Abstract
Abstract The harriers (Accipitridae: Circus) represent a unique group of raptorial birds due to their hunting behaviour and their facial ruff and prominent facial disc. During previous studies it was suggested that harrier species may have other convergent features shared with owls like asymmetric or enlarged ear openings related to sensitive hearing capabilities. In this study, cranial asymmetry was done using SAGE (Symmetry and Asymmetry of Geometric Data) software. 32 skulls of 3 species (Western Marsh Harrier (Circus aeruginosus) n=8, Montagu’s Harrier (Circus pygargus) n=10, Hen Harrier (Circus cyaneus) n=14) were photographed, digitized and assigned with 2D landmarks with TpsDig software. The variables were analysed based on Generalized Procrustes analysis. The morphometric data showed cranial asymmetry of harriers. This asymmetry should rather be explained by foraging strategies as the results are corresponding to the exceptionally good hearing of these species among diurnal raptors.
Highlights
The harriers (Accipitridae: Circus) represent a unique group of raptorial birds due to their particular hunting behaviour (Redpath 1992), low, quartering flight, exceptionally light wing loading, their mating pattern of polygyny
Procrustes ANOVAs revealed that directional asymmetry was greater than fluctuating asymmetry (Table 2) reflecting the coordinates in caudal and lateral view
The first principal components (PCs) described the variation in shape, the second the relative orientation of the temporal wing which showed that Circus species differ considerably in their degree of asymmetry (Figure 2)
Summary
The harriers (Accipitridae: Circus) represent a unique group of raptorial birds due to their particular hunting behaviour (Redpath 1992), low, quartering flight, exceptionally light wing loading, their mating pattern of polygyny. In their external appearance, harriers are one of the most recognisable Accipitrinae raptors due to their external features such as facial ruff and prominent facial disc (Simmons 2000). As in many other avian groups, food supply is one of the main factors influencing the ecology of raptors. Population densities of raptors are often limited by food supply (Newton 1980, Franklin et al 2000), which influences the number of specialist hunters being more likely to fluctuate annually when dependent on a fluctuating food source (Hamerstrom 1969, Korpimäki & Norrdahl 1991, Butet & Leroux 1993)
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