Golden Gate Raptor Observatory
Data from Fall Hawk Counts at the Marin Headlands, California
Compiled by Nathan K. Elliott and edited by Allen M. Fish
Data collected through the efforts of hundreds of dedicated GGRO staff, interns, and volunteers—thank you!
July 2016
Citation: Elliott, N.K., and A.M. Fish. 2016. Golden Gate Raptor Observatory Hawkwatch Trends and Graphs—Data from fall hawk counts at the Marin Headlands, California. GGRO Report. 24 pp.
For additional copies, write [email protected]
2 Table of Contents
Front Matter 2 - 3
Timing Profiles by Species 4 - 13 Simplified Species Profiles 8 - 9 Explanation 4 - 5 Species Profiles 6 - 12 Making the Graphs 13
Yearly Trends in Sightings 14 - 22 Explanation 14 Yearly Species Trends 15-21 Making the Graphs 22
Notes Pages 23
GGRO Contact Info 24
3 Timing Profiles by Species
WHEN DO DIFFERENT RAPTORS PASS BY HAWK HILL? These graphs show how often a specific raptor has been sighted on any day of the season (normalized as raptors per hour of effort, or RPH). Historically, the peak of "Peak" is Sep. 24th, with an average RPH of a whopping 95!
Each species (and often age/sex categories within a species) has a unique migration profile which generally lines up with what we expect. Note that abundant species have smoother curves than rare species due to the number of data points available.
INTERPRETING THE GRAPHS The following graphs show how our raptor sightings per hour (y-axis) break down throughout the year (x-axis), presenting a “migration” (or more appropri- ately, “raptor activity”) timing profile for each species.
Days/hours counted are shown opposite. Raptor species charts are dis- played left to right by taxonomy, starting with total raptor sightings on the up- per left of the next page. Note that y-scales are relative and not the same be- tween species!
SPECIFIC TRENDS Each species has a unique migration profile. Some species start strong and finish weak, like the Prairie Falcon. Others follow the exact opposite trend, most notably the Rough-legged Hawk. Many species peak mid-season, dis- playing both broad peaks (e.g. the Red-shouldered Hawk) and narrow peaks (e.g. the Broad-winged Hawk). The Red-tailed Hawk and Merlin are unique in displaying double peaks, one in mid to late September and the other in late October.
Within species, the trends become even more complicated. Though the American Kestrel is fairly constant throughout the year, more females are sighted early while more males are seen late. And many species, most nota- bly the accipiters and Redtails, have a higher proportion of adults late-season. For some species (e.g., SSHA and COHA), juveniles peak before adults, while the opposite is true for others (e.g., BWHA).
4 Number of Days Included in Analysis
30 25 20 s y a 15 D 10 5 0 14-Aug 28-Aug 11-Sep 25-Sep 9-Oct 23-Oct 6-Nov 20-Nov 4-Dec Yearless Date
Total Hours Spent on Hawk Hill
160 140 120 100 sr u o80 H60 40 20 0 14-Aug 28-Aug 11-Sep 25-Sep 9-Oct 23-Oct 6-Nov 20-Nov 4-Dec Yearless Date
Note: Many species are now counted by age/sex categories that were previ- ously recorded as undocumented (e.g., BAEA and FEHA). The raptor per hour values shown for these new age/sex categories have been normalized for the amount of effort spend counting during years in which they were a valid choice. This allows direct comparison with those age/sec categories that have been counted for the entire study period though the sample size is lower.
5 Migration Profiles by Species
All Raptors 120 Total 100
80
60
40 SightingsPer Hour 20
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Turkey Vulture 25 Total 20
15
10
SightingsPer Hour 5
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Osprey 0.9 Total 0.8 0.7 0.6 0.5 0.4 0.3
SightingsPer Hour 0.2 0.1 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
6 Migration Profiles by Species
White-tailed Kite 0.35 Total 0.3
0.25 0.2 0.15
0.1 SightingsPer Hour 0.05 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Bald Eagle Juvenile 0.3 Subadult 0.25 Adult Undoc 0.2
0.15
0.1 SightingsPer Hour 0.05
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Norther Harrier Juvenile 3 Adult Female 2.5 Adult Male Undoc 2
1.5
1 Sightings Per Hour Sightings 0.5
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
7 Migration Profiles by Species
Total Sharp-shinned Hawk Juvenile 35 Adult 30 Undoc 25 20 15
10 Sightings Per Hour 5 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Cooper's Hawk Juvenile 18 Adult 16 Undoc 14 12 10 8 6
Sightings Per Hour 4 2 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Northern Goshawk Juvenile 0.07 Adult 0.06 Undoc 0.05 0.04 0.03
0.02 Sightings Per Hour 0.01 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
8 Migration Profiles by Species
Total Red-shouldered Hawk Juvenile 3.5 Adult 3 Undoc 2.5 2 1.5
1 Sightings Per Hour 0.5 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Broad-winged Hawk Juvenile 3.5 Adult 3 Undoc 2.5 2 1.5
1 Sightings Per Hour 0.5 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Swainson's Hawk Juvenile 0.12 Subadult 0.1 Adult Undoc 0.08
0.06
0.04 Sightings Per Hour 0.02
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
9 Migration Profiles by Species
Total Red-tailed Hawk Juvenile 25 Adult Undoc 20
15
10
Sightings Per Hour 5
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Ferruginous Hawk Juvenile 0.18 Adult 0.16 Undoc 0.14 0.12 0.1 0.08 0.06
Sightings Per Hour 0.04 0.02 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Rough-legged Hawk Juvenile 0.1 Adult 0.09 Undoc 0.08 0.07 0.06 0.05 0.04 0.03
Sightings Per Hour 0.02 0.01 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
10 Migration Profiles by Species
Total Golden Eagle Juvenile 0.35 Subadult 0.3 Adult 0.25 Undoc 0.2 0.15
0.1 Sightings Per Hour 0.05 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total American Kestrel Female 2 Male 1.8 Undoc 1.6 1.4 1.2 1 0.8 0.6
Sightings Per Hour 0.4 0.2 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Merlin 1.2
1
0.8
0.6
0.4
Sightings Per Hour 0.2
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
11 Migration Profiles by Species
Total Peregrine Falcon Juvenile 1.8 Adult 1.6 Undoc 1.4 1.2 1 0.8 0.6
Sightings Per Hour 0.4 0.2 0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total Prairie Falcon 0.12
0.1
0.08
0.06
0.04 Sightings Per Hour 0.02
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
Total All Raptors 120
100
80
60
40 Sightings Per Hour 20
0 Aug.14 Aug.28 Sep.11 Sep.25 Oct.9 Oct.23 Nov.6 Nov.20 Dec.4 Day of Year
12 Migration Profiles by Species
MAKING THE GRAPHS These graphs were generated using all 24 years of GGRO hawkwatch data collected using a regularized quadrant system. Sightings of a given species/ age/sex combination were totaled by the month and day on which they were observed and then averaged by the total number of hours spent counting a given species/age/sex categoryon that day. The resultant data are per hour, allowing comparisons between disparate seasons and days.
Do note that many species are now counted by age/sex categories that were previously recorded as undocumented (e.g., BAEA and FEHA). The raptor per hour values shown for these new age/sex categories have been normalized for the amount of effort spend counting during years in which they were a valid choice. This allows direct comparison with those age/sec categories that have been counted for the entire study period though the sample size is lower.
These graphs use a running average to present smoother curves that are easier to interpret and less influenced by abnormally high or low counts. The date range of August14th through December 5th was chosen to ensure that each day had at least 5 years of data behind it (the very early and very late season are less consistently covered, as can be seen in the "Days Counted" graph). Check out the dips mid-season in both days and hours counted-- that's due to fog and other bad weather!
13 Yearly Trends in Sightings
INTERPRETING THE GRAPHS These graphs show how the number of raptor sightings (y-axis) have changed across 30 years of GGRO hawkwatch data (x-axis). Yearly sightings are on the left y-axis, and raptors per hour on the right y-axis with year across the bottom. Actual number of sightings are plotted as dark blue diamonds for each year with a trendline (5th degree polynomial) shown as a lighter blue dashed line to capture the overall changes in sightings over tie. Note that the trendline is for illustrative purposes only and is not a marker of statistically significant trends.
GGRO’s data collection has changed over time and important dates are marked on the plots. In particular, 1989 saw the institution of the quadrant sys- tem which was then regularized in 1992. The mid-1990s had further refine- ment of counting methods, particularly the presence of expert observers in North Quadrant and a better standardization of how to count (and recount) the more common species. For illustration, see how the “Total Raptors” and “Turkey Vulture” counts (top of opposing page) increase fairly steadily until the mid-1990s, when they begin to oscillate like one would expect. 2010 saw the lowest counts in recent years due to construction and counting from Slacker Hill rather than Hawk Hill.
CHANGES ACROSS 30 YEARS OF GGRO HAWKWATCH As you know, long-term monitoring sites like the GGRO Hawkwatch on Hawk Hill are very important for picking up changes in species populations. They are important early-warning mechanisms that can pick up on signs of environmen- tal trouble, as in the case of DDT in the 1970s.
At the GGRO, our data set now goes back 30 years, letting us draw some con- clusions about the overall health of raptor populations on the Pacific Coast. The majority of species seen from Hawk Hill have either shown slight to moder- ate increases or remained constant (no significant and/or substantial trend).
The Peregrine Falcon is one of the positive exceptions. Irrespective of normal inter-annual variations, Peregrine Falcon sightings have been increasing at the average rate of 4% (of our maximum sightings) per year (linear regression, ANOVA P<0.001) though have recently levelled off. As a result, the 2015 Hawkwatch saw about five times as many Peregrines and four times as many Redshoulders as the 1986 Hawkwatch!
14 Yearly Trends in Sightings
All Raptors 40,000 35,000 30,000 25,000 20,000 15,000
Total Sightings 10,000 5,000 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Turkey Vulture 12,000
10,000
8,000
6,000
4,000 Total Sightings 2,000
0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Osprey 160 140 120 100 80 60
Total Sightings 40 20 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
15 Yearly Trends in Sightings
White-tailed Kite 250
200
150
100 Total Sightings 50
0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Bald Eagle 14 12 10 8 6
Total Sightings 4 2 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Norther Harrier 1,600 1,400 1,200 1,000 800 600
Total Sightings 400 200 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
16 Yearly Trends in Sightings
Sharp-shinned Hawk 7,000 6,000 5,000 4,000 3,000
Total Sightings 2,000 1,000 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Cooper's Hawk 4,000 3,500 3,000 2,500 2,000 1,500
Total Sightings 1,000 500 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Northern Goshawk 10 9 8 7 6 5 4
3 Total Sightings 2 1 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
17 Yearly Trends in Sightings
Red-shouldered Hawk 900 800 700 600 500 400 300 Total Sightings 200 100 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Broad-winged Hawk 800 700 600 500 400 300
Total Sightings 200 100 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Swainson's Hawk 14 12 10 8 6
Total Sightings 4 2 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
18 Yearly Trends in Sightings
Red-tailed Hawk 14,000 12,000 10,000 8,000 6,000
Total Sightings 4,000 2,000 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Ferruginous Hawk 80 70 60 50 40 30
Total Sightings 20 10 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Rough-legged Hawk 80 70 60 50 40 30
Total Sightings Total 20 10 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularize Year Hawkwatc
19 Yearly Trends in Sightings
Golden Eagle 45 40 35 30 25 20 15 Total Sightings 10 5 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
American Kestrel 800 700 600 500 400 300
Total Sightings 200 100 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Merlin 300
250
200
150
100 Total Sightings 50
0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
20 Yearly Trends in Sightings
Peregrine Falcon 300
250
200
150
100 Total Sightings 50
0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
Prairie Falcon 18 16 14 12 10 8 6 Total Sightings 4 2 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
All Raptors 40,000 35,000 30,000 25,000 20,000 15,000
Total Sightings 10,000 5,000 0 1986 1990 1994 1998 2002 2006 2010 2014 Quadrants Quadrants Slacker Hill Instituted Regularized Year Hawkwatch
21 Yearly Trends in Sightings
THE CYCLICAL NATURE OF OUR SIGHTINGS As you have likely noticed, the almost all of these graphs seem to al- ternate pretty steadily between high and low counts. Such inter- annual cycles are to be expected when monitoring any population, but unfortunately make analysis much more difficult—it’s often hard to sort out an actual trend when the year-to-year variation between sightings is so extreme. It’s also interesting to note that the cycles of different species often don’t line up, which begs some interesting questions.
MAKING THE GRAPHS Data were first truncated by date, using August 14-December 14 to weed out especially early or late counts. Seasonal rph was included in addition to season totals to account for any variation in hours count- ed by year. For the most part, this difference was negligible, but there are usually a few years in which the difference is visible for each spe- cies.
22 Notes
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Golden Gate Raptor Observatory 1064 Fort Cronkhite Sausalito, CA 94965
(415) 331-0730 www.ggro.org
July 2016
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