by recent experimental evidence showing that the upper beak elevates only minimally in woodpeckers compared to other birds (Lyons et al. 2023).

These observations indicate that the mesethmoid plays a complex functional role in woodpecker cranial architecture, not only as structural support beneath the frontal bones but also as a central component in the integration of impact resistance and kinetic capabilities in Picidae.