Nesting performance of Peregrine Falcons in Colorado, Montana, and Wyoming, 2005-2009.
Abstract: We monitored 256 Peregrine Falcon (Falco perigrinus) nest-sites, accumulating 852 site-years in Colorado, Wyoming, and Montana during 2005-2009. The sites included 42 selected by the U.S. Fish and WiLdlife Service for its monitoring program in 2006 and 2009. Annual nest occupancy rates ranged from 75 to 100% and varied as much as 10% among years in each state, and 25% among states. Nest success was 77% overall (n = 687), but differed as much as 25% among states in 2009. Reproduction rate was 1.8 young/pair for 687 nesting attempts where outcome was known, and annual state averages ranged from 1.2 to 2.2 young/pair. We discovered or were alerted to 77 pairs at new locations, suggesting that future searches will be successful. Overall, 353 nesting locations in the three states combined had been recorded at the end of the 2009. Wide variations among years in occupancy, nest success, and reproduction underscore the necessity of long-term monitoring of Peregrine Falcons on a regional, rather than a state, perspective.
Article Type: Report
Subject: Nest building (Research)
Peregrine falcon (Behavior)
Authors: Enderson, James H.
Oakleaf, Robert J.
Rogers, Ralph R.
Sumner, Jay S.
Pub Date: 03/01/2012
Publication: Name: The Wilson Journal of Ornithology Publisher: Wilson Ornithological Society Audience: Academic Format: Magazine/Journal Subject: Biological sciences Copyright: COPYRIGHT 2012 Wilson Ornithological Society ISSN: 1559-4491
Issue: Date: March, 2012 Source Volume: 124 Source Issue: 1
Topic: Event Code: 310 Science & research
Geographic: Geographic Scope: United States Geographic Code: 1USA United States
Accession Number: 285207251
Full Text: The Peregrine Falcon (Falco peregrinus) in Colorado, Montana, and Wyoming declined to seven known pairs on territory (all in Colorado) in 1974-1976 and, despite annual searches of historical sites, only 16 pairs were known in 1985, including one pair each in Wyoming and Montana (Cade and Burnham 2003:table 8.1). Counts after 1985 showed a resurgence of nesting pairs. This increase was probably enhanced by: (1) the release in 1976-1984 of 238 juvenile peregrines from cages on cliffs (hacking) and fostering to wild adults in Colorado, 62 hacked peregrines in Wyoming, and 24 hacked peregrines in Montana (Burnham et al. 1988:569); (2) natural reproduction by known and unknown pairs in the central Rocky Mountains; and (3) suspected, but undocumented, dispersal of peregrines into the region from elsewhere, for example, the Colorado Plateau and the southwestern United States.

Counts of active peregrine nests in the contiguous United States after 1985 showed range-wide increases (Enderson et al. 1995). The U.S. Fish and Wildlife Service (USFWS) de-listed the species in 1999 (USDI 1999). The final ruling was based in part on recent rapid increase in nesting pairs. Accounts of peregrines in the Rocky Mountains after 1999 showed upward trends in numbers in several western states (Cade and Burnham 2003:135). Northern Utah was the exception as counts showed no increase.

The USFWS developed a protocol in 2001 to monitor nest sites every 3 years (USDI 2003). The first monitoring was in 2003 (Green et al. 2006), and the second in 2006 (M. G. Green, unpubl. data), and the third in 2009. We studied not only sites selected for monitoring by the USFWS but included many others in this work. Monitoring by us in 2005, 2007, and 2008 was not part of the USFWS post-delisting program but followed the same protocol.

The goals of our work were to document occupancy rates and reproduction by Peregrine Falcons in Colorado, Montana, and Wyoming during 2005 to 2009. We compared our findings with those from other studies from before or after delisting of the Peregrine Falcon in this and nearby regions.


Study Area.--We monitored 50 nest-sites in central and southwestern Colorado, including the Front Range and the upper reaches of the Arkansas, Colorado, Rio Grande, and San Juan rivers. We monitored 119 nest-sites in Montana, mostly in the drainages of the Bighorn, Bitterroot, Clark Fork, and Yellowstone rivers. We studied 87 peregrine nest-sites in Wyoming in the western two-thirds of the state including the Yellowstone National Park region and the Bighorn Mountains (Fig. 1). We also searched previously vacant historical nest-sites, potential nest-cliffs, and cliffs within these regions where peregrines had been seen by casual observers to ascertain if nesting pairs were increasing.


Site Selection.--The USFWS randomly selected 57 historical nest sites in Colorado, Wyoming, and Montana as a part of a national monitoring program in 2003, 2006, and 2009 (USDI 2003). We included data from 15 each in Wyoming and Montana, and 12 in Colorado of the 57 sites selected. Our total sample included many sites not designated by the USFWS (total sites: 157 in 2005, 177 in 2006, 167 in 2007, 149 in 2008, and 202 in 2009). We accumulated 852 site-years, 82 of which included sites selected by the USFWS and 770 that were not part of that program.

Work in each state was independent, and there were differences in how we selected sites to minimize bias in our sampling. A major portion of all known sites in Montana was studied each year. The 15 USFWS randomly selected sites were studied in Wyoming in 2006 and 2009 plus five others in each of three regions (Yellowstone National Park, east of the Continental Divide, and west of the Continental Divide). Randomly chosen sites substituted for the USFWS sites in the other years. About one third of the 27 USFWS designated sites in Colorado were studied each year in addition to many other sites near the USFWS sites or along routes to those sites.

We also searched cliffs where no peregrine had ever been reported. These cliffs were not selected randomly or systematically, and most had been previously searched for peregrines. We used the same techniques as at selected sites. The extent of these searches varied by year and by state.

We studied between 78 (n = 83) and 96% (n = 119) of all nest sites known at the beginning of each season in Montana plus sites discovered opportunistically in a survey year. We studied between 33 (n = 29) and 85% (n = 64) of all nest-sites known in Wyoming including subsets of randomly selected sites in three geographic regions. We studied between 20 (n = 27) and 30% (n = 44) of all sites known in Colorado at the beginning of each season.

Occupancy and Reproduction.--Nesting localities were observed at least once early in the nesting period (1 Apr-15 May) to locate active territories, defined as areas containing a nest within the home range of a pair (Steenhof and Newton 2007). We made at least one other visit (15 Jun-30 Jul) to territories to document reproductive success. Additional visits were made when earlier observations were inconclusive. We considered adult behavior and the condition and abundance of excrement, and searched possible alternate sites before interpreting each nesting situation. We met the requirements of the USFWS monitoring protocol (USDI 2003). We returned a second day for further searching if no peregrine was seen after 3-5 hrs of steady viewing. We considered a site vacant when no peregrine was seen in several hours of observation on each of at least 2 days. Peregrines may have been overlooked causing us to erroneously declare the site vacant when it was not; the occupancy rates we report may be minimum values.

Nest-sites, usually cliffs, rocky outcrops, or tall river banks were observed with binoculars and spotting scopes at distances of 200 m to 1.5 km but for best viewing, we sought to observe from 300-500 m, which was sufficiently close to hear peregrines yet provided a broad view. We did not climb to nests. The authors were the principal field observers, and each had more than 30 yrs of peregrine nest survey experience, mainly in their respective regions. Other competent observers (Acknowledgments) made <10% of all observations.

Reproduction rate was defined as the mean number of young of an assigned age produced by all pairs on territory where nesting outcome was ascertained (Steenhof and Newton 2007). We used 28 days of age, following the USFWS monitoring protocol. Counts of young on the ledge, or after fledging were made when all young were seen simultaneously, or when other young, temporarily out of view, could be taken into account. Young may still have been overlooked and our reproduction values may be minimum estimates.


Increase of Recorded Territories.--Discovery of cliffs with new pairs occurred each year in all states (Table 1). We searched 11 cliffs in Colorado with no record of peregrines and two had pairs. Other workers involved in wildlife studies reported 11 pairs at newly used cliffs, 2005-2009, and also found two pairs at long-vacant historical sites. We found 18 new pairs in Wyoming including seven by follow-up checks on reports from others, seven by helicopter in wildlife surveys, and four by casual checks of likely cliffs. We did not count all cliffs examined. We searched 50 historical sites in Montana vacant through 2004 and found pairs at 11. Additionally, 33 pairs were found in other searches and through casual reports.

Occupancy Rate.--The percent of nest-sites studied that were occupied by a pair of peregrines (occupancy rate) varied between 75 (Montana in 2009) and 100% (Wyoming in 3 years) (Table 2). Annual occupancy rates in each state had a range of ~10%, and the 5-year means for each state differed by nearly as much. In no year was the number of surveyed sites fewer than 27 in any state. No trend in occupancy rates was apparent.

Nest Success.--The percent of pairs that produced at least one young (success rate) where nesting outcome was known usually varied over the years in each state by <10% (Table 3). Colorado was exceptional in 2009 where, remarkably, 26 of 28 pairs (93%) produced at least one young 28 days or older. Nest success varied among states by as little as 4% (2007) to as much as 25% (2009), the latter because of the exceptional year in Colorado. About one in three nesting attempts failed in Wyoming in 2008 and 2009, and in Colorado in 2005, the low end of the range. In the three states collectively, 2005-2009, 530 of 687 nesting attempts (77%) succeeded.

Reproduction Rate.--The extremes of annual reproduction rate we found were between 1.2 and 2.2 young/pair (Table 3). These values were in Colorado in 2008 and 2009, respectively, but the latter value was also recorded in Montana in 2 of 5 years. Collectively, 687 nesting attempts in the three states during 2005-2009 produced at least 1,219 young, 28 days or older (1.8 young/pair). No trend among years was apparent.


The discovery of new sites and re-use of historical sites revealed in this study have been ongoing. Pairs found at new, or formerly vacant historical nest-sites in Colorado, increased from 15 in 1985 to 118 in 2001 (Craig and Enderson 2004). Similarly, 21 territories, all studied in 1963-1965, 1973-1975, and in 2004 had twice the occupancy rate (87%) in the latter year compared to the earlier periods (Enderson 2005). We predict more pairs will be found because of this history of discovery in all three states, and the numerous tall cliffs yet to be searched.

We cannot fully explain the variation in occupancy between years within states, or the difference among states. Results for each state were comparable because in all cases we applied the USFWS nest visit and data collection protocols. Territories we studied included regular (used by a pair every year) and irregular (used only in some years) territories (Steenhof and Newton 2007). The proportion of the two types of nest-sites monitored in any year was unknown and was probably not constant because some territories were selected randomly for study and newly discovered territories of unknown history were variously included. Between-year variation in occupancy rate would tend to be inversely related to the proportion of regular territories in study samples. A downward trend in occupancy rate over several years could signal a population decline if only regular territories were studied.

Other studies have found wide variation in occupancy rate. Occupancy rates in Colorado, 1990-2001, varied between ~70 and 90% with differences between consecutive years as great as 10% (Craig and Enderson 2004:fig. 46). Similarly, rates varied between 64 and 86% in Washington State, 1990-2001, with differences between consecutive years as great as 12% (Hayes and Buchanan 2002:table 4). The USFWS national monitoring results for 2003 show a rate of 87% for 90 territories in Colorado, Montana, Utah, and Wyoming (Green et al. 2006), and 81% for 91 territories in the same states in the draft report for 2006 (M. G. Green, unpubl, data). Occupancy rate may not be a sensitive indicator of population change because of considerable between-year variation.

The vagaries of adverse weather may explain some variation in nest success. Wet weather, in 2009, probably caused nest failure at three sites in the Bighorn Mountains of Wyoming, a phenomenon reported elsewhere (Olsen and Olsen 1989). The nest-sites we studied were distributed across a large region and adverse weather in any year would not likely affect nesting in all states, or even in parts of states, to the same extent.

Annual nest success in Washington, in comparison, varied widely between 40 and 80% (mean = 61%, n = 460) in 1990-2001 (Hayes and Buchanan 2002). Success may be inflated in that study because young of all ages were recorded. Annual nest success in Idaho, 2005-2009, was between 52 and 83% (mean = 71%, n = 129) (Moulton 2009); only young ~33 days of age or older were recorded. The success rate for 70 pairs in Colorado, Montana, Utah, and Wyoming in the 2003 USFWS monitoring survey was 74% (Green et al. 2006), and the draft report for 2006 gave a success rate of 70% for 70 pairs (M. G. Green, unpubl, data).

Wide variation in reproduction rate among years has been reported. Reproduction rate in Colorado ranged from 1.4 to 2.1 (mean = 1.7) young/pair (40-70 nesting attempts/year) during 1995-2001 (Craig and Enderson 2004). Reproduction rate for 10 pairs in 2004 averaged 2.1 young/attempt (Enderson 2005). Reproduction rate in Washington in 1990-2001 varied from 1.0 to 2.2 young/pair, but may have been biased upwards because young of all ages were reported (Hayes and Buchanan 2002). In Washington, 449 young were counted in those years in 679 nesting attempts (mean = 1.5). Reproduction rates during annual counts in the same period in Idaho averaged between 1.0 and 2.5 (6 to 15 nesting attempts/year) (Moulton 2009). The 2005-2009 average in Idaho was 1.6 young/pair for 129 nesting attempts. The 2003 USFWS national survey found a reproductive rate of 1.5 young/ pair in Colorado, Montana, Wyoming, and Utah (Green et al. 2006) and 1.4 in 2006 (M. G. Green, unpubl, data). The 2000 Canadian national peregrine survey reported a mean of 2.5 young per pair (n = 23) in Alberta south of 58[degrees] N latitude, a population where some pairs were adjacent to those in Montana (Rowell et al. 2003). We conclude that annual reproduction in the range of 1.4 to 2.0 young/pair on territory was usual. Wide year-to-year fluctuations were common, perhaps caused partly by adverse weather.

Reproduction measured in our study seems robust, but factors such as adult mortality, age at first reproduction, and immigration, all usually unknown, combine to affect population change. Craig et al. (2004) modeled these components based on values from Colorado during 1989-2001. They predicted an average reproduction of 1.7 young/pair would yield a population growth rate of 3 to 8% per year, the range resulting from the actual values accepted for the other components. That prediction coincides with our discovery of new pairs each year at cliffs where none had been seen in earlier searches.


This work was supported in part by funds from the U.S. Fish and Wildlife Service administered by the Montana Fish, Wildlife and Parks Department, the Wyoming Game and Fish Department, and the Colorado Division of Wildlife, and partly by funds arising within these agencies. K. C. Haynam and Adam Shreading helped with fieldwork in Montana. Observers in Wyoming included Terry McEaneaney, D. R. Mutch, S. M. Patla, D. W. Smith, and L. A. VanFleet. Michelle Cowardin, David Klute, K. M. Potter, and Michael Reid participated in Colorado. We are grateful for their help. We thank Colleen Moulton for permission to use recent information from Idaho. Reviews by M. R. Fuller and C. M. White improved this paper.

Received 19 March 2011. Accepted 14 August 2011.


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(1) Department of Biology, Colorado College, 14 East Cache La Poudre Street, Colorado Springs, CO 80903, USA.

(2) Wyoming Game and Fish Department, 260 Buena Vista Drive, Lander, WY 82520, USA.

(3) Centmont BioConsultants, P. O. Box 63, Winifred, MT 59489, USA.

(4) Montana Peregrine Institute, P. O. Box 317, Arlee, MT 59821, USA.

(5) Corresponding author; e-mail:
TABLE 1. Cumulative number of known Peregrine Falcon territories
on record at de-listing in 1999 and in recent years.

                                                 No.    Annual
Year   Colorado   Montana   Wyoming   Totals   gained   gain (%)

1999    93 (a)       42      50 (a)    185
2004    129          75      72        276
2005    131          83      75        289       13        5
2006    133          94      85        312       23        8
2007    134          99      87        320        8        3
2008    136         106      89        331       11        3
2009    144         119      90        353       22        7

(a) Cade and Burnham (2003: table 8.1).

TABLE 2. Occupancy rates of Peregrine Falcons at nest-sites
in Colorado, Montana, and Wyoming, 2005-2009.

          Nest-sites        Sites
           studied        with pairs   Occupancy (%)

Year    CO    MT   WY   CO   MT   WY   CO   MT    WY

2005    29    64   64   27   52   64   93   81   100
2006    36    80   61   34   67   61   94   84   100
2007    27    86   54   23   68   51   85   79    94
2008    27    93   29   23   74   29   85   80   100
2009    44   112   46   41   84   41   93   75    89
Means                                  90   80    97

TABLE 3. Nest success and reproductive performance of Peregrine
Falcons in Colorado, Montana, and Wyoming, 2005-2009.

                 Number          Successful      Success
              of pairs (a)       pairs (b)       rate (%)

Year         CO    MT    WY   CO    MT    WY   CO   MT   WY

2005         24    49    64   16    40    45   67   82   70
2006         32    66    61   24    58    44   75   88   72
2007         17    68    51   12    51    36   71   75   71
2008         11    67    29    8    54    19   73   81   66
2009         28    79    41   26    69    28   93   87   68
Totals      112   329   246   86   272   172

Means (d)                                      77   83   70

                 Number            Young
                of young        per pair (c)

Year         CO    MT    WY    CO    MT    WY

2005         36    94    99   1.5   1.9   1.5
2006         53   147   101   1.7   2.2   1.7
2007         28   108    75   1.6   1.6   1.5
2008         13   125    45   1.2   1.9   1.6
2009         61   176    58   2.2   2.2   1.4
Totals      191   650   378

Means (d)                     1.7   2.0   1.5

(a) Pairs for which reproductive outcome was known.

(b) Pairs producing at least one young 28 days of age or older.

(c) Based on all pairs of known outcome, successful or not.

(d) Mean success rate based on the total number of successful
pairs. 2005-2009. divided by the total number of pairs.
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