Facultative Zygodactyly in the Black-shouldered Kite (Elanus axillaris): A comparative study of the morpho-functional adaptations of raptor feet

Abstract

<p>The study of the occurrence of facultative zygodactyly in the Blackshouldered Kite (<i>Elanus axillaris</i>) has been undertaken using several novel approaches, combined with traditional ecological methods and field observations. This unusual foot morphotype enables a bird to rotate and laterally position the outer digit (digit IV) at a much wider placement than most raptors. This thesis will explore the trait by looking at lesser studied morphological features and traits that are present in the digits of the kite in the context of predatory ecology and behaviour, compare them with both raptors and non-raptorial taxa to identify physical features and possible functional benefits it may provide. Since most raptors including the kite use feet and digits equipped with talons to procure prey, <b>Chapter 2</b> presents an ecological-based approach to understand how the kite’s diet differs from other co-ocurring diurnal raptor species that share a similar dietary ecology. It was found that although there was minimal dietary overlap with another small, diurnally active, hovering raptor – the Nankeen Kestrel (<i>Falco cenchroides</i>) – small mammals dominated the diet of the kite (House Mouse <i>Mus musculus</i>), indicating a strong dietary preference and possible physical adaptations of the kite that may have developed in concert with its extremely restricted dietary niche. <b>Chapter 3</b> compares four key foot morphotypes including raptors and non-raptors, to investigate and quantify if the kite’s feet are different from other raptors in terms of digit flexibility. It was found that not only are raptorial digits more flexible and able to have a wider grasping area (when compared to non-raptors), the kite had greater digit flexibility especially with the ability to move the outer digit (digit IV) to a wider lateral placement than other diurnal raptors within the same family (Accipitridae), and was similar to that of the owls and Osprey. The following two chapters (<b>Chapter 4</b> and <b>Chapter 5</b>) focus on the specific structures of raptor digits – including the ventral macrostructures and surface microstructures that characterise the digits of the kite, and compare them with both raptors and non-raptorial taxa to investigate the differences among these structures between groups and elucidate if any differences are predatory based, or allometrically correlated. Toepad sizes and structures were found to be functionally related to prey category (prey size related to predator body mass) and the kite was found to have well developed toepads typical of raptors but a more uniform contact surface area, which could be advantageous for providing even distribution of forces across the digits. Further, the papillae microstructures of raptors also differed when compared with non-raptorial groups, with the papillae morphology of the kite toepads sharing features of both owls and falcons. Finally the biomechanics of the ungual bone of digit I are analysed in <b>Chapter 6.</b> The talon of the hallux (digit I) of several raptors across a range of predatory and non-predatory species were analysed and studied to determine if the bone shape and biomechanical performance was based on evolutionary relatedness, allometry, or if it had been influenced by diet preferences. It was found that the talon shape was largely influenced by the size of the prey regularly captured; and that the shape also distributed the stresses around the bone during a grasping event which was most evident in those species that captured large prey. The black-shouldered kite shared similar morpho-space and biomechanical performance to other small-mammal predators, particularly the owls. In conclusion, it can be shown both theoretically and via field observations that the black-shouldered kite is able to engage in facultative zygodactyly, and additional anatomical structures of the digits themselves may also facilitate this foot morphotype. Findings presented here pave the way for other avenues of investigation and warrants further and closer observations of free-living <i>Elanus</i> kites including non-Australian species.</p>