UC Berkeley

Researchers have sought to understand the link between the avian skull and dietary preferences for centuries by studying vision, brain size, beak shape, and the complex evolutionary relationship of the beak relative to the cranium. One group of birds of recurring interest to researchers are the raptors. The term ‘raptor’ colloquially encompasses three orders: Accipitriformes, Falconiformes, and Strigiformes. This study provides a comprehensive comparison of species among the diurnal raptors: Accipitriformes and Falconiformes, including the skulls of 150 species. It is one of the largest studies of the overall morphology of these two orders to date and lays the groundwork for further research into the diet, function, anatomy, taxonomy, and morphological diversity of living and extinct raptors. All three chapters rely on the use of 2D geometric morphometrics to draw conclusions about shape variation in the skulls of raptors.

In Chapter 1, I investigated the effect of the rhamphotheca on the shape of the bird beak in raptors by comparing the shape of the beak with and without a rhamphotheca in the same species. This chapter provides answers to the question of whether a combination of skulls with and without the rhamphotheca can be used in geometric morphometric analyses of skull shape. The rhamphotheca adds length and creates a sharper point in the beaks of raptors, but the extent to which the shape of the beak changes compared to the bone underneath had not been investigated. A geometric morphometric analysis of five different species of raptors from different orders and families found that the rhamphotheca does differ significantly from the shape of the underlying bony beak in every case. This has implications for the study of bird beaks when the rhamphotheca isn’t available, as is often the case when looking at fossil birds. These results also resulted in the exclusion of rhamphotheca from the analyses of skulls in chapters two and three.

In Chapter 2, I explored the general morphology of the raptor skull using 2D landmarks of skulls from the Accipitriformes and Falconiformes. This chapter details the morphology of the raptor skull, what features scavenging members of each order have in common, and how scavengers and non-scavengers differ morphologically. Scavenging has a significant effect on the overall shape of the skull, driving the evolutionary difference between the families and subfamilies of the Accipitriformes and Falconiformes. The results of this chapter strongly suggest the presence of a feeding guild structure within the accipitrid and cathartid vultures and point to the progressive evolution of ‘vulture-like’ traits in the Polyborinae subfamily of the Falconiformes.

In Chapter 3, I analyzed the scavenging members of each order in greater detail, to discuss and reevaluate the guild structures proposed by Kruuk (1967) and Hertel (1994). I also evaluate the inclusion of the caracaras into this guild structure, based on geographical distribution and known overlap with the vultures of the Accipitriformes. Chapter 3 also discusses the shape of the vulture skull through time by including fossil specimens from the La Brea and McKittrick tar pits. Using the knowledge about existing scavenger guild structures, I apply this knowledge to fossil and extinct species of raptors and scavengers of the Pleistocene. The statistical results from this study support the existence of the guild structures proposed by Hertel (1994), with only minor differences with respect to which species fit within each guild. The caracaras may comprise their own guild structure independent of cathartid and accipitrid vultures, but more observational data needs to be collected in order to draw firm conclusions.