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THE WILSON JOURNAL OF ORNITHOLOGY • Vol. 124, No. 3, September 2012 
TABLE 1. Continued. 
Family/Species 
Number of mixed- 
species bird flocks 
Number of individuals/ More common within raixeil-species 
mixed-species bird flock (mean) bird flocks than apart from them 
Parulidae 
Setophaga fusca 
1 
I 
Myioborus ornatus 
21 
1-4 (2.4) 
Myiotlilypis luteoviridis 
1 
3 
Icteridae 
Cacicus chrysonoim 
2 
2-5 (3.5) 
Fringillidae 
Chlorophonia pvrrhophrys 
2 
2 (2.0) 
garornis squamiger). Golden-crowned Tanager 
(I rid iso mis ru/ivertex). Blue-backed Conebill 
(Conirostrum silticolor). White-handed Tyrannu- 
let (Mercocerculus stictopterus), and Masked 
Flowerpiercer ( Diglossa cyanea). We observed 
at least one of these species in 43 of the 64 flocks, 
and we once observed all five in a single flock. 
We cannot be sure whether one of these potential 
nuclear species was present unobserved in the 
other 21 flocks, or whether some other species 
then had that role. Some of the potential nuclear 
species varied in their proportion of occurrence in 
mixed-species bird flocks along the elevation 
gradient (Table 3). For example, the Golden- 
crowned Tanager was more common in mixed- 
species bird flocks in the higher elevation hand 
(3,300-3,450 m). while the Masked Flowerpiercer 
was more common in flocks of the lower 
elevation band (3.000-3.150 m). 
Other species, besides the five potential nuclear 
species, that occurred only in one positive co¬ 
occurrence also had some qualitative traits that 
seem to be related with a cohesive role. For 
example, the Black-headed Hcmispingus ( Hemi- 
sping us vertical is) and Black-backed Bush Tana¬ 
ger (Urothruupis stolzjnanni) were followed by 
other species on several occasions, contrasting 
with the Golden-crowned Tanager and the Blue- 
backed Conebill that seem to be followers, while 
the Pearled Treerunner was observed both fol¬ 
lowing and being followed. The White-throated 
Tyrannulet {Mecocerculus leucophrys). Golden- 
crowned Tanager, and Blue-backed Conebill in 
some mixed-species bird flocks often gave contact 
calls while foraging. The Golden-fronted White- 
start (Myiohortts omaius). White-handed Tyran- 
nulel. Blue-backed Conebill, and Pearled Tree- 
runner foraged more often with conspicuous 
maneuvers such as sally and hang. The negative 
association among the White-throated Tyrannulet 
and other species is suggestive of competitive 
interaction but we cannot rule out these negative 
associations are due to chance based on the 
available data. Wc consider it important that the 
Masked Mountain Tanager (Buthraupis wet- 
morei), a vulnerable and rare species (Arbelaez- 
Cortes and Baena-Tovar 2006). was observed in 
the paramo within two mixed-species bird Docks 
accompanied by tanagers and finches while 
foraging in branches at mid-level vegetation, 
apparently following the Black-backed Bush 
Tanager. 
DISCUSSION 
The number of species participating in mixed- 
species bird flocks in our study area is within the 
range reported for other Andean sites (10— 7 5 
species) (Poulsen 1996. Herzog et al. 2002. 
Bohdrquez 2003. Rodriguez 2003, Arbelaez- 
Cortes et al. 2011a), and represent 38% of the 
total species known from this area (Arbclaez- 
Cortes et al. 2011b: EA-C and OHM-G. pcTS. 
obs.j. Our documented species participation ts 
below that for other Andean forests iRemsen 
1985. Poulsen 1996. Arbelaez-Cortes et al. 
201 la). Twenty-one species observed by a- also 
have been recorded in mixed-species bird flock'- 
in another site at high elevation (3,200-3,800 m) 
in this Andean region (Rodriguez 2003). Grib 
nine species were shared with mixed-species bird 
flocks from mid-elevations (2,200-2.600 nn 11,1 
the same mountain slope (Arbelaez-Cortes cl 
201 la). These differences are related to species 
composition among communities and indicate 
mixed-species bird flocks reflect the species :” u 'i 
of one locality (Hutto 1994. Peron and Crochet 
2009). However, four species: Pearled Treerun- 
ner. Masked Flowerpiercer, Slaty Brush f |[,L ’ 1 
