|About the trend lines:|
The trend lines have not been tested for significance. Their intent is to provide an overview of the general trend. Negative slopes represent an overall decline, and positive slopes represent an overall increase in numbers. R2 can loosely be interpreted as the proportion of the variation in the data that can be attributed to year to year differences. When this number is low, confidence in the equation is low. Note that the graphs with more scatter have lower R2 because the pattern of numbers is less like a straight line. We are using the trend lines to help us see overall patterns, not to get the best fit equation for prediction of next year's numbers. In this way the trend line can help us to answer questions such as: "Are the numbers declining?" or "Has the population been stable?" or "Has the population recovered from the use of DDT?"
Data is for the total number of hawks of this species for each year. Our totals do not necessarily represent equal numbers of days or hours. However, for most species the total may be more representative than per day or per hour values, since those values could be impacted by days of observation outside the typical migration time for the species or hours when weather inhibits migration. Among the many other sources of variation are different weather patterns from year to year, and the presence of local hawks which confound the count when they rise up to escort migrants through the territory.
The graphs show two trend lines: 1971 - 2008, and 1999 - 2008. The Hook Mountain Hawk Watch began in 1971, and the points with blue markers indicate the totals for all these years. The points that show red squares around the blue markers represent the last ten years, showing the trend in this last decade.