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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123. No. 2. June 2011 
FIG. 1. Heights of trees in stands used by Kirtland’s Warblers. White bars are means of all trees from previous studies 
in Michigan; Probst and Wcinrich (P & W 1993) measured 10 wildfire sites, five unbumed natural-regeneration sites, three 
jack pine plantations, and three plantations dominated by red pine; Bocetti (1994) measured 11 wildfire sites and 10 (jack 
pine) plantation sites. Gray bars are from 160 trees measured at a used site in Adams County, Wisconsin, and include 107 
red pine. 33 northern pin oak/black oak, and 21 juck pine. Error bars are 95% confidence intervals. 
more likely to return. Breeding has been success¬ 
ful with the cowbird trapping that occurred at our 
site, and this population appears to be increasing. 
However, the long-term population dynamics in 
this habitat type are unknown. 
We recognize the limitations of results based on 
one stand containing the territories of six males, 
but our data provide important additional infor¬ 
mation on the lull range of habitats successfully 
used by Kirtland's Warblers (see also Probst and 
Weinrich 1993). Jack pine is thought to be a 
requirement for Kirtland's Warbler and natural 
recruitment of jack pine occurred on all occupied 
stands in Adams County. The density and 
percentage of jack pine on our Kirtland’s Warbler 
stand, to our knowledge, is the lowest reported in 
the literature. J, R. Probst (pers. comm.) observed 
that 2-3 red pine sites used in Lower Michigan 
were larger, and had fewer jack pines, than our 
Site. Mayfield (1960:16) reported birds nesting in 
1951 in “red-pine plantations where there were 
few if any jack pines”, but did not elaborate 
further. 
Our measurements of pine density and total tree 
density are low compared to sites used for 
breeding in Michigan, although plantation sites 
typically have fewer trees than wildfire sites 
(Probst 1988, Probst and Weinrich 1993). Stands 
with fewer than 2.500 trees/ha have been 
considered marginal Kirtland's Warbler habitat 
(Probst and Hayes 1987). Houseman and Ander¬ 
son (2002) tound tree densities greater than ours 
in plantations (2.890 jack pines/ha and 3.345 total 
trees/ha), and on burned sites at Mack Lake (8.578 
jack pines/ha and 8.950 total trees/ha). Bocetii 
(1994) tound an average of 7,000 jack pines/ha on 
wildfire sites; her measurement of 2.000 jack 
pines/ha on plantation sites was comparable to our 
total tree density. However, tree densities alone do 
not adequately define habitat suitability, because 
variable tree-spacing can greatly affect canopy 
cover in a stand, and the wider spacing of trees at 
