The Wilson Journal of Ornithology 124(1): 127-132. 2012 
NESTING PERFORMANCE OF PEREGRINE FALCONS IN COLORADO, 
MONTANA, AND WYOMING, 2005-2009 
JAMES H. ENDERSON,'- s ROBERT J. OAKLEAF, 2 
RALPH R. ROGERS; AND JAY S. SUMNER 4 
ABSTRACT.—We monitored 256 Peregrine Falcon {FalCO pcrigrimiK ) 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 21X19. Annual nest occupancy rates ranged from 75 to 100% and varied as much as 
UKt among years in each state, and 25% among states. Nest success was 77% overall (n = 687). but differed as much as 
m among states in 2009. Reproduction rate was 1.8 young/pair for 687 nesting attempts where outcome was known, and 
annual stare 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. Received 19 Match 
2011. Accepted 14 August 2011. 
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 1085. 
including one pair each in Wyoming and Montana 
iC'ade 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 
troin 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 reproduc- 
’' on b .V known and unknown pairs in the central 
R'Xky Mountains; and (3) suspected, but undoc¬ 
umented. dispersal of peregrines into the region 
horn elsewhere, for example, the Colorado 
bateau and the southwestern United States. 
Counts of active peregrine nests in the contig¬ 
uous United States after 1985 showed range-wide 
increases (Enderson et al. 1995). The U.S. Fish 
J nd Wildlife Service (USFWS) de-listed the 
species in 1999 (USD1 1999). The final ruling 
Was based in part on recent rapid increase in 
nesting pairs. Accounts of peregrines in the Rocky 
Department of Biology. Colorado College. 14 East Cache 
UPoutlre Street, Colorado Springs, CO 80903. USA, 
Wyoming Game and Fish Department, 260 Buena Vista 
frju-. Lander. WY 82520. USA. 
Ccntmont BioConsultants. P. O. Box 63. Winifred. MT 
59489, USA. 
Montana Peregrine Institute. P. O. Box 317. Arlee. MT 
59821. USA. 
Corresponding author; 
e -mail: jenderson@coloradocollege.edu 
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 (he 
same protocol. 
The goals of our work were to document 
occupancy rates and reproduction by Peregrine 
Falcons in Colorado. Montana, and Wyoming 
during 2005 lo 2009. We compared our findings 
with those from other studies from before or after 
delisting of the Peregrine Falcon in this and 
nearby regions. 
METHODS 
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 
127 
