Accipiter ID study

Beginning in 1995, the hawkwatch teams have participated in an interesting study that provides a quantitative measurement of the statistical reliability of the age, sex and species identification of one of the more difficult judgments that the observers must make. In overall appearance, the Sharp-shinned Hawk (Accipiter striatus) and the Cooper's Hawk (Accipiter cooperii) are remarkably similar.

The Cooper's Hawk, which is somewhat larger than a pigeon, is larger than the Sharp-shinned Hawk, which is somewhat larger than a jay. However, female hawks are larger than the males. In the western populations, the male Cooper's partially overlap in size with the female sharpies. Moreover, accurate judgments in size are difficult to make in the field. Consequently, other factors are generally used to make a positive identification. The differences most easily observed in the field are:

• Flight characteristics: The most commonly observed flight behavior is a repeated flap-flap-glide for both species. The wingbeat of the sharpie is very quick compared to the Coop.
• Head position: The head of the Cooper's hawk extends well beyond the line connecting the leading edge of the wings (carpal line), while the sharpie's head does not.
• End-of-the-tail: The tail feathers of the sharpie are very-nearly all the same length, giving it a squared-off appearance, or, if fanned, the corners are right angles. The outer tail feathers of the Cooper's hawk are shorter than the inner tail feathers, giving the tail a rounded appearance; if fanned, the corners appear greater than 90-degrees.

One can be fooled by the viewing angles, missing tail feathers, wing positions, and general inexperience, so as many factors as possible should be evaluated.

The accipiter ID study works as follows. The process begins when the banding blind nearest to Hawk Hill captures an accipiter. Shortly before releasing the bird, the banders radio the Hawkwatch leader that a release is about to occur. The leader gathers all hawkwatchers who can be spared to watch the release point. The banders radio a countdown for the release time. The hawkwatchers watch the released bird for as long as is practical, and then fill out a form documenting their judgments as to the species, sex and age, and what factors they used to reach this judgment ;"unidentified" is also an acceptable ID. The hawkwatchers don't have to identify themselves on the form. During the first year for the study, the results were not released until the end of the year. In successive years, after the forms are collected, the banders radio the details on the bird that was released.

The first year's results showed that male sharpies and female coops were correctly identified in a high percentage of the time. However, identification of female sharpies and male coops were incorrect nearly fifty percent of the time. In the second year, identification reliability improved in all categories, with the female sharpies and male coops being correctly identified most of the time, but the number of "unidentified accipiters" increased greatly. Clearly, learning the results of the first year of the study made the hawkwatchers much more careful with the certainty of their judgments.

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