130 
THE WILSON JOURNAL OF ORNITHOLOGY • Vo/. 124. No. 1. March 2012 
TABLE I. Cumulative number of known Peregrine Falcon territories on record at de-listing in 1999 and in recent years. 
Year 
Colorado 
Moniana 
Wyoming 
Totals 
No. gained 
Annual gain (%) 
1999 
93 a 
42 
50* 
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 (20O3:table X. 1 1 . 
Colorado was exceptional in 2009 where, remark¬ 
ably, 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), (he 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. 
DISCUSSION 
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 USEWS 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 und 
Newton 2007 ). The proportion of the two types ol 
nest-sites monitored in any year was unknown and 
was probably not constant because some territo¬ 
ries were selected randomly for study ami 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% «:th 
differences between consecutive years as great a> 
10% (Craig and Enderson 2004:fig. 46). Similar !' 
rates varied between 64 and 86% in Washingt 1 " 
TABLE 2. Occupancy rates of Peregrine Falcons at nest-sites in Colorado. Montana, and Wyoming. 2005-2009. 
Nest-sites studied 
Year 
CO 
MT 
WY 
2005 
2006 
2007 
2008 
2009 
Means 
29 
36 
27 
27 
44 
64 
80 
86 
93 
1 12 
64 
61 
54 
29 
46 
Sites with pairs 
CO 
MT 
WY 
27 
52 
64 
34 
67 
61 
23 
68 
51 
23 
74 
29 
41 
84 
41 
co 
93 
94 
85 
85 
93 
90 
Occupancy ('*) 
MT 
WY 
81 
84 
79 
80 
75 
80 
100 
100 
94 
100 
89 
97 
