Use of body mass, footpad length, and wing chord to determine sex in Swainson's Hawks

Many studies of avian ecology require an expedient means to determine sex, and the use of molecular techniques has provided an effective and accurate means to determine sex of raptors in the field (Sarasola and Negro 2004, Donohue and Dufty 2006). Sometimes investigators need to rely on morphometric measurements to determine sex of monochromatic species such as Swainson's Hawks (Buteo swainsoni) because they lack funds or facilities to use molecular techniques or they are analyzing extant data. Discriminant analyses conducted on morphometric measurements have been effective for sexing many raptor species (e.g., Edwards and Kochert 1986, Bavoux et al. 2006, Donohue and Dufty 2006). Sarasola and Negro (2004) recently reported on the effectiveness of seven morphometric measurements as means to identify sex of Swainson's Hawks on the hawk's austral summer areas in Argentina. They proposed a combination of the length of forearm, wing chord, and tail as an effective means to determine sex, with an overall accuracy of 93%. However, Sarasola and Negro (2004) did not assess the effectiveness of using footpad length (sometimes called toe-pad) to classify sex of these hawks. Footpad length and mass were quite effective in sexing Golden Eagles (Aquila chrysaetos), a species with a degree of dimorphism similar to that of Swainson's Hawks (Snyder and Wiley 1976), with an overall accuracy of 98% (Edwards and Kochert 1986). We here report the utility of using footpad length and mass, as well as wing chord length, as a means of sexing Swainson's Hawks on the nesting grounds.