Morphology of the feeding apparatus in shorebirds (Charadriiformes) : a comparative analysis : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science in Zoology at Massey University, Palmerston North, New Zealand
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2022
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Massey University
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Abstract
Shorebirds (Charadriiformes) have developed diverse foraging strategies to feed in different niches. These foraging strategies are associated with morphological variation in their feeding apparatus. This variation among species is complex, with different morphological features reflecting different aspects of foraging ecology. These traits are also presumed to relate to the evolutionary history of the species. Here I attempt to untangle the mystery of the development of diverse feeding apparatus in shorebirds through a series of morphological and phylogenetic comparative analyses using 20 species. First, I used landmark-based geometric morphometrics to compare skull shapes between species. The results showed that the major variation was the bill length, and there were also other variations. The results suggest that bill and cranial shapes vary phylogenetically, but do not suggest that visual foragers have larger eyes than tactile foragers. Second, I compare the interspecific variation of bill forms, including the length and curvature of bills, and the bony sensory pit under the rhamphotheca. Individuals of the family Scolopacidae (sandpipers and allies) possess various bill-tip organs with large numbers of concentrated sensory pits. The variety of bill shapes and sensory pits is suggested to relate to different foraging behaviours. Moreover, I test the relationship between distal rhynchokinesis (the ability of some species to bend the tip of the upper mandible) and bill forms. The results suggest that rhynchokinesis may enhance the grabbing capability of some species with bill-tip organs. Third, I documented and compared the morphology of tongues and palates, including the gross anatomy of oropharyngeal cavities and scanning electron microscopy of micro tongue tip spines. Tongue lengths generally increased with mandible lengths, though there were some exceptions. The spines were significantly associated with small size (light weight) and with a short distance between the tongue tip and the mandible tip. The variety of the forms of tongue tip spines may relate to different foraging behaviours and diets. Finally, I used the taxonomic distribution of three traits (sensory pits, micro tongue tip spines and distal rhynchokinesis) to test for a role of phylogeny on the evolution of these traits. Different evolutionary models were tested with the traits, and the phylogenetic signal was measured to determine which model best fit the partial phylogeny of charadriiform shorebirds. The results suggest that sensory pits were present in the common ancestor of all shorebirds, but that the number of pits has rapidly accumulated during the evolution of the Scolopacidae. Micro tongue tip spines were suggested to be absent at the root and to have independently evolved twice across the partial phylogeny. Distal rhynchokinesis was present in some species throughout the phylogeny, and it was suggested to be lost more rapidly than it was gained. Together, these results provide new insights into how the fundamental structures of the feeding apparatus have been modified during the diversification of the shorebirds.