North American Birds

VOLUME 68 NO4 2015

A Quarterly Journal of Ornithological Record Published by the American Birding Association. The mission of the journal is to provide a complete overview of the changing panorama of our continent’s birdlife.

Issue link:

Contents of this Issue


Page 118 of 123

565 V o l u m e 6 8 ( 2 0 1 5 ) • N u m b e r 4 Wa r b l e r s i n i n d i a n a signifcance of those slopes, such as confdence intervals on the slopes and p-values. Regressions were performed directly on the raw data without any intermediate steps (e.g., taking the mean ar- rival date of each year and regressing those), because the only way to get a slope properly representative of all the data is to regress the individual observations. This procedure automatically com- pensates for the data increase in recent years. Each record was plotted on a scatter diagram with the observa- tion date on the y-axis and the year on the x-axis. A linear regres- sion curve was then generated for these data-points (Figures 1-3). These fgures provide examples of the 16 plots. Figure 1 illustrates the species with the fewest number of data points. Figure 2 pro- vides a typical example of the majority of the graphs. Figure 3 displays the species with the maximum number of data points. In Figure 1, the regression line slope is -0.1846±0.0989, indicating that today's Connecticut Warblers are arriving an aver- age of 1.85 days earlier than they did a decade ago. Slope of the trend line in Figure 2 is -0.2649 ±0.0546 days/ year. The negative slope reveals that today's Cape May Warblers are passing through Indiana earlier than in former years, by about 2.6 days per decade. Data for the most numerous species in this study are plotted in Figure 3. The slope of the curve is -0.0889±0.0200, indicating an arrival about 0.9 days earlier than ten years ago. The regression line slope for Palm Warbler is the second lowest of the study group. Results Negative slopes were found for all 16 warbler species in the study; the results are summarized in Table 1. The next-to-last column is a 95% confdence interval for the slope. The last column, p-value, is the probability that the actual slope is not negative (i.e., smaller is statistically more signifcant). Slope values range from a minimum of -0.0606 for Wilson's War- bler to a maximum of -0.3883 for the Black-throated Green. Al- though the graphed data points are widely spread, the large number of points strengthens the statistical argument. The combined group slope (based on 67,942 data points) is -0.2380 (bottom value in Table 1), suggesting that, on average, today's migrant warblers are arriving some 4.76±0.23 days earlier than they did two decades ago. Conclusions The answer to the question posed by the study appears to be an unequivocal yes, as all 16 species studied are arriving earlier than in 1996. It also indicates that data from routine birding can yield valuable results. Literature cited Cotton, P. A. 2003. Avian migration phenology and global climate change. Proceedings of the National Academy of Sciences of the United States of America 100: 12219-12222. Crick, H. Q. P. 2004. The impact of climate change on birds. Ibis 146 (Supplement 1): 48-56. Miller-Rushing, A. J., T. L. Lloyd-Evans, R. B. Primack, and P. Satz- inger. 2008. Bird migration times, climate change, and chang- ing population sizes. Global Change Biology 14: 1959-1972. Mills, A. M. 2005. Changes in the timing of spring and autumn migration in North American migrant passerines during a pe- riod of global warming. Ibis 147: 259-269. Strode, P. K. 2003. Implications of climate change for North Amer- ican wood warblers (Parulidae). Global Change Biology 9: 1137- 1144. n 1995 1997 1999 2001 2003 2005 2006 2008 2010 2012 2014 2016 10 Jun 31 May 21 May 11 May 2 May 1995 1997 1999 2001 2003 2005 2006 2008 2010 2012 2014 2016 4 Jun 22 May 10 May 28 apr 16 apr 1995 1997 1999 2001 2003 2005 2006 2008 2010 2012 2014 2016 4 Jun 13 May 21 apr 30 Mar 8 Mar Figure 1. Connecticut Warbler: a plot of record date versus year for 382 individuals. Figure 2. Cape May Warbler: a plot of record date versus year for 1737 individuals. Figure 3. Palm Warbler: a plot of record date versus year for 16,672 individuals.

Articles in this issue

Archives of this issue

view archives of North American Birds - VOLUME 68 NO4 2015