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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 123, No. 3. September 2011 
and all 13 psittacine species regularly joined at 
least one of the three types of mixed species 
aggregations. These aggregations were not ob¬ 
served away from clay licks except for casual 
associations of large macaws at fruiting trees 
(Gilardi and Munn 1998; A. T. K. Lee, pers. 
comm.). Group sizes at clay licks were much 
greater than those of birds away from clay licks 
(Gilardi and Munn 1998) and <1% of lick use 
was by lone individuals. These data suggest that 
birds adopt novel behavioral strategies when 
using clay licks. 
Stratification by size was clear among the 
mixed species aggregations: (1) large macaws, (2) 
large parrots and small macaws, and (3) parakeets 
and small parrots (Table 1). Species may segre¬ 
gate by size as heavier species take flight slower, 
accelerate slower, and have wider turning radii 
making them stragglers when mixed species 
groups flee from aerial predators. Direct compe¬ 
tition should also favor size stratification as 
aggressive interactions are common on clay licks: 
smaller species are usually displaced by larger 
species but numerically dominant smaller species 
can exclude larger species if the size difference is 
not too great (Burger and Gochfeld 2003). Thus, 
predation may select against larger species joining 
smaller ones while competition may select against 
smaller species joining larger ones. This may 
explain the relative uniformity of body size 
among aggregation members. 
The cost of ‘false alarms’ may also be 
important in shaping foraging behavior and 
aggregation composition, as disturbances reduce 
foraging efficiency (Sirot 2006. Beauchamp and 
Ruxton 2007). Over 90% of the flights from the 
lick in our study had no apparent cause suggesting 
a high rate of false alarms. Smaller species have a 
higher risk of predation and expend less energy 
each time they fly from the lick, and should have a 
lower alarm threshold, give more unnecessary 
alarm calls, and have correspondingly higher rate 
of departures from the lick. Members of an 
aggregation often respond to alarms as a group 
and larger species may have greater energy 
expenditure when using the lick alongside more 
‘flighty’ smaller species. This also favors forma¬ 
tion of aggregations of similar sized individuals. 
The coloration ol the species in each aggrega¬ 
tion was similar; the three large macaw species in 
flight were a mix of red, blue, green, and yellow 
whde t hc large parrot and parakeet aggregations 
were composed predominantly of green birds with 
dark green, blue or black heads and primaries. The 
formation of homogeneous groups (in size and 
color) is predicted where predators attacking 
groups focus on visually aberrant individual 1 . 
(Landeau and Terborgh 1986. Thcodorakis 1989. 
McRobert and Bradner 1998. Hoare et al, 2000 1 . 
The large macaws that occasionally join the 
large parrot aggregation are a notable exception to 
the tendency for visually similar individuals to 
join together on clay licks (see also Mee et al. 
2005). However, when large macaws join the 
large parrots they usually do not integrate into the 
center of the group. Instead, they use the highest 
parts of the lick, ~3 m above the center of the 
aggregation, where the soil quality is inferior(i.e., 
50 to 75% less sodium), but where they have the 
best chances for rapid escape (Brightsmith et al. 
2008; DJB, unpubl. data). The large macaws at 
Tambopata Research Center spent <10% of their 
total lick use in the presence of the large parrot 
aggregation, In addition, large macaws rarely join 
parrot aggregations at other clay licks and instead 
usually use licks during the late mornings and 
afternoons (Burger and Gochfield 2003; DJB. 
unpubl. data). Why large macaws join parrot 
groups is unclear, but it may be because their 
large size makes them vulnerable to a smaller 
number of raptor species, and because early 
morning is the only time when lick use ts 
temporally predictable. Large macaws during the 
rest of the day may wait near the lick for up to 
3 hrs before a group successfully initiates lick use 
White-bellied Parrots were least likely to join 
mixed species groups and were the most visually 
distinct small parrot at the site. They are green 
with a bright yellow head when seen in flight from 
above and behind while all other local species are 
green and have green, dark blue or black heads 
However, both the large macaws and White- 
bellied Parrots are likely using the best of the 
available options for lick use and probably benefit 
from joining mixed species groups as even 
oddballs' receive protection from predators when 
group sizes are sufficiently large (Landeau and 
Terborgh 1986). 
Lick use aggregations similar in structure t' 1 
those at Tambopata have been documented by 
Burger and Gochfeld (2003) at a lick 250 km \o 
the west and by DJB at numerous other licks 
throughout southeastern Peru. The similarities in 
behavior observed across these localities suggest 
that generalizations discussed here may apply 10 
avian aggregations at many geophagy sites. 
