SHORT COMMUNICATIONS 
825 
TABLE 1. Responses (mean ± SE) of Yellow-headed Blackbird nest owners to Clay-colored Sparrow (control) and 
Marsh Wren models presented at 10 nests. 
Model 
Response* 
Nest owner 
Sparrow 
Wren 
P-value 1 * 
<2 m 
Female 
10.3 ± 0.9 
9.8 ± 1.2 
0.69 
Male 
4.2 ± 1.2 
5.7 ± 1.6 
0.11 
Close passes/contacts 
Female 
4.0 ± 2.1 
8.4 ± 4.4 
0.63 
Male 
0.5 ± 0.3 
2.8 ± 1.3 
0.13 
Squawk 
Female 
4.5 ± 1.5 
4.7 ± 1.7 
1.00 
In nest 
Female 
4.0 ± 1.4 
3.2 ± 1.3 
0.38 
1 Calegtnes of distance, squawk calls, and titling in nest were quantified in IQ-sec intervals in which nest owners engaged in that behavior for a maximum of 12 
intervals. Close passes/contacts were measured by the number of times the behavior occurred within the trial. 
T«o-uiied Wilcoxon signed-rank tests. 
significant damage by females was to the wren 
model, and involved sitting on the model’s back, 
pulling apart the model and. in one case, pulling 
off the model's head. Males tended to spend more 
lime within 2 m of the wren model with more 
close passes and contacts directed at ihe wren than 
the control model (Table 1). 
Females and males differed in amount of time 
within 2 m of the wren model (Wilcoxon signed- 
rank test, P = 0.05) and sparrow model (P — 
0.03) with females spending more time within 2 m 
of both models. Females gave more close passes/ 
contacts than males to both models, but the 
number did not differ significantly between males 
and females toward the wren (P = 0.22) or the 
sparrow (p = 0.13: Table 1), probably due to 
small sample sizes. 
Yellow Warbler .—Females responded during 
most trials, whereas males rarely responded (25% 
of trials), and their responses were not quantified. 
Males that did respond did not give alarm calls, 
pass close to or contact the model, or sit in the 
nest in response to the model. The primary 
responses of females towards both sparrow and 
wren models were presence within 2 m and perch 
changes, although these behaviors did not differ 
between models (Table 2). High-intensity nest 
defense responses such as close passes and 
contacts were rarely given and only in response 
to the wren. Two female Yellow Warblers 
responded with close passes and another contact¬ 
ed the wren model (Table 2). Chips were given 
occasionally but not at high rates and did not 
differ between models. Females spent more time 
in the nest in response to the wren and this 
difference approached significance (Table 2). 
DISCUSSION 
Yellow-headed Blackbirds apparently recog¬ 
nized the Marsh Wren as a threat to their nest and 
males in particular responded more aggressively 
to the wren model than to the control. Our results 
agree with those of Bump (1986), who found that 
blackbirds responded aggressively to vocaliza¬ 
tions of Marsh Wrens. FYevious experiments 
showed that Yellow-headed Blackbirds chase 
TABLE 2. Responses (mean ± SE) of female Yellow Warblers to Clay-colored Sparrow (control) and House Wren 
models presented at 12 nests. 
Model 
Response* 
Sparrow 
Wren 
/’-value 1 ’ 
<2 m 
9.5 ± 1.0 
10.5 ± 0.8 
0.43 
Close passes/contact 
0.0 
0.7 ± 0.4 
0.56 
Chip 
2.8 ± 1.7 
1.1 ± 0.5 
In nest 
1.1 ± 0.6 
2.5 ± 1.1 
0.13 
p ereh change 
8.4 ± 2.0 
10.8 ± 2.7 
0.57 
■ Categories of distance, chip calls, and sitting in nest were quantified in 10-sec intervals in which nes, owners engaged in tha, behavior for a maximum of .2 
■tovah. Close passes/contacts and perch changes w ere measured by the number of times the behavior occurred within the ml. 
c Two-tailed Wilcoxon signed-rank tests. 
Too many zero differences to perform paired-test. 
