Postnatal cranial ontogeny and growth strategies in the black‐tailed gullLarus crassirostrisbreeding on Kabu Island, Aomori, Japan

Abstract

Although cranial morphology of modern birds has been an important subject of evolutionary studies, detailed information on postnatal ontogeny of the avian skull remains very scarce. Herein, a wild population of the black-tailed gull (Larus crassirostris) breeding on Kabu Island was examined to explore relationships between their growth strategies and ontogenetic changes in the cranial shape. By examining growth series covering a major part of the postnatal ontogenetic period, it was clarified that the typical form of the adult larid skull was produced through dynamic proportional changes among cranial structures after hatching. Cranial structures related to the oral capacity and deglutition (e.g., some elements of hyoid bones) attained the adult sizes before fledging, whereas other structures apparently reached the adult sizes well after leaving the nest. The palatal width exhibited positively allometric growth against the skull volume in chicks in the early nestling stage, followed by a period of negative allometry in the later growth stage. A nestling gull has to acquire swallowing ability by the time of fledging at the latest because this ability is apparently essential for a food intake without parental aid. In addition, because nestlings mainly feed on foodstuffs supplied by their parents, previous studies suggested the possibility that siblings within a nest compete for food resources. Under this scenario, early acquisition of swallowing abilities, as indicated by the allometric patterns of relevant cranial structures, may be an adaptive trait that enhances survival of nestlings.

Common Patterns of Skull Bone Fusion and Their Potential to Discriminate Different Ontogenetic Stages in Extant Birds

“…In comparison, the linear measurements reflect interspecific allometric variation (against the geometric mean) much better than ontogenetic allometric variation (against RSP). However, on an intraspecific level, the analysis of morphometric measurements is still an appropriate approach to document changes of size and proportions during ontogeny (see e.g., Carrier and Leon, 1990;Köppl et al, 2005;Bennett, 2008;Hanai et al, 2021).…”

“…However, major osteological developments happen during the short period of postnatal growth, e.g., the ossification of the sternum (Heers et al, 2021) and articulation facets in long bones (Picasso and Barbeito, 2017) or the fusion of bones in the skull, axial skeleton, manus and tarsus (Heers and Dial, 2012;Plateau and Foth, 2020). When compared with embryogenesis, the postnatal development is only poorly documented so far (e.g., Hogg, 1977Hogg, , 1984Winkler, 1979;Carrier and Leon, 1990;Starck, 1998;Köppl et al, 2005;Turvey and Holdaway, 2005;Tumarkin-Deratzian et al, 2006;Bennett, 2008;Heers and Dial, 2012;Bailleul et al, 2016;Hanai et al, 2021). Consequently, the age/ontogenetic stage of specimens in ornithological collections is often poorly documented.…”

“…Consequently, the age/ontogenetic stage of specimens in ornithological collections is often poorly documented. However, to study postnatal ontogeny and potential heterochronies in birds this information is crucial (see e.g., Hanai et al, 2021). Most of the approaches compared changes in skeleton proportions (tarsal and ulnar length, skull width and length) of post-hatchling individuals (e.g., Carrier and Leon, 1990;Köppl et al, 2005;Bennett, 2008;Hanai et al, 2021).…”

“…However, to study postnatal ontogeny and potential heterochronies in birds this information is crucial (see e.g., Hanai et al, 2021). Most of the approaches compared changes in skeleton proportions (tarsal and ulnar length, skull width and length) of post-hatchling individuals (e.g., Carrier and Leon, 1990;Köppl et al, 2005;Bennett, 2008;Hanai et al, 2021). In addition, the ontogenetic stage of a bird was determined in several case studies based on the bone textures (e.g., Tumarkin-Deratzian et al, 2006;Watanabe, 2018), degree of bone pneumatization (e.g., Winkler, 1979;Hogg, 1984), plumage generations (e.g., Dwight, 1900;Humphrey and Parkes, 1959;Köppl et al, 2005) and bone fusion (e.g., Hogg, 1977;Bailleul et al, 2016), but never compared and verified on an interspecific level.…”