Project Owlnet - Sex Criteria for Northern Saw-whet Owls - Boundaries for assignment of sex in Northern Saw-whet Owls - Northern Saw-whet Owl Sex Criteria - A quick look at a large sample of measurements - Project Owlnet - sexing owls - determining sex of owls - Northern saw-whet owl, aegolius acadicus - morphometrics - David F. Brinker

Now that we have workable criteria for determining sex in Northern Saw-whet Owls a few banders have wondered what the extremes are on weight and wing chord measurements. To address this I’ve summarized 2,450 pairs of wing chord and weight measurements obtained from unknown sex saw-whet owls, most of which were banded in the states of Maryland and West Virginia. The points representing these individuals are plotted on the figure. (Click the figure for a larger image) There are also a series of lines to help illustrate various points.

The green line is the boundary of 95% certainty for the separation of females from unknown sex individuals and males; all points to the right of and above the green line are considered female. The blue line is the boundary of 95% certainty for the separation of males from unknown sex individuals and females; all points to the left of and below the blue line are considered male. All of the points that lie between the green and blue lines are unknown sex individuals.

There are two sets of red and yellow lines that delimit the extreme portions of the distribution. Individuals in the upper left and lower right corners are the most unlikely pairs of measurements. The yellow lines represent additional zones that include more individuals to show how uncommon these extreme owls are. The owls in the lower right corner, to the right of and below the red line, are unusually short-winged females. Individuals in the lower right corner represent only 0.6% of the 2,450 owls. Moving the lower limit for weight from 90 down to 85 grams (the yellow line) only increases this group of unusually short-winged owls to 1.5% and might also include a few rare heavy males. Likewise, the owls in the upper left corner, to the left and above the red line, are unusually long-winged males and might also include a rare starving female. Individuals in the upper left corner represent only 0.3% of the 2,450 owls. Moving the upper limit for weight from 80 up to 85 grams (the yellow line) only increases this group of unusually long-winged, low weight owls to 2.1%.

So what does all of the above mean to a bander? Understanding the distribution will help alert banders to measurement pairs that are unlikely and should be double checked for measurement or weighting errors. This information can be used to assist in setting reasonable limits for error checking by Band Manager. For example, I have set my limits for female wing chords at 130-150 and weight at 85-165. For males my limits are 120-140 for wing chords and 65-85 grams for weight. I recommend these same limits for most eastern banders. The disparity in weight ranges is because females put on considerable weight related to breeding, while males do not. During autumn migration female weights generally range between 85-110 grams. The recommended limits will pass most of the owls that you assign sex to while pointing out those with the most unusual wing chord and/or weight values. You can also use the distribution to select your own limits, or better yet, plot your own data and evaluate it in the same manner.

Finally, for anyone taking blood samples to verify the validity of the sex criteria, the most interesting owls to sample would be long-winged males (upper left corner), short-winged females (lower right corner), unknown sex owls (between the blue and green lines), and those owls very near the green and blue lines. Taking blood samples for sex determination from owls solidly in the male and female regions will generally be a waste of effort and money because very little additional knowledge is likely to be gained from these reliably male/female owls.