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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124. No. 3. September 2012 
I he Wilson Journal of Ornithology 124(3):626-629. 2012 
Asymmetries in Mobbing Behavior Among Nuclear Flockmates 
Jason R. Coulter 1 *• 
ABSTRACT.—Tufted Titmice (Baeolophus bicolor ) 
and Carolina Chickadees ( Poecile carolinensis) often 
occur together in mixed species flocks during the non¬ 
breeding season and. as nuclear species, often initiate 
mobbing bouts. We compared the mobbing behavior of 
Tufted Titmice and Carolina Chickadees and. specifi¬ 
cally, their tendency to approach five potential preda¬ 
tors. We exposed flocks of chickadees and titmice to 
study skins of live species of raptors in 2008; raptors 
were categorized as either low-threat (rarely preying on 
chickadees or titmice) or high-threat (more likely to 
prey on chickadees or titmice). We noted the distance of 
closest approach by titmice and chickadees during trials, 
and whether a chickadee or titmouse spent more lime 
within 5 m of the raptor, Titmice were more likely to 
remain within 5 m of both low (P = 0.0008) and high- 
threat [P = 0.0015) raptors. Titmice approached low- 
threat raptors closer than chickadees (P 0.014). There 
was no difference in the mean distance of closest 
approach by chickadees and titmice during high-threat 
trials {P = 0.34). Titmice generally approached and 
remained closer to raptors during mobbing bouts than 
chickadees, possibly because larger titmice (~ 21 g) are 
more likely targets of aerial predators than smaller 
chickadees (— 1 1 g). Titmice may be willing to take 
greater risks because the potential benefits (reduced risk 
of predation) are greater if mobbing causes potential 
predators to leave an area. Received 30 September 2011. 
Accepted 14 April 2012. 
Mixed-species flocks during the non-breeding 
season in the southeastern United States are often 
composed of nuclear species including Tufted 
Titmice ( Bcteoiophus bicolor) and Black-capped 
(■Poecile atricupillus) or Carolina (P. carolinensis) 
chickadees and attendant species (Moynihan 
1962). Nuclear species direct group movements 
toward food, maintain flock cohesion, and are 
typically the first to approach potential predators 
and initiate mobbing bouts (Dolby and Grubb 
1999, Templeton et al. 2005, Bartmess-LeVasseur 
et al. 2010). 
1 Department of Biological Sciences. Eastern Kentucky 
University. Richmond. KY 40475. USA. 
C urrent address: School of Agricultural, Forest, and 
Environmental Sciences. CIcmson University. Clemson SC 
29634, USA. 
'Corresponding author; e-mail: jcourte@demson.edu 
and Gary Ritchison 1 
Mobbing can be beneficial, causing a predator 
to leave an area, but such behavior can also be 
costly, including the risk of being injured or killed 
lor birds that closely approach a predator (e.g.. 
Denson 1979, Sordahl 1990). The costs and 
benefits of mobbing may vary among species 
depending on the identity of the predator. For 
example, because larger predators tend to take 
larger prey (Vezina 1985), larger raptors may 
represent more of a threat to larger species in 
mixed-species flocks. Roth and Lima (2007) 
lound that Cooper's Hawks ( Accipiter cooperii) 
generally prefer larger avian prey, while Roth 
cl al. (2006) found that Sharp-shinned Hawks (A. 
st rial us) typically do not target prey species 
weighing <20 g. Thus, the potential benefits of 
mobbing larger raptors may be lower for smaller 
species in mixed-species flocks (e.g.. Carolina 
Chickadees: ~ 11 g; Mostrom et al. 2002) Ilian 
for larger species (e.g.. Tufted Titmice: — 21 g; 
Grubb and Pravasudov 1994). 
Both Tufted Titmice and Carolina Chickadees 
utter 'chiek-a-dce* alarm calls when mobbing 
perched aerial predators. Carolina Chickadees vary 
the number of ‘dee* notes per call to potentially 
convey information to conspecillcs about the threat 
posed by different predators (Soard and Ritchison 
2009). Tufted Titmice vary the rale at which ’dee' 
notes are uttered which may convey information 
about predator threat; this variation in call rate may 
also deter predators by causing them to overesti¬ 
mate the number of titmice present (Courier and 
Ritchison 2010). Thus, the alarm calls of Tufted 
Titmice may differ functionally from those of 
chickadees, conveying information to conspecillcs 
and 'misinformation' to predators. These species 
also differ in size and other aspects of their 
mobbing behavior may also differ. Our objective 
was to compare the mobbing behavior of Tufted 
Titmice and Carolina Chickadees responding to 
different species of raptors. We examined their 
spatial relationships relative to potential predators 
because proximity to predators, i.e.. approach 
distances and time spent near potential predators, 
may be related to risk (Krams et al. 2010) or 
willingness to accept greater potential costs. 
