Results
In assigning the bird species to foraging niches, we have followed procedures proposed by Pigot et al. (2020) based on a standardized protocol for foraging niche delimitation. Thirty niches were identified for all of the approximately 10,000 bird species of the world. Of these six major foraging niches were categorized as Aquatic with three more chosen by us to accommodate the 49 water bird species of this study, darters and dippers excluded. The 12 land bird species could be grouped into two niches: Ground Feeding and Aerial/Sally (Table 2).
We found a significant difference between the deflection parameter medians of aquatic (260.106) and land (1595.106) bird foraging niches (p < 0.001), thereby demonstrating an evolutionary distinction between these groups. There was no significant difference between the deflection parameters of the two land bird foraging niches (p = 0.600), Ground Feeding (1228.106) and Aerial/Sally (2864.106), which indicates similarity among land birds. The aquatic bird species however, expressed significant differences between Aquatic Dive (328.106) and Aquatic Surface (37.106) (p = 0.012), Aquatic Surface and Aquatic Plunge (935.106) (p < 0.001), and Aquatic Surface and Aquatic Perch (276.106) (p = 0.001). These results show that differences in feather microstructure are identifiable with respect to differences in aquatic niches.
In order to determine if the foraging niches for aquatic and land birds explain feather microstructure while accounting for phylogenetic relationships, a phylogenetic ANOVA approach was followed. The degree of group aggregation was determined in order to establish if the ANOVA methodology would be affected by the association between the independent variable, i. e. foraging niche, and the phylogeny. The results indicated no significant group aggregation for either aquatic birds (r = 0.468 and p = 0.122) or land birds (r = 0.650 and p= 0.120) which confirmed the statistical reliability of the ANOVA approach. The phylogenetic ANOVA results demonstrated that there is a significant difference in feather microstructure between aquatic bird groups (p = 0.001), regardless of phylogenetic relatedness. In contrast, no significant difference in feather microstructure for the two land bird feeding niches was observed (p = 0.971).