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THE WILSON JOURNAL OF ORNITHOLOGY • Vol 123, No. 4, December 2011 
Marsh Wrens and competitively exclude them 
from their territories (Leonard and Pieman 1986). 
Female Yellow-headed Blackbirds responded at 
similar levels of intensity to the wren and the 
control. Two of 10 blackbird females tested 
damaged the wren, pulling it apart. The same 
females were less aggressive toward the sparrow. 
These findings demonstrate individual variation in 
responses, which is common in studies of nest 
defense (Grim 2005, Campobello and Scaly 
2010). Past experience with wrens or the age of 
the female may have a role in recognition and 
response. Larger sample sizes may have strength¬ 
ened our case for specific recognition of wrens as 
threats by blackbirds. Male blackbirds appeared to 
discriminate between wrens and the control, 
responding more intensively to the former (Ta¬ 
ble I). Females were more attentive to the nest 
than males and had more close passes and contact 
to both models. Female blackbirds may respond 
aggressively to any small species near their nests. 
Marsh Wrens frequently destroy Yellow-headed 
Blackbird nests (Pieman and Isabelle 1995) and 
any small species encountered near the nest may 
be regarded as a potential threat. 
respond aggressively to wren models. We ob¬ 
served the most intense levels of nest defease 
(close passes/contacts) by blackbirds in seven of 
the 10 Marsh Wren presentations. We do no: 
know the incidence of egg destruction experi¬ 
enced by Yellow Warblers from House Wiens m 
our study area. Most Yellow Warblers did not 
respond to House Wrens with their typical 
predator response (e.g.. chip alarm calls). Some 
females, however, responded aggressively to the 
wren model, whereas others sat in their nest. The 
incidence of nest destruction by wrens and use of 
nest protection behavior as a deterrent to nest 
destruction by Yellow Warblers requires further 
investigation. 
ACKNOWLEDGMENTS 
We thank S. A. Gill and P. M. Gricef for assistance in the 
field, the Director of the Della Marsh field Station 
(University of Manitoba) for excellent accommodation 
and support, and the Portage Country Club for permission 
to conduct some work on their property. 
LITERATURE CITED 
Yellow Warblers responded similarly to House 
Wrens and the control in time close to the model, 
alarm calls, and perch changes. Yellow Warblers 
consistently give chip calls in response to egg and 
nestling predators, and seer calls toward cowbirds 
(Gill and Sealy 1996), but they gave few alarm 
calls when presented with wrens in the present 
study. However, female Yellow Warblers spent 
more time in the nest when the wren model was 
present nearby. This could represent a form of 
nest protection behavior as described by Hobson 
and Sealy (1989). Researchers in previous studies 
showed that Yellow Warblers rush to sit in the 
nest and give seet alarm calls when presented with 
cowbird models (e.g., Hobson and Sealy 1989 
Gill and Sealy 1996). Five of 12 Yellow Warblers 
in our study sat in their nests in response to the 
use Wren model and in only one case did the 
female chip immediately prior to entering the 
The differences between warblers and black¬ 
birds in level of response toward wrens may be 
relmed to the frequency of nest destruction Ly 
gies ° r d ; fferenCes in nest defense strate- 
BlacmT" ^ tW ° SpCCies - Yellow-headed 
by mS w PCnenCe frequent nesl destruction 
Wcnrm I98Tp nS 31 ° Ur SU,dy Si ' e < L ™nard and 
0 1986, Pieman and Isabelle 1995) and they 
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