Strigiformes

Abstract: The eyes of vertebrates show adaptations to the visual environments in which they evolve. For example, eye shape is associated with activity pattern, while retinal topography is related to the symmetry or 'openness' of the habitat of a species. Although these relationships are well documented in many vertebrates including birds, the extent to which they hold true for species within the same avian order is not well understood. Owls (Strigiformes) represent an ideal group for the study of interspecific variation in the avian visual system because they are one of very few avian orders to contain species that vary in both activity pattern and habitat preference. Here, we examined interspecific variation in eye shape and retinal topography in nine species of owl. Eye shape (the ratio of corneal diameter to eye axial length) differed among species, with nocturnal species having relatively larger corneal diameters than diurnal species. All the owl species have an area of high retinal ganglion cell (RGC) density in the temporal retina and a visual streak of increased cell density extending across the central retina from temporal to nasal. However, the organization and degree of elongation of the visual streak varied considerably among species and this variation was quantified using H:V ratios. Species that live in open habitats and/or that are more diurnally active have well-defined, elongated visual streaks and high H:V ratios (3.88-2.33). In contrast, most nocturnal and/or forest-dwelling owls have a poorly defined visual streak, a more radially symmetrical arrangement of RGCs and lower H:V ratios (1.77-1.27). The results of a hierarchical cluster analysis indicate that the apparent interspecific variation is associated with activity pattern and habitat as opposed to the phylogenetic relationships among species. In seven species, the presence of a fovea was confirmed and it is suggested that all strigid owls may possess a fovea, whereas the tytonid barn owl (Tyto alba) does not. A size-frequency analysis of cell soma area indicates that a number of different RGC classes are represented in owls, including a population of large RGCs (cell soma area >150 µm(2)) that resemble the giant RGCs reported in other vertebrates. In conclusion, eye shape and retinal topography in owls vary among species and this variation is associated with different activity patterns and habitat preferences, thereby supporting similar observations in other vertebrates.

Abstract: We examined four correlates of hunting mode, dichotomized as active search (AS) and sit-and-wait (SW), in five assemblages of sympatric hawks and owls. Examining twenty-four hawk and ten owl species we found that: (1) Linearized wing loadings are light for kites, harriers, and eagles (which are primarily AS), and heavy for buteonines, falcons, and accipiters (which are mixed-mode hunters using both AS and SW techniques). Primarily AS-owls (mainly strigines and tytonines) have light linearized wing loadings, whereas SW-owls (mainly bubonines) are more heavily loaded. AShawks do not differ significantly from AS-owls, nor do SW-hawks from SW-owls; (2) Among hawks, diet breadth increases from accipiters through harriers, to falcons, eagles, buteonines, and to kites; and among owls, from strigines through bubonines to tytonines. No significant differences in diet breadth are detected between AS and SW hawks (or owls), nor between single-mode and mixed-mode hawks (or owls); (3) Among hawks, the ratio prey mass/body mass (PM/BM) increases from harriers through falcons to buteonines, accipiters, eagles, and to kites, and from tytonines through strigines to bubonines, among owls. No significant differences in the ratios PM/BM are found between AS and SW hawks (or owls); (4) Energetic costs incurred per prey capture are greater with the AS than with the SW mode in three out of four cases analyzed, but information on energetic gains are available for only one. Here, AS rendered a larger difference between benefit and cost than did SW; in the other three cases that differential was not known, but several advantages associated to AS were claimed to offset its high energetic cost. Our main conclusions are: (1) Calculation of linearized wing loading of a hawk (or owl) enables prediction of its main hunting mode; (2) Although use of AS or SW techniques (singly or in combination) seems not to constrain access of raptors to prey resources (in terms of diversity and size), further studies focusing on single subfamilies are likely to detect differences in prey use by raptors using different techniques; (3) Little can be predicted about the extent of hunting modes used by different raptors, until the associated energetic gains are evaluated.

Abstract: Species considered raptors are subjects of monitoring programs, textbooks, scientific societies, legislation, and multinational agreements. Yet no standard definition for the synonymous terms “raptor” or “bird of prey” exists. Groups, including owls, vultures, corvids, and shrikes are variably considered raptors based on morphological, ecological, and taxonomic criteria, depending on the authors. We review various criteria previously used to define raptors and we present an updated definition that incorporates current understanding of bird phylogeny. For example, hunting live vertebrates has been largely accepted as an ecological trait of raptorial birds, yet not all species considered raptors are raptorial (e.g., Palm-nut Vulture [Gypohierax angolensis]), and not all raptorial birds are considered raptors (e.g., skuas [Stercorariidae]). Acute vision, a hooked bill, and sharp talons are the most commonly used morphological characters for delineating raptors; however, using those characters as criteria may cause confusion because they can be vague and exceptions are sometimes made. Old World vultures, for example, are in the family Accipitridae along with hawks and eagles, and thus are usually considered raptors despite their lack of sharp talons. We define raptors as species within orders that evolved from raptorial landbirds (Telluraves) in which most species maintained raptorial lifestyles. Raptors are therefore all species within Accipitriformes, Cathartiformes, Falconiformes, and Strigiformes. Importantly, we believe that seriemas (Cariamiformes) should also be considered raptors. Our definition combines phylogeny with morphology and ecology, and avoids ambiguity associated with owls, vultures, and shrikes. Establishing a common definition of raptors should improve interpretability across studies and lessen ambiguity of research and management recommendations.