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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 4. December 2011 
recorded responses of Yellow Warblers to freeze- 
dried House Wren models placed at their nests to 
examine if they respond more intensely to the 
wren model than an innocuous control. 
METHODS 
Study Site .—We tested active nests of Yellow 
Warblers in June 1991 and nests of Yellow¬ 
headed Blackbirds from late May to early June 
1992 on and around the Delta Marsh Field Station 
(University of Manitoba), along the south edge of 
Lake Manitoba. Yellow Warblers nested in the 
forested dune ridge that separates Lake Manitoba 
Irom Delta Marsh; Yellow-headed Blackbirds 
nested in Delta Marsh. House and Marsh wrens 
nest commonly in the study area. 
Mode la. We presented active nests of Yellow¬ 
headed Blackbirds with freeze-dried Marsh Wrens 
and active nests of Yellow Warblers with freeze- 
dried House Wrens (hereafter, models). We used a 
freeze-dried Clay-colored Sparrow (Spizella pal¬ 
lida) model as a control in both experiments. We 
chose the Clay-colored Sparrow as the control 
because n is similar in size to the wrens but the 
sparrow does not threaten the nests of either 
species tested. Clay-colored Sparrows do not nest 
hM hC M f Tu St ° r ma ' Sh bul s,lou,d be ^miliar to 
bo h blackbirds and warblers because they nest in 
upland habitats adjacent to nest sites of these 
species (Hill and Sealy 1994). We used 
models of each species to reduce the effects*’of 
pseudorephcation and to replace damaged models. 
F.eld Procedures .-We performed the tests in 
he mid-morning or afternoon during the incuba¬ 
tion stage of each species. We chose to test our 
would'be" ^ mCuba,ion sta S e whe ” females 
would be more attentive to the nest and more 
hkely to detect the model. We clipped models to 
y ve | etatl °n so they were level with the 
nests and faced them from a distance of 0.5 m 
Tur Oh e nest 0WnefS Were absenL We recorded 
r observations at a distance of at least 15 m 
or <Yell0W ' headed Blackbird) 
or hidden behind vegetation (Yellow Warbler) 
Yellow-headed Blackbirds nesting within the 
vicinity of roads are habituated to ears parked or 
near in theix V !*** l ° US when we P ai *ed 
near their nests. Yellow Warblers are fairlv 
and'™ dLtT ° bSCrVCrS in our ' ,l % area 
don to the limited visiWIity'ofTbJind where^Cds 
perched at a distance may be missed. Timim 
began when the first nest owner returned to thl 
vicinity of the nest. Each nest was tested with boil 
models, sequentially. We assigned orderoEmodc 
presentation randomly for the first nest and then ir 
alternate order to ensure the same number of nr-n 
would receive the control first as those :L 
received the wren first. We left a 15-mia test 
period between model presentations. We assuca! 
this time period would be adequate to ink* 
effects of carry-over aggression as it has beer 
used in previous studies of nest defense in our 
study area (Briskie and Sealy 1989). 
Defensive responses included; (1) approach 
within 2 m of the model. (2) close passes me 
or contact with the model, (3) alarm calls, (h 
perch changes (Yellow Warblers only), and if 
time in the nest. Distance, alarm calls, and time id 
the nest behaviors were measured as the number 
of 10-sec intervals in which nest owners engaged 
in that behavior. Close passes or contacts and 
perch changes were measured as the actual 
number of times they occurred in the trial, Female 
Yellow-headed Blackbirds give the squawk alarm 
call (Sealy et al. 1998), whereas Yellow Warblers 
use chip calls toward nest predators (Hobson and 
Sealy 1989. Gill and Sealy 2003). 
We initially presented each model for 5 rain to 
Yellow-headed Blackbirds (n — 7) and Yellow 
Warblers (n = 5). How'ever, we reduced the trial 
to 2 min in three of the presentations to minimize 
damage to the models by aggressive individual 
Yellow-headed Blackbirds. The most intense 
defense behaviors (e.g., close passes and contact) 
for Yellow-headed Blackbirds and Yellow War- 
biers occurred only in the first 2 min of the trial' 
75 and 100% of the time, respectively 0u f 
analyses are based on the first 2 min of model 
presentations for all nests tested for both species 
We used nonparametric statistics because our data 
were not normally distributed. All tests were two- 
tailed because we were also interested in re^ 1 ' 
that may be counter to our predictions (Lombard! 
and Hurlbert 2009). 
RESULTS 
Yellow-headed Blackbird .—Female Yellow¬ 
headed Blackbirds reacted to the models more 
frequently than males but did not responu 
significantly differently to the wren compared 
w'ith the sparrow (Table 1). Females speni similar 
amounts of time close to both models and alarm 
called in response to both models. The most 
