Enderson et al. • PEREGRINE FALCON NESTING PERFORMANCE 
131 
TABLE 3. Nest success and reproductive performance of Peregrine Falcons in 
Wyoming, 2005-2009. 
Colorado. Montana, and 
Yea 
Number of pairs" 
Successful pairs h 
Success rale (%) 
Number of young 
Young per pail" 
CO 
MT 
WY 
CO 
MT 
WY 
CO 
MT 
WY 
CO 
MT 
WY 
CO 
MT 
WY 
2005 
24 
49 
64 
16 
40 
45 
67 
82 
70 
36 
94 
99 
1.5 
1.9 
1.5 
2006 
32 
66 
61 
24 
58 
44 
75 
88 
72 
53 
147 
101 
1.7 
2.2 
1.7 
2007 
17 
68 
51 
12 
51 
36 
71 
75 
71 
28 
108 
75 
1.6 
1.6 
1.5 
2008 
11 
67 
29 
8 
54 
19 
73 
81 
66 
13 
125 
45 
1.2 
1.9 
1.6 
2009 
28 
79 
41 
26 
69 
28 
93 
87 
68 
61 
176 
58 
2.2 
2.2 
1.4 
Totals 
112 
329 
246 
86 
272 
172 
191 
650 
378 
Means* 3 
77 
83 
70 
1.7 
2.0 
1.5 
! Pairs for which reproductive outcome was known. 
’ Pairs producing at least one young 28 days of age or older. 
c Based on all pairs of known outcome, successful or not. , 
0 Mean success rale based on the total number of successful pairs. 2005-2009, divided by the total number of pairs. 
Slate. 1990-2001. with differences between con¬ 
secutive 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 slates 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 
-009. probably caused nest failure at three sites in 
the Bighorn Mountains of Wyoming, a phenom¬ 
enon reported elsewhere (Olsen and Olsen 1989). 
The nest-sites we studied were distributed across a 
• a rge 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 compar¬ 
ison. varied widely between 40 and 80% (mean — 
6 1%,/? = 460) in 1990-2001 (Hayes and Buchanan 
-002). Success may be inflated in that study 
because young of all ages were recorded. Annual 
nest success in Idaho. 2005-2009, w as between 52 
and 83% (mean = 71%.//= 129) (Moulton 2009); 
°nly 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 
7 0% 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). Repro¬ 
duction 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 voting 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 na¬ 
tional peregrine survey reported a mean ol 2.5 
young per pair (n — 23) in Alberta south of 58 
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 
