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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 2. June 2012 
was 43% during the post-fledging period, ac¬ 
counting for two-thirds of all mortality observed 
in the first year of life (Davies and Restani 2006). 
Similarly, post-fledging mortality of Ferruginous 
Pygmy-Owls (Glaucidium hrasilianum ) was 37% 
(Proudfoot and Johnson 2000). However, mortal¬ 
ity is only one aspect of the post-fledging ecology 
of birds. The developmental and behavioral 
components of a species' post-fledging strategy 
also have direct implications on individual fitness. 
For example, extent of parental investment affects 
survival and recruitment of offspring, as well as 
survival and future reproduction of the parents 
(Williams 1966. Trivers 1972, Roff 1992). Simi¬ 
larly, tradeoffs involved in learning to forage 
independently while avoiding predators place 
strong selective pressure on fledglings during the 
post-fledging stage (Lack 1954. Arnold and Wade 
1994). Post-fledging strategies vary among species, 
but their characteristics are seldom explicitly 
described. 
The Northern Pygmy-Owl is among the least 
studied of North American strigids and little is 
known about the post-fledging ecology of the 
species. Our objectives were to describe: (I) 
behaviors, (2) movements, and (3) developmental 
patterns of family groups of the Northern Pygmy- 
Owl from fledging through initiation of natal 
dispersal. 
METHODS 
Study Areas. —We conducted research on two 
study areas in the Rocky Mountains of Montana 
and Idaho. One study area was in the Lewis and 
Clark National Forest adjacent to the Rocky 
Mountain Front in northern Montana (47 52' N, 
112 41' W). and encompassed the western 
portions of Teton, Pondera, Lewis and Clark, 
and Glacier counties. Elevations ranged from 
~ 1,300 to 2,500 m and topographic relief 
was extreme. Average annual precipitation at the 
nearest station of record (Gibson Dam; 1,399 m 
asl) was 43.2 cm and average annual snowfall was 
185.4 cm. The average daily high and low 
temperatures in January were 0.4 and -10.8 C, 
respectively. The average daily high and low 
temperatures in July were 26.1 and 8 2 C 
respectively (NOAA 2011). Vegetative ’ cove^ 
was predominately lodgepole pine (Pinas con - 
tortd) and Douglas-fir (Pseudotsaga menriesii) 
forest interspersed with stands of quaking aspen 
(/ opulus tremuloides). Engelmann spruce (Picea 
engelmannii) was common in more mesic areas. 
and subalpine fir ( Abies lasiocarpa) occurred at 
higher elevations. Riparian vegetation was pre¬ 
dominately black cottonwood ( Populus tricho- 
ctirpa ), willow {Salix spp.). alder (Alnus spp.), and 
silverberry ( Elaeagnus commutata). 
The second study area was in north-central 
Idaho (46 49' N, I 16 54' W) and included 
portions of Latah, Benewah, and Clearwater 
counties. Ownership was —50% federal (Clear¬ 
water National Forest) and 50% state and private 
industrial forest lands (mostly Potlatch Corpora¬ 
tion). Elevations ranged from -500 to 1.600 m. 
The study area was transitional foothills between 
the Palouse Prairie to the west and the Rocky 
Mountains to the cast, and had more moderate 
topography than the Montana study area. The area 
was wanner and more mesic than the Montana 
site with mixed mesic forests predominating. 
Average annual precipitation at the nearest station 
of record (Potlatch; 841.2 m asl) was 62.2 cm and 
average annual snowfall was 114.8 cm. The 
average daily high and low temperatures in 
January were 2.2 and -6.1 C, respectively. The 
average daily high and low temperatures in July 
were 28.2 and 7.6 C, respectively (NOAA 2011). 
Grand fir ( Abies grand is) was the dominant 
conifer, but Douglas-fir and western larch (Larix 
occidcntalis) were common stand associates on 
xeric south-facing aspects. Riparian communities 
were dominated by western red cedar (Thuja 
plicata), alder, and willow. 
Field Procedures. —We recorded fledging date 
and brood size at all successful nests located during 
the study (n 16). Fledging was directly observed 
at one nest, occurred within a I -day period between 
visits at 13 nests, and was estimated based on 
developmental stage of fledglings at two nests. We 
used tail length and flight ability of fledgling 1 ' as 
criteria for estimating developmental stage. Obser¬ 
vations of known-age fledglings indicated that at 
5-6 days after fledging rectrices were —25-30% of 
full length and flight ability was markedly 
improved relative to the first day after fledging, 
rectrices were -50% of full length at 10 days, and 
rectrices were full grown by 18-20 days. Brood 
size was estimated based on counts 1-2 days prior 
to fledging at four nests, within I day after fledging 
at 10 nests, and within 8 days after fledging at two 
nests. Brood reductions could have occurred prior 
to counts; therefore, brood sizes are minimum 
estimates. 
We attached radio transmitters to both adults 
from each of four family groups occupying 
