2004, Pages 16-19
Prepared by Michael G. Figgs, President, LREP, Inc.
5.3.4 Historic Winter Raptor Data
18.104.22.168 Dowe Flats Winter Raptor Study
With respect to this Report, the key information from the Dowe Flats winter raptor study is that:
- Approximately 90% of 2,018 responses to disturbance events occurred when the disturbance was within 200 meters of the raptor (p.100 & 106).
- Raptors had the greatest responses, in order of increasing significance, to movingvehicles, stopped vehicles and pedestrians (p.80, 98, 103 & 105).
- There was a strong positive correlation between the numbers of prairie dog raptors present in a given area, and the size of prairie dog population in that same area (p. 43, 59, & 60). So long as prairie dogs were present, the three prairie dog raptor species tolerated chronic human presence in the form of occupied vehicles, but not pedestrians (p. 69).
22.214.171.124 Christmas Bird Counts Data from the Boulder and Longmont Christmas Bird Counts (CBC) from 1980 to 2001 for winter raptors is located in Appendix 3B (Ed. not included).
Trends for winter raptor populations are described below. Long term trends were determined by averaging the first two and last two years of the 22 year period, and noting any significant fluctuations in between. Short term trends were determined by comparing the last three years of data to the previous three years, and any notable fluctuations in the final 8 years of the 22 year period.
- Bald Eagle--has increased dramatically over the long term (up over 1000% in the Boulder CBC, and over 2000% in the Longmont CBC). The short term trend indicates that the population may be reaching a plateau. This species' population tends to track with prairie dog populations, but the correlation in recent years is not as strong as the Ferruginous Hawk.
- Northern Harrier--the population may fluctuate dramatically on an annual basis but over the 21 year period the population appears stable.
- Accipiters--all three species appear to have stable winter populations over the 22 year period, with a few dramatic, short term fluctuations.
- Red-tailed Hawk--winter populations have tripled (Boulder CBC) to quadrupled (Longmont CBC) over the long term, and the population does not appear to have as much annual fluctuation as other raptor species. The short term trend is for continuing increase in the Boulder CBC and stable in the Longmont CBC.
- Ferruginous Hawk--the long term trend is up (plus 68% in the Boulder CBC and plus 188% in the Longmont CBC); however the three year short term trend is down for both CBC's. The downward trend may due to milder winters in recent years that would not force additional hawks into habitat along the Front Range where the climate is milder than on the high plains. The downward trend appears to be a local phenomenon as range-wide monitoring indicates the population is stable or rising slightly (National Audubon Society 2002, Sauer, et al. 2001). Winter populations peaked m both CBC's in the early 1990's and then dropped after plague spread through the prairie dog population in the county in 1994 and 1995. The population in the Boulder CBC also had a peak in 1985, and dropped after a plague outbreak reduced the prairie dog population north of Boulder in 1986.
- Rough-legged Hawk--this is the only raptor species to display a clear, long term downward trend (down 12% in the Boulder CBC, and 72% m the Longmont CBC). The short term three year trend continues the overall downward trend. It should be remembered that Roughlegged Hawks migrate great distances from the Canadian and Alaskan arctic, and may not have moved into Colorado in the usual numbers during recent mild winters. The downward trend appears to be a local phenomenon, as CBC data throughout North America indicates a stable population (National Audubon Society 2002).
- Golden Eagle--similar to the Bald Eagle, this species has staged a dramatic increase over the long term (up 448% in the Boulder CBC, up over 1200% in the Longmont CBC). Of the three prairie dog raptors, this species shows the least positive correlation with prairie dog populations.
- American Kestrel--appears stable in the Boulder CBC and increasing in the Longmont CBC; short term trend is up on both CBC's.
- Merlin, Peregrine Falcon, Prairie Falcon--long term and short term trends are stable. It is the opinion of the author that the increase of Bald and Golden Eagles, although real, is over represented in the CBC data. Observations of eagles are supposed to be cross-checked to eliminate duplicate sightings, but in practice this is rarely done. According to 2001 CBC data there were 89 Bald Eagles and 57 Golden Eagles in the two count circles, which is a fairly obvious over counting of eagles. A similar bias may impact other raptor species that have undergone significant population fluctuations (this bias can work in the reverse, and thus exaggerate population decreases).
126.96.36.199 BCNA Winter Raptor Survey
Results of the BCNA project include (with comparisons to CBC data in parentheses):
- Rise and decline of a significant Bald Eagle and Ferruginous Hawk population centered in the Boulder Reservoir--Haystack Mountain area in 1985-86. The population decline followed an outbreak of plague in the local prairie dog population. This area includes the southern 2.5 miles of the Boulder Feeder Canal study area.
- County-wide decline of Ferruginous Hawks, Red-tailed Hawks, and Bald Eagles in 1993- 94 after another outbreak of plague. This decline was not as significant as the 1985-86 population decline at the Boulder Reservoir--Haystack Mountain area. (Both CBC's similarly track the declines of Bald Eagles and Ferruginous Hawks, but not Red-tailed Hawks, which actually increased in both CBC's in 1994 and 1995.)
- The near total loss of prairie dog habitat east of McCaslin Boulevard in southeastern Boulder County, with a corresponding significant decline in Ferruginous Hawks and Bald Eagles.
- A decline in Bald Eagles and Ferruginous Hawks in eastern Boulder County beginning before the 1993-94 plague outbreak. (This result does not track with CBC results, where Ferruginous Hawks in both CBC's peaked in 1993, and then declined after the outbreak of plague. By 1996 in the Boulder CBC, and 1997 in the Longmont CBC, Bald Eagle counts had rebounded and passed the 1993-94 peaks.)
- Winter populations of Rough-legged Hawks, Northern Harriers, and American Kestrels remained steady or increased slightly from 1990-96. (This trend was consistent with CBC data for Northern Harriers and American Kestrels, but Rough-legged Hawks displayed a downward trend, particularly in the Longmont CBC. However, the strongest downward trend dates to 1996 in the Boulder CBC, and 1998 in the Longmont CBC, at the end of the reporting period of the BCNA study.) There appears to be a discrepancy between the CBC data, which shows a clear increase or stable populations for almost all species of winter raptors, and BCNA data which indicates a decline of Ferruginous Hawks and Bald Eagles. However, this seeming contradiction is resolved by looking at the time frames for the respective data sets. The CBC data presented herein dates to 1980, when most winter raptor populations were relatively low (excepting species of accipiters and falcons), whereas the BCNA data set is only fully represented from the early 1990's, when many winter raptor populations were at or near their peak. So both perspectives may be considered correct. The CBC data set simply provides a longer term perspective.
188.8.131.52 Winter Raptor Data Summary Although there is some discrepancy in the historical winter raptor data presented above, certain trends and findings are quite evident.
- The winter populations of Bald Eagles, Red-tailed Hawks, and Golden Eagles in Boulder County have increased dramatically over the past 22 years.
- The other winter raptors have generally maintained stable populations in the long term, with short term fluctuations being fairly common.
- Only the Rough-legged Hawk has shown any evidence of long term winter population decline, but this trend has primarily occurred during a series of fairly mild winters.
- Ferruginous Hawk populations most closely track the fluctuations of prairie dog populations. Bald Eagles moderately track prairie dog populations, and Golden Eagles appear to have a small but essentially insignificant positive correlation with prairie dog populations. These trends are remarkable when it is considered that from 1980-2000 the human population in Boulder County rose from approximately 190,000 to 290,000 (Colorado Department of Local Affairs 2002).