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THE WILSON JOURNAL OF ORNITHOLOGY • VoL 124. No. September 2012 
We found no difference in the mean distance of 
closest approach by chickadees and titmice during 
either control (Z = 0.1, P = 0.50) or high-threat 
trials (Z = 0.4, P = 0.34). However, titmice 
approached predators closer than chickadees 
during low-threat trials (Z = 2.2, P = 0.014). 
The mean distance of closest approach during 
low-threat trials was 1.4 ± 0.3 m for titmice and 
3.2 ± 0.6 m for chickadees, whereas (lie mean 
closest approach distance during high-threat trials 
was 1.7 ± 0.2 m for titmice and 2.4 ± 0.5 m for 
chickadees. 
DISCUSSION 
Titmice in our study generally approached and 
remained closer to predators than chickadees. 
One possible explanation is that larger titmice 
are dominant to smaller chickadees (Waite and 
Grubb 1988) and, as a result, chickadees may 
remain some distance from titmice to avoid 
possible aggression. Another possible explana¬ 
tion. however, is that larger titmice may be more 
likely targets of aerial predators than smaller 
chickadees and, it so, titmice may mob with 
greater intensity because those predators repre¬ 
sent a greater threat to titmice than chickadees. 
Gehlbach (1994), in support of this hypothesis, 
examined the behavior of 17 species of song¬ 
birds that mobbed Eastern Screech-Owls In 
Texas and found a significant positive correla¬ 
tion between how often a species mobbed 
screech-owls and how often screech-owls preyed 
upon those species. Previous studies have re¬ 
vealed that Cooper's Hawks, for example, 
generally prefer larger avian prey (Roth and 
Lima 2007), and Sharp-shinned Hawks typically 
do not target prey species weighing <20 g (Roth 
et al. 2006). Larger titmice may also be less 
maneuverable than smaller chickadees (Dial et al. 
2008), possibly increasing their vulnerability to 
aerial predators and providing additional incen¬ 
tive for titmice to aggressively mob predators 
(Cresswell 1994a, b; Flasskamp 1994; Courier 
and Ritchison 2010). Nocedal and Ficken (1998) 
reported that Bridled I ittniee ( Baeolophus wolt- 
weberi) in similar mixed-flock contexts in the 
American southwest aggressively mobbed North 
ern Pygmy-Owls ( Glaucidium gnoma), some¬ 
times approaching the owls as close as 30 cm. 
Intense bouts ol mobbing behavior that include 
close approaches to predators are adaptive 
strategies that deter predators (Pettifor 1990) 
and cause raptors to change their roost locations 
(move-on hypothesis; Sunde et al. 2003, Hen- 
drichsen et al. 2006). 
Titmice generally approached closer to preda¬ 
tors and remained closer for longer periods than 
chickadees, but the difference in approach dis¬ 
tance by these two species with high-threat 
predators was not significant. Small raptors such 
as Sharp-shinned Hawks and Eastern Screech- 
Owls are unlikely to prey on birds as small as 
chickadees, but they almost certainly represent a 
greater potential threat to chickadees than large 
raptors including Red-tailed Hawks and Great 
Horned Owls. Thus, for chickadees, the benefit of 
more vigorous mobbing behavior (i.e., small 
raptors leaving an area in response to vigorous 
mobbing) may outweigh the potential costs (i.e.. 
risk ol predation by closely approaching a 
potential predator). 
Nolen and Lucas (2009) examined the mob¬ 
bing behavior of Tufted Titmice. Carolina 
Chickadees, and White-breasted Nuthatches 
{Sinn carolineuxis) and found that chickadees 
and nuthatches mobbed an Eastern Screech-Owl 
model with greater frequency and intensity, and 
typically approached the model closer than 
titmice. We examined the combined responses 
ol chickadee and titmice to three high-threat 
predators; titmice in our study exhibited a 
stronger response to Eastern Screech-Owl. spend¬ 
ing more time within 5 m than chickadees during 
seven ol eight trials and, on average, approaching 
closer (mean approach distances = 0.7 m for 
titmice and 1.6 m for chickadees). One possible 
explanation for this apparent difference in 
responses of titmice and chickadees is that we 
used study skins to elicit mobbing behavior, 
whereas Nolen and Lucas (2009) presented a 
model combined with playback of the screech- 
owl monotonic trill. Nolen and Lucas (2009) and 
others (Lind et al. 2005) noted a predator's 
behavior may influence mobbing behavior. A 
calling screech-owl likely represents less of a 
threat than a silent owl that may be actively 
hunting and titmice may respond less aggres¬ 
sively to the former than the latter. 
ACKNOWLEDGMENTS 
We lhank the Kentucky Ornithological Society for 
linancial support. C. M. Soard for assistance in developing 
our methods, L. A. Courter for help with field work, and N. 
Santangelo. C. E. Braun, and an anonymous reviewer for 
helpful comments. 
