200 
THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 2, June 2011 
Peninsula of Michigan since 1995 (Probst et al. 
2003) and in Ontario. Canada since 2007 (Richard 
2008). 
Occasional observations of Kirtland's Warblers 
have been reported in Wisconsin. Early observa¬ 
tions were of single birds during migration (e.g., 
Taylor 1917), and there were only nine confirmed 
observations prior to 1978 (Tilghman 1979). 
Singing males were occasionally found in young 
jack pines in Wisconsin beginning in 1978 during 
the breeding season. Statewide surveys of jack pine 
stands on sandy soils in 1978 and 1988 turned up 
two and eight singing males, respectively (Tilgh¬ 
man 1979, Hoffman and Abernathy 1988). Three 
singing males were discovered in a red pine ( Pinus 
resinosa) plantation in Adams County, Wisconsin 
in May 2007. Subsequent intensive searches in 
2007 revealed al least eight, males, three females, 
and three nests at that site, but without evidence of 
fledging (Trick et al. 2008). 
Typically sites are only occupied by Kirtland's 
Warbler when trees are young (1.5-5 in in height; 
Mayfield 1992). Pines in these stands generally 
retain green needles on branches low to the 
ground (Mayfield I960. Probst and Weinrich 
1993). Occupied stands are also typically charac¬ 
terized by both dense pine thickets and open 
grassy areas. 
Kirtland’s Warbler habitat was historically 
generated and maintained primarily by wildfires, 
but forest fragmentation and fire suppression 
drastically reduced the extent of this habitat type 
(Mayfield 1992). Currently, the majority of Kirt¬ 
land’s Warblers breed in jack pine plantations 
created specifically for the species with die pines 
planted in opposing sine-wave patterns that 
alternately create thickets and openings (Donner 
et al. 2008). Nesting has been infrequently 
reported in red pine plantations (Mayfield I960, 
Anderson and Storer 1976. Walkinshaw 1983, 
Probst and Weinrich 1993), but with few details. 
Occupancy of red pine-dominated areas has been 
considered rare (Huber et al. 2001). 
We began monitoring the pioneering popula¬ 
tion of Kirtland’s Warblers in Wisconsin once the 
first individual was documented there. The 
objectives ol this paper are to: (I) document site 
occupancy and nesting success in the first 2 years 
of colonization, and (2) evaluate the quality of the 
habitat patches used by the warblers by measuring 
vegetative parameters at the site. This information 
will be helpful for future conservation efforts for 
mis endangered species. 
METHODS 
Study Area. —We studied Kirtland’s Warblers 
in Adams County, Wisconsin, in commercially- 
owned red pine plantations. Jack pine and 
northern pin oak/black oak ( Quercus ellipsoida- 
lis/Q. vetutina: hereafter ‘ ‘oak’ ’) were common at 
the sites, while black cherry' ( Primus serntina) and 
bur oak ( Q. ntacrocarpa) were uncommon. Stands 
consisted of rows in which red pines were 
generally spaced ~2 ni apart and featured 
irregular grassy openings where pines failed to 
survive. Ground cover consisted of sedge (Carex 
pensylvanica ), grasses ( Andropogon spp. and 
Panicum sp.), blueberry ( Vaccinium angustifo- 
lium ). forbs. pine needles, mosses and lichens, and 
downed wood from past timber harvest. Soils of 
occupied stands were sandy, and conditions in 
neighboring stands ranged from recently clear-cut 
to 20-m-tafl trees. 
Site Occupancy .—We surveyed sites in Adams 
County, Wisconsin known to have been occupied 
by Kirtland's Warblers in previous years from 
mid-May to mid-July 2008 and 2009. We walked 
areas of suitable habitat listening and looking for 
Kirtland’s Warblers, starting about dawn and 
ending in late morning when singing subsided or 
the wind increased. We also used song playbacks 
to search apparently suitable stands within 16 km 
of previously occupied sites. We used mist nets 
and song playbacks to target-net, capture, and 
color-band males (Refsnider et al. 2009. Trick et 
al. 2009); spot-mapping (Bibby et al. 2000) was 
used to estimate the extent of males’ territories. 
Nest Monitoring. —We followed males to locate 
females and followed both males and females to 
locate nests. We systematically searched to locate 
nests, but this was generally less effective than 
follow ing birds and detecting cues from the birds. 
We minimized approaches to nests and generally 
monitored nests from a distance (4-14 m) to 
ascertain nest stage and verify they were active. 
We visited nests at estimated fledging dates to 
record success (fledged young). We assumed that 
all young present the last time we looked in the 
nest had fledged. 
Habitat Characteristics. —We collected data 
from 10 to 12 August 2009 at the stand in Adams 
County. Wisconsin that had most birds (6 males, 7 
females, and 8 nests in 2009). We sampled 40 
vegetation points spaced 30 m apart on three 
evenly-spaced transects in the core area of the 
stand used by Kirtland’s Warblers, spanning the 
