Geometric morphometrics reveal interspecific and sexual differences in bill morphology in four sympatric planktivorous petrels

Abstract

Variation in morphological structures may indicate the existence of ecological differences between species or sexes. In birds, the bill is one of the structures most affected by selection pressures because it is directly involved with several biological functions, particularly the acquisition of food. In this study, we combined geometric morphometrics and comparisons of linear measurements to assess the presence of sexual dimorphism and differences in bill shape and size of four species of abundant zooplanktivorous seabirds that coexist during their breeding season in Southern Ocean ecosystems [blue petrel (Halobaena caerulea), Antarctic prion (Pachyptila desolata), common diving petrel (Pelecanoides urinatrix) and South Georgia diving petrel (P. georgicus)]. The results showed that bills of Antarctic prion and blue petrel differed noticeably in size and shape from those of the two diving petrels, whereas those of common diving and South Georgian diving petrels overlapped in bill shape. These differences may be due to diet segregation or factors such as diving strategy, vocalizations or olfactory development. In addition, there was sexual dimorphism in bill shape or size in all four species. However, we could not directly attribute these differences to selection pressures because of the lack of studies comparing feeding ecology between sexes in these seabirds. These results reveal relationships between the bill shape and key factors of the ecology of these species, and demonstrate the usefulness of geometric morphometrics for detecting interspecific and sexual differences in species of otherwise sexually monomorphic petrels.

Stage-Dependent Niche Segregation: Insights From a Multi-Dimensional Approach of Two Sympatric Sibling Seabirds

“…In South Georgia, the co-existence of CDP and SGDP appears to induce similar exploitative competition during the incubation period, with an isotopic niche segregation arising from SGDP constrained to diving deeper than CDP (Navarro et al 2014). Similar to Kerguelen, this potential inter-speci c competition may result in SGDP pushing the trade-off between prey size and catch towards larger prey (Reid et al 1997), which would be facilitated by their wider bill compared to CDP (Trallero et al 2019). However, while body size difference appears to be the main factor driving competitive exclusion, and has been largely documented in various cases in both terrestrial and marine environments (Wearmouth and Sims 2008), our understanding of such predator-prey interactions will remain unclear without direct observation of foraging behaviour in the eld.…”

Variation among species and populations in bill shape and size in three planktivorous petrels

“…Any pictures that were not standardized in this way were excluded from morphological analyses. In addition, three biometric measurements were taken in the field: maximum bill depth, bill depth at nares and culmen length as shown in Trallero et al (2019). All measurements were made using digital callipers (± 0.1 mm) (Fig.…”

“…Given its versatile role, the bill is subject to strong selective forces and it shows wide morphological plasticity and divergence (Schluter 2000;Van Oordt et al 2018). Many studies have pointed out the association between bill shape and size, and diet and niche differentiation (Nebel et al 2005;Bull 2006;Trallero et al 2019), presumably driven by interspecific competition and natural selection (Grémillet et al 2004;Navarro et al 2013;Corman et al 2016).…”

“…Geometric morphometric analyses represents a precise tool for measuring shape variation in anatomical features (Adams et al 2013), providing insights into relationships between morphological structures and ecological strategies (Adams 1999;Zelditch et al 2004;Farré et al 2016b), including in seabirds (Foster et al 2008;Navarro et al 2009;Berns and Adams 2010;Trallero et al 2019). With this approach, it is possible to accurately compare biological form in statistical terms, while accounting for the non-shape variation (size, orientation, deformation etc.)…”

“…(Adams et al 2004;Zelditch et al 2004). In a previous study (Trallero et al 2019), we quantified differences among species and sexes in bill morphology of Antarctic prions, blue petrels, and two species of diving petrels (Pelecanoides spp.) all breeding at South Georgia, South Atlantic.…”